Q. J. R. Meteorol. SOC.(1998), 124, pp. 2035-2071
Microstructures of low and middle-level clouds over the Beaufort Sea
By PETER V. HOBBS* and ARTHUR L. RANGNO
University of Washington, USA
(Received 20 November 1996; revised 1 September 1997)
SUMMARY
Airborne measurements in low and middle-level clouds over the Beaufort Sea in April 1992 and June 1995
show that these clouds often have low droplet concentrations (<I00
and relatively large effective droplet
radii. The highest average droplet concentrations overall were measured in altocumulus clouds that formed in
airflows from the south that passed either over the North American continent or were from Asia. Droplet concentrations in low clouds tended to be higher in April than in June. The low clouds in June occasionally contained
drops as large as 35 p m diameter; in these clouds the collision-coalescence process was active and produced
regions of extensive drizzle. Cloud-top droplet concentrations were significantly correlated with aerosols beneath
their bases, but appeared to be relatively unaffected by aerosols above their tops. Anthropogenic sources around
Deadhorse, Alaska, increased local cloud droplet concentrations.
Ice particle concentrations were generally low in April, but high ice particle concentrations were encountered
in June when cloud-top temperatures were considerably higher. On two days in June, tens per litre of columnar
and needle ice crystals were measured in stratocumulus with top temperatures between -4 and -9 "C. Ice particle concentrations were poorly correlated with temperature ( r = 0.39) but, for the two data sets as a whole, the
concentrations of ice particles tended to increase with increasing temperature from -30 to -4.5 "C. Ice particle
concentrations correlated better with the size of the largest droplets ( r = 0.61).
The most common mixed-phased cloud structure encountered was a cloud topped by liquid water that precipitated ice. Liquid-water topped clouds were observed down to temperatures of -31 "C. They are likely common
in the Arctic, and may play an important role in the radiation balance of the region.
Temperature lapse rates in the clouds were generally complex, reflecting either layering, due to differential
advection, or radiational effects. In these cases, vertical profiles of liquid-water content and effective cloud droplet
radius did not vary systematically with height above cloud base, as they do in well-mixed clouds. However, when
the temperature in a cloud decreased with height at or very near the pseudo-adiabatic lapse rate, the cloud liquidwater content (and various measures of the broadness of the cloud droplet size distribution) generally increased
monotonically with height until very close to cloud top.
For clouds consisting entirely of droplets, or droplets and ice crystals, cloud coverage increased by about 10%
when the definition of a cloud was changed from 10 to 5 droplets per cubic centimetre. For clouds containing ice
particles, cloud coverage andor cloud depth increased by about 40% when the definition of a cloud was changed
from 1 to 0.1 ice particles per litre.
The significance of these observations with respect to the effects of clouds on the radiation budget of the
Arctic, and the potential for modification of arctic clouds by pollution, are discussed.
KEYWORDS:Airborne measurements Arctic clouds Cloud droplet concentrations Ice particles Mixedphase clouds
1. INTRODUCTION
Increasing concern over the possibility of global climate change has heightened interest in the factors that affect clouds in climatically pivotal areas such as the Arctic (e.g.
Goody 1980; Abelson 1989; Ackerman et al. 1986). Changes in either the areal coverage
or radiative properties of arctic layer clouds could modify the arctic climate and ice pack
and potentially affect global climate (e.g. Schlesinger 1986; Walsh and Crane 1992; Curry
et al. 1996).
Information on the structures of arctic clouds is relatively scant. Most previous microstructural measurements for summer were obtained in June 1980 during the Arctic
Stratus Experiment (ASE) (e.g. Tsay and Jayaweera 1984; Herman and Curry 1984; Curry
and Ebert 1992) when stratocumulus clouds were widespread (Warren et al. 1988). Cloud
measurements in the Arctic at other times of the year by Witte (1968) and Jayaweera and
Ohtake (1973) suggest that cloud structures are fairly simple and homogeneous over large
areas, with cloud liquid-water content (LWC) generally increasing with height above cloud
* Corresponding author: Department of Atmospheric Sciences, University of Washington, Box 35 1640, Seattle,
Washington 98195-1640, USA.
2035
2036
P. V. HOBBS and A. L. RANGNO
base. However, the ASE measurements showed deviations from this simple picture. For
example, Herman and Curry (1984) and Curry (1986) found that the clouds sampled in a
single aircraft flight in the Arctic in June varied appreciably in both depth and areal extent.
Further, cloud liquid-water profiles were sometimes observed to be more complicated than
those reported for stratus and stratocumulus clouds in lower latitudes (e.g. Neiburger 1949;
Warner and Newnham 1952), or for arctic stratus clouds by Tsay and Jayaweera (1984).
Curry (1986) also observed some complex lapse rates in arctic stratiform clouds.
The results of Witte (1968), Jayaweera and Ohtake (1973), Tsay and Jayaweera (1984)
and Herman and Curry (1984) suggest that neither secondary ice particle production processes (which produce unusually high ice particle concentrations at relatively high temperatures) nor precipitation production by the collision-coalescence process operate in arctic
stratus and stratocumulus clouds. Curry (1986) reported that the concentrations of cloud
particles (diameters >60 pm) measured in the ASE did not exceed 0.001 cm-3 in arctic
stratus (i.e. drizzle was not observed).*
In this paper we present airborne measurements of the microstructures of low and
middle-level stratiform clouds obtained in 99 profiles and horizontal legs through clouds
over the Beaufort Sea in the vicinity of Deadhorse, Alaska (70.20"N and 148.47"W), from
3 to 27 April 1992 and from 3 to 15 June 1995. We will be concerned mainly with the
observations made in June, but the April data are used for comparisons. We also compare
our data with those obtained in the ASE in order to put all three data sets in a climatological
context.
AND PROCEDURES
2. INSTRUMENTATION
All of the measurements presented in this paper were obtained aboard the University
of Washington's Convair C-13 1A research aircraft. The airborne instrumentation used
to characterize aerosols and cloud microstructures have been described by Hobbs et al.
(1991), Rangno and Hobbs (1991), and Hegg et al. (1995, 1996).
The primary instruments that were used to measure the microstructure of liquidwater clouds in our June 1995 study were a Gerber Particle Volume Monitor-100 (PVM100; Gerber et al. 1994), a King liquid-water meter (King et al. 1978), and a Particle
Measuring Systems (PMS) Forward Scattering Spectrometer Probe (FSSP- 100).The same
instruments were used in our April 1992 study, except that the PVM-100 was not aboard.
A Johnson-Williams (J-W) liquid-water meter was aboard the aircraft for both the June
and April field studies.
All of the liquid-water probes underwent extensive wind-tunnel tests at the National
Research Council facility in Ontario, Canada. The FSSP-100 and King probe performed
well in these tests, except that the FSSP-100 showed a slight fall-off in LWC at air speeds
above about 50 m s-' .The true air speeds at which measurements were collected from the
C- 131A ranged from 70-80 m s-l in stratus and stratocumulus clouds to 85-95 m s-' in
mid-level clouds. However, the field data provided no evidence of a fall-off in measured
LWC with sampling speed. Since the wind-tunnel tests showed that the J-W meter measured only about one third of the LWC measured by the other probes, a correction factor
was applied to retrieve more accurate LWC values from the J-W meter.
Liquid-water contents from the Gerber, King and FSSP-100 showed good agreement.
The average LWC measured by these three probes in the June flights were 0.063,0.067,
* In the Glossary of meteorology (Huschke 1989) 'drizzle' is defined as drops of 200-500 p m diameter. We refer to
drops between 50 and 200 p m diameter as embryonic drizzle drops since, under appropriate conditions, such drops
can grow rapidly to drizzle drops.
BEAUFORT SEA CLOUDS
2037
and 0.095 g mP3,respectively. Large momentary differences in LWC were observed occasionally,probably due to the different response times of the instruments. To aid comparison
between the April and June measurements, we report here LWC from the King probe alone.
To avoid contamination of the LWC measurements by ice crystals, the LWC was set to
zero for all probes when the FSSP-100 indicated particle concentrations G3 ~ m - ~ .
If ice particles are present, the concentrations of droplets recorded by the FSSP can
be in error by several droplets per cubic centimetre (Gardiner and Hallett 1985). However,
this possibility for erroneous measurements in the mixed-phase clouds we sampled was
virtually eliminated by requiring that FSSP concentrations be at least 5
and that the
other liquid-water probes indicate a sustained deviation from zero.
Effective cloud partide radii were computed from spectral data obtained from the
PMS FSSP-100 combined with the PMS Optical Array Probe OAP-200. The size channels
of the OAP-200 were moved up by one channel (20 p m diameter) to account for undersizing of particles at aircraft speeds (Baumgardner 1987). In glaciated and mixed-phase
clouds, the effective cloud particle radius obtained from these two instruments include
contributions from solid particles (assumed spherical). This contribution significantly increases the effective cloud particle radius over what it would have been in the presence of
droplets alone (e.g. King 1993).
The Gerber PVM- 100 probe provides measurements of the effective cloud droplet
radius by ratioing the volume of the droplets to their surface area. When flying in mixed
clouds, ice particles produce noise in the PVM-100 signal; however, this noise can be
excluded so that the true effective cloud droplet radius is obtained. The contributions of
solid particles to the effective cloud particle radius will be shown later (see Tables 2 and
3 ) , when we list some simultaneous values in mixed clouds derived from the PVM-100
(with the noise from ice excluded) and the FSSP-100 and PMS-100 probe (which include
droplets and ice).
ice particle concentrations were determined from the PMS 2-D cloud probe, and
occasionally from the Optical Ice Particle Counter (OIPC; Turner and Radke 1973; Turner
et al. 1976) when two-dimensional (2-D) data were not available. The concentrations are
limited to those particles with maximum dimensions 2 100 p m for the 2-D probe, and
particles larger than about 150 p m for the OIPC.
Ice masses and ice paths were determined from crystal types and degrees of riming,
as estimated from PMS 2-D cloud-probe imagery, using empirical relationships given by
Locatelli and Hobbs (1974). in some cases, clouds were deemed not to have contained ice
if the habit of the crystals found in them indicated that they had fallen from higher levels.
For example, some altocumulus clouds with tops warmer than -10 "C contained stellar
crystals. Since stellar crystals grow at temperatures below - 10 "C, they must have formed
in colder clouds and have been transported to the altocumulus.
Cloud coverage and cloud type were estimated by a meteorological observer aboard
the C- 131A aircraft and, in post-analysis, from video recordings made aboard the C-13 1 A.
3 . CLOUDS
SAMPLED AND DEFINITIONS
Figure 1 shows the research area and the general location of the 23 flights made in
April 1992 and June 1995. In all, the flights provided 99 vertical profiles and horizontal
legs through various stratiform clouds.
Our nomenclature for cloud types follows that of the World Meteorological Organization (WMO; 1956, 1975). We refer to clouds with bases at or below 2 km above ground
level (a.g.1.) as 'low' clouds (e.g. stratus or stratocumulus clouds). However, due to the
difficulty of precisely determining when stratus clouds transition into stratocumulus, we
2038
P. V. HOBBS and A. L. RANGNO
Figure 1. Location (dotted area) where the flights took place.
will refer to these low clouds as ‘stratus or stratocumulus’ when a positive identification
was not possible. Clouds above 2 km a.g.1. are termed altocumulus (a thin cloud, mainly
composed of droplets) or altostratus (a deep cloud with a lofted base consisting of cloud
and precipitation-sized ice particles), depending on their visible and microstructural attributes (e.g. Lewis 1951; Plank et al. 1955; WMO 1956, 1975). Cirriform clouds were
often seen, but only one was sampled. Whether clouds merged, and whether or not they
precipitated, was determined from visual observations, airborne video records, and cloud
microstructural measurements.
We define nimbostratus (Ns) and altostratus (As) clouds as those composed solely of
cloud and precipitation ice particles, and with FSSP-100 concentrations ( 5 ~ m - When
~.
droplet concentrations 3 5 cmW3were encountered in Ns clouds below 2 km a.g.l., we refer
to them as ‘embedded stratocumulus’. If such regions were found above 2 km a.g.1. they
are referred to as ‘embedded altocumulus’. In most cases, when flying above 2 km a.g.1.
in deep ice clouds, it is not known whether precipitatiodcloud ice reached to low enough
levels, or to the ground, for the designation nimbostratus. Hence, these situations will be
referred to as ‘Ns/As’ clouds.
Some cloud situations peculiar to the Arctic are not adequately described by conventional cloud reports. For example, as pointed out by Curry et al. (1990), ice crystal hazes,
which are endemic in the Arctic in the coldest half of the year, are not well reported in
synoptic observations. For example, observers are not required to report the cause of an
obstruction to visibility if the horizontal visibility is greater than 10 km, even though an
ice crystal haze may be present. In this paper, we have tried to overcome this difficulty
by referring to situations where ice crystals are present (either as a visual haze or are apparent in the airborne measurements), but are too low in concentrations or vertical extent
to constitute a ‘cloud’, as ‘ice crystal hazes’. However, if a surface-based ice crystal haze
extended above 2 km a.g.1. andor produced a noticeable shading on its underside when
BEAUFORT SEA CLOUDS
TABLE 1. CLOUDMICROSTRUCTURE
Criterion
(a)
(b)
(c)
(4
(el
Minimum concentration
of particles indicated on
the FSSP-100 ( ~ m - ~ )
1
10
5
10
5
2039
CRITERIA USED TO DEFINE A CLOUD
Minimum concentration (per litre) of
precipitation particles with diameter 2 100 pm
and/or
andor
and
and
and
0.1 (glaciated regions permitted)
1 (glaciated regions permitted)
0.1 (glaciated regions not permitted)
1 (glaciated regions not permitted)
<O.l (mostly ice-free)
viewed toward the sun (i.e. optical depths approaching 3-4), we classified it as Ns/As.
The closest WMO cloud description (WMO 1969) of ice crystal hazes (although at quite
different heights) is cirrostratus nebulosus, which is, in essence, a uniform ice crystal haze
typically associated with high-level, aging ejecta from mid-latitude cyclones. We suspect
that many of the ice crystal layers encountered by us in the Arctic during the April study
were ejecta, or the remains of previously denser ice clouds from cyclonic systems.
To summarize, ice crystal hazes, as defined in this paper, are identical to glaciated As
and Ns, except they did not cause a significant dimming of sunlight.
Where does a cloud begin and end? Surprisingly, no formal definition exists. Moreover, rarely are the criteria used to define the presence of a cloud mentioned in research
papers* Yet, the average microphysical and radiative properties, and the areal extent and
depth of a cloud, depend on how a cloud is defined, particularly when cloud droplet or ice
particle concentration are low.
In this paper we report on the dimensions and some average microphysical properties
of arctic clouds using five different criteria for defining a cloud (Table 1). For clouds
consisting entirely of droplets, or mixed-phase clouds, we define a cloud as being present
if either the concentration of droplets with diameter 2 2 p m exceeds 5 cmP3(as measured
by the FSSP-100) and the existence of water was confirmed by one of the other three
liquid-water probes, or if the concentration of precipitation-sized (2100 pm maximum
diameter) particles (ice or water) equalled or exceeded 0.1 per litre (as measured by the
PMS 1-D and 2-D cloud and precipitation probes). For clouds consisting solely of ice or
containing both droplets and ice particles, we will explore the sensitivity of cloud coverage
and cloud depth to changing the definition of a cloud from one with a minimum ice particle
concentration of 1 per litre to 0.1 per litre (as measured by the PMS 2-D cloud probe).
The vertical profile measurements described in this paper were collected randomly,
in the sense that neither thicker nor thinner cloud regions were specifically targeted for
measurement.
4. WEATHER
AND SYNOPTIC SITUATIONS
April 1992 was a synoptically active month in the Beaufort Sea, with the first half
of the month very cold and frequented by the passage of several strong upper lows or
short-wave troughs from the north-west or north. Upper winds were brisk, often exceeding
20 m s-l. However, at mid-month, a striking circulation change occurred in which a strong
upper ridge protruded north-westward from Canada accompaniedby a sharp warming trend
aloft and at the surface, and a general weakening of the 500 hPa flow to < 10 m SKI.For
* In a survey of 30 papers between 1977 and 1992 that dealt with airborne cloud measurements, only two were
found that mentioned how the clouds studied were defined in terms of measured microphysical properties.
2040
P. V. HOBBS and A. L. RANGNO
example, the 500 hPa temperatures for our flights on 3 , 7, 9, 10, 12, and 13 April were
all equal to or less than -40 "C, with surface temperatures ranging from -20 to -28 "C.
In contrast, after the circulation change at mid-month, the 500 hPa temperatures ranged
from -27 to -35 "C, and surface temperatures from -10 to -15 "C. Overall, when the
500 hPa heights on the days on which our April flights took place are averaged, they lie
near the median for a 30-year sample (see section 7).
In the early half of the month, weak surface low-pressure centres accompanied the
upper troughs tracking south-eastward from the Beaufort Sea, bringing changeable surface
winds as they moved across the research area. However, with the mid-month change and
weakening of the upper-air circulation, and strengthening of upper troughs and cyclones in
the North Pacific, the surface pressure remained generally lower in the interior of Alaska
through the remainder of April. Therefore, north to north-easterly winds prevailed during
the latter half of the month.
Flights did not take place in April on the worst weather days. In general, the cold days
on which flights took place in the first half of the month were characterized by thin droplet
clouds, usually sliver thin perlucidus-type layers or isolated stratus fractus clouds, and
varying depths of surface-based but transparent ice crystal clouds. Layer ice crystal clouds
were also encountered. No cloud obscured the sun's disk during the flights in the first part
of April. The second, and warmer, period of the month was marked by a sharp increase
in stratus and stratocumulus (droplet) clouds, many of which were just thick enough to
obscure the disk of the sun.
In June 1995 a large blocking anticyclone developed over north-west North America
as our field study began; its persistence produced south to south-east airflow from about
850 to above 500 hPa (1.5 to 6 km a.g.1.) during most of the period of our measurements.
This circulation pattern produced frequent intrusions of dense and precipitating middle
and high-level clouds from the south associated with several short waves from the Gulf of
Alaska, and enhanced subsidence following the passage of short-wave troughs due to the
amplifying effect on synoptic-scale subsidence of downslope flow in the lee of the Brooks
Range to the south (Fig. 1). Also, the 500 hPa heights over north-west Canada were above
normal for the period. Thus, stratus and stratocumulus clouds were often suppressed, and
completely absent on some days, which is very unusual over the Beaufort Sea in June
(e.g. Warren et al. 1988). Nevertheless, a very shallow northerly to north-easterly flow of
moist air, which is characteristic of the northern coast of Alaska in summer, was present on
most days even when clouds were not present. However, the flights in June 1995, in which
clouds were sampled, generally took place on days that were cooler aloft than normal, and
under general cyclonic conditions at the surface or aloft, especially at the beginning of our
field project.
5. RESULTS
( a ) Sensitivity of the horizontal extent and depth of the clouds to the microphysical
criteria used to define a cloud
To investigate the sensitivity of our statistical results to changing the definition of a
cloud, in terms of its microphysical parameters, we used five definitions (see Table 1).
The most liberal cloud definition was that listed as (a) in Table 1. Criterion (b) was used
by Hobbs et al. (1980), Hobbs and Rangno (1985, 1990), and Rangno and Hobbs (1991,
1994). Criteria (c)-(e) exclude 'clouds' solely composed of precipitation particles (liquid
or frozen), but criteria (a) and (b) do not. For each of the five criteria a sampling duration of
at least 2 s (about 150-180 m in path length) was required before the designation 'cloud'
or 'non-cloud' was applied. This excluded some small wisps from being defined as clouds.
204 1
BEAUFORT SEA CLOUDS
3
600 c
v
5
g so0 -
el
s
2 400-
2
300-
-.-,g
>
.*
200-
6
100 -
0
I
I
3
4
I
5
I
6
I
I
11
14
IS
June 1995
June 1995
3
4
5
6
1
1
1
4
I
June 1995
Figure 2. The effects of changing the definition of a cloud on (a) the total path length flown in cloud, (b) the
average liquid-water content, and (c) the effective cloud particle radius. The diamonds, squares, triangles, circles
and crosses represent, respectively, criteria (a)-(e) in Table 1.
Inclusion of precipitation particles in the criteria for defining a cloud affects both cloud
width and, particularly, cloud depth. Thus, both the horizontal and the vertical dimensions
of precipitating clouds depend on which of the definitions listed in Table 1 is used, For
example, altostratus, which by definition is a precipitating cloud, will have a lower cloud
base if the most sensitive measure of ice particle concentrations (i.e. criterion (a) in Table 1)
is used to define its presence.
Figure 2(a) shows the effects of using the various criteria given in Table 1for defining
a cloud on the total derived path length flown in clouds during June 1995. As can be seen,
changing the criteria for concentrations measured with the FSSP-100 from 5 to 10 cmP2
(glaciated clouds excluded) increases the amount of cloud flown in by less than 10%.
However, when clouds composed of ice only are considered (e.g. many As/Ns clouds),
and the criteria for an ice cloud is changed from 0.1 to 1 per litre (i.e. from criteria (a) to
(b)), the amount of cloud flown in increases by about 40% overall. Thus, in June 1995,
2042
P. V. HOBBS and A. L. RANGNO
tenuous ice clouds (i.e. those having indicated ice particle concentrations between 0.1 and
1 per litre) were a large portion of the cloudscape.
Figure 2(b) shows that the various criteria for a cloud listed in Table 1 can also change
the average LWC, and associated parameters such as droplet concentrations, by tens of per
cent. Lowering the concentration of ice required to qualify as an ice cloud from 1 to 0.1 per
litre lowers the average LWC. This is because of the large amounts of tenuous ice clouds
in the Arctic that lack droplets (i.e. are glaciated). The higher the droplet concentration
required to qualify as a cloud, the higher the average LWC (reflecting perhaps the tendency
for appreciable droplet concentrations to be located in updraught regions).
Figure 2(c) shows the effects of the cloud criteria listed in Table 1 on the effective
cloud particle radius. A secular trend is seen in the data: as the weather warmed during
June 1995, and droplet concentrations tended to increase, there was a general trend toward
smaller effective cloud particle radii. Also noticeable is the significant increase in effective
cloud particle radius when clouds composed entirely of ice are included (see diamonds and
squares in Fig. 2(c)). In these cases, the average effective cloud particle radii are generally
several micrometres larger than those for mixed-phase or all-water clouds.
The most frequent mixed-phased cloud structure encountered in both April 1992 and
June 1995was a cloud topped by liquid water that precipitated ice particles. Liquid-water
cloud tops were observed at temperatures as low as -31 "C (although most of the clouds
encountered at temperatures below -25 "C were comprised solely of ice crystals).
For the purpose of evaluating the measurements presented here on liquid and mixedphase clouds in the Arctic, we used criteria (c) in Table 1 (viz. a droplet concentration of at
least 5
and, for those clouds solely composed of ice, a concentration of 0.1 per litre
of particles with diameters 2100 pm).The latter value was chosen because it appeared to
correspond best with visible ice crystal hazes. Results of a detailed study of the sensitivity
of cloud dimensions to the various microphysical definitions of a cloud given in Table 1 may
be found on our World Wide Web home page (http://cargsun2.atmos.washington.edu/).
( b ) Vertical temperature structures and cloud microstructures
In the Arctic the circumpolar vortex is relatively strong, so that arctic stratus clouds
can be subjected to large dynamic influences associated with rapid changes in upper-level
flow.Temperatureprofiles measured aboard the C- 131A on the first six flights in June 1995
are shown in Fig. 3. These profiles demonstrate both the day-to-day changes in temperature
and the large horizontal variations that can occur in the temperature profile during a flight
of just 4 h duration. For example, on 3, 6, and 14 June large variations in the temperature
profile were measured over distances of less than 300 km. These changes are proxies for
the large changes in the vertical motion fields that enhance and erode arctic clouds.
The low clouds that we sampled in the Arctic often contained multiple lapse rates.
The simplest lapse rate was moist adiabatic. However, most clouds had lapse rates that
were more stable, including inverted lapse rates, either throughout the depth of the cloud or
over some sub-region of the cloud. Lapse rates are usually pseudo-adiabatic in the Arctic
over an appreciable depth only if air colder than the ice-covered surface is advected into
the region, for example, after a cold front has passed (Tsay and Jayaweera 1984). In these
situations the average and maximum values of the liquid-water parameters usually increase
monotonically with increasing height above cloud base. However, in the present study, this
situation occurred on only two days (3 and 4 June 1995) following an intrusion of cold air
aloft.
The majority of the clouds we sampled had complex lapse rates. In regions where
the lapse rate steepened with height above cloud base, the liquid-water parameters tended
to increase with height. When the lapse rates were inverted or isothermal, liquid water
(a) 3 June 1995
(b) 4 June 1995
3-
2 -
1 -
0
Temperature
1
I
("C)
Temperature
("C)
4
3
2
1
5
-10
-5
0
5
Temperature (OC)
Temperature
(OC)
10
15 0-15
-10
-5
0
Temperature
5
10
("C)
Temperature
(OC)
Figure 3. Temperature profiles measured from the aircraft on six flights in June 1995.The various profiles shown
in each panel were measured at different times and different locations during the flight.
tended to decrease with height, or was erratic. The most common lapse rate encountered
was one in which the lower portion of a cloud had an approximately moist adiabatic lapse
rate and the upper portion an inverted lapse rate; this gave the impression of a stratus or
stratocumulus cloud atop which a radiation fog had formed. The liquid-water profile in
the upper portions of such clouds resembled that sometimes seen in valley fogs (e.g. Jiusto
1981); droplet sizes diminished with increasing height in these clouds.
Figure 4 shows some representative vertical profiles of cloud droplet concentrations.
Adjacent layers, separated by only tens of metres (approximately the widths of the lines in
P. V. HOBBS and A. L. RANGNO
2044
4
1 (b)
3 June 1995
(4
4 June 1995
i-
2
2
P
3 June 1995
3 r
F
-1
1
6 June 1995
5 June 1995
.-2
5
2
1 c
0 '
A
I
4
P
0
0
100
200
300
400
Droplet Concentration (cm-3)
500
1
1
14 June 1995
.
1
200 300 400 500
0
100
Droplet Concentration (cm-3)
Figure 4. Vertical profiles of droplet concentrations measured on various days near Deadhorse and over the
Beaufort Sea: (a) 1042-1052 LDT 3 June, Deadhorse; (b) 111@1128 LDT 3 June, Beaufort Sea; ( c ) 1331-1419 LDT
3 June, Deadhorse; (d) 1114-1 127 LDT 4 June, Beaufort Sea; (e) 1715-1724 LDT 5 June, Beaufort Sea; (f) 17021750 LDT 6 June, Beaufort Sea; (g) 130&1455 LDT 11 June, Beaufort Sea; and (h) 1146-1220 LDT 14 June,
Deadhorse. The locations given above are those at the lowest flight level. LDT = local daylight time = GMT -9
hours.
BEAUFORT SEA CLOUDS
P
v
II
**
h
1°1
4 ' .
X
2045
**
**
X
X
** &
x x
*
X
8
e
4
x
X
*
*
I
xs
0.1 -
X
x
0.01
Ll
-40
-35
-30
-25
-20
x
-15
-10
-5
Minimum Temperature in Profile ("C)
0
0
5
I
A
1
I
I
10
15
20
25
30
I
I
35
40
Threshold Diameter (pm)
Figure 5. Relationships between average ice particle concentrations and (a) minimum cloud temperatures
(T',,) for those profiles for which T,,, < -4 "C, and (b) the threshold diameter for Tmin< -4 "C for those profiles
having a droplet cloud at the top of the profile. Crosses for April 1992 and diamonds for June 1995.
Fig. 4), often exhibited substantially different droplet concentrations (e.g. Figs. 4(e)-(h)).
This indicates that closely adjacent layers were not mixing with one another. In many
cases the 'trapping' of thin cloud layers is indicated by slightly stable layers (e.g. compare
Fig. 3(f) with Fig. 4(f)). In other cases the reason for the confinement of layers, and lack
of mixing between layers, is not obvious because the lapse rates are virtually pseudoadiabatic (e.g. compare Fig. 3(a) with Fig. 4(b)). In some cases cloud layers separated
by short distances merged together for a time, as indicated by sudden increases in LWC
while flying horizontal legs. In these cases the LWC and effective cloud droplet radius
increased with height above cloud base, as commonly observed in mid-latitude stratus and
stratocumulus clouds. However, because such regions constituted a minority of the droplet
clouds in which we flew, the average cloud properties measured were overwhelmed by the
essentially non-mixed clouds.
In well-mixed stratiform clouds, the LWC generally increases regularly with height
above cloud base until very near cloud top (e.g. Neiburger 1949; Warner and Newnham
1952). However, significant trends in LWC (and other measures of liquid water, such as
effective cloud droplet radius) with increasing height above cloud base were not apparent
in about half of the clouds sampled. The reason appears to be the complex nature of the
temperature lapse rates in the clouds (Fig. 3), which often produced many thin layers rather
than a homogenous, well-mixed, cloud.
( c ) Ice particle concentrations
Figure 5(a) shows the relationship between our measurements of average ice particle
concentrations and cloud-top temperatures for all of the data from April 1992 and June
1995. The surprising result is that, overall, the ice particle concentrations increased with
2046
P.V. HOBBS and A. L. RANGNO
increasing temperatures ( r 2 = 0.39). This is because the highest average ice particle concentrations were observed in the warmest-topped clouds. There are several reasons for the
considerable scatter in the data. First, very thin ice clouds at very low temperatures can
contain relatively few ice particles, particularly when they are dissipating and far removed
from their origins. This is often the case in the Arctic, which is likely a 'dumping ground'
for stratiform clouds from mid-latitudes, and is affected by weak synoptic systems. Second, ice crystals with maximum dimensions <lo0 pm, which were not measured, may
contribute more to total ice particle concentrations in colder clouds than in warmer clouds.
Third, the concentrations of ice nuclei in the air at higher altitudes may have been less
than at lower altitudes. However, the most likely explanation for the high ice particle concentrations at the higher temperatures is that broader droplet spectra were encountered in
the moderately supercooled clouds than in the strongly supercooled clouds (see Table 2),
thereby providing a necessary condition for the production of high ice particle concentration by secondary ice particle production processes (e.g. Mossop 1985a; Hobbs and
Rangno 1985; Rangno and Hobbs 1994). This view is supported by the results, shown in
Fig. 5(b), which show that the correlation between the average ice particle concentrations
and the threshold drop diameter (DT)*
is 0.61.
( d ) Stratus and stratocumulus clouds
Table 2 summarizes our microstructural measurements for low clouds (stratus and
stratocumulus) and middle-level clouds (altostratus and altocumulus).
The stratus and stratocumulus clouds that we sampled were always less than 700 m
thick. Therefore, those that had bases near the surface always had tops below 1 km a.g.1.
Stratus and stratocumulus clouds sampled in the vicinity (within -30 km) of Deadhorse,
Alaska, usually had substantially higher droplet concentrations than those sampled farther
out over the Beaufort Sea (Fig. 6). This was no doubt due to anthropogenic pollution from
Deadhorse (primarily associated with the local oil industry). The pristine arctic stratus and
stratocumulus clouds over the Beaufort Sea had an average droplet concentration of only
35 ~ m - while
~ , those in the vicinity of Deadhorse had an average concentration of 60 cmP3.
The corresponding average maximum droplet concentrations were 55 and 110 ~ m - re~ ,
spectively. Figure 6 shows the comparison between average droplet spectra measured in
arctic stratus and stratocumulus clouds under pristine conditions, and those measured in
the vicinity of Deadhorse. It can be seen that the primary effect of the local pollution was
to increase the number of smaller droplets, particularly those below 20 p m diameter. The
effective cloud droplet radius was influenced by the higher droplet concentration, being
reduced from 10 p m in the pristine clouds to 9 p m in clouds around Deadhorse. This confirms the conclusion of Hegg et al. (1996) that arctic stratiform clouds are very susceptible
to modification by cloud condensation nuclei (CCN). Comparison of our measurements of
clouds affected by local pollution in the Deadhorse area with those over the Beaufort Sea
could provide an indication of the potential effects of widespread pollution in the Arctic.
Drizzle drops (200-500 p m diameter) were measured in portions of about one-half of
the stratus and stratocumulus clouds that we studied in June 1995; drizzle drops were not
encountered in April 1992. The highest average concentration of drizzle drops measured
over a path length of 1 km was 21 per litre (on 4 June). Ice particles were observed in some
portion of all of the low-level clouds that had tops of -4 "C or colder; they were found in
one stratus/stratocumulus cloud with a cloud top as warm as -3 "Cnear Deadhorse. All
of the clouds with top temperatures warmer than - 10 "C that contained ice also contained
* The threshold diameter ( D T )for a spectrum is such that the cumulative concentration of droplets with diameters
2DT is 3 cm-3 as measured by the FSSP (Hobbs and Rangno 1985).
As/Ns
Sc per
Ice haze
27 April Sc per
(1553) Ice haze
Summary for Apnl
1992
26April
(1552)
(mi)
14 April
(1544)
I 8 April Scper
(1 545)
Ice haze
19 April Ac/As/Ns
(1546)
As/Ns
Ns
s c per
Ice haze
21 Apnl Scper
(1547) Ice haze
21 April Scper
(1548) Ice haze
23April Scper
(1549) Ice haze
24April Scper
(1550) Ice haze
24 Auril Ice haze
13April
(1543)
12April
(1542)
10April
(1541)
Sc per
Ice haze
As/Ns
Ns
Sc per
As/Ns
Ns
s c per
AsNs
Ns
St fra
Ice haze
NS
Date
(flight
Cloud
number) category
Ns
3Apnl
(1538)
Scper
Ice haze
Sc-lofted
Ns
7April
(1539) Ice haze
9 April Ac/As/Ns
(1540)
AsNs
95 (42)
0.2 (0.5)
130(115)
0.9 ( I )
0 (0.2)
27 (26)
1.2 (1.2)
60 (20)
0 (0.3)
55 (38)
0 (0.3)
170 (95)
0.3 (0.5)
65 (61)
0.2 (0.5)
0 (0.1)
0.2 (0.4)
120 (75)
0.I (0.5j
120 (40)
0.1 (0.4)
27(201
1334
1559
2118
745
534
87 1
260 1
480
7782
65
4197
501
3665
428
1806
1469
819
776
2354
1507
52 I
1877
24
I30
0
0
I10
0
0
35
0
0
50
0
180
0
95
0
100
I
0
17
1
65
83
1
1
310
4
280
t
4
I10
4
140
4
310
4
240
4
1
140
340
4
490
4
O.OO(0.00)
0.07 (0.05)
O(O.01)
0.03 (0.03)
0 (0)
0.01 (0.01)
0(0)
0c0.01~
0 (0)
0.02 (0.02)
0 (0)
0.00
0
0.02
0
0
0.07
0
0.02
0
0
0
0.03
0
0
0.03
0.02
0.06
0.03
0
0.3
0.09
0.14
0
0.02
0.01
0.02
0.01
0.07
0.04
0
-
0.11
0.02
6
14
7.9
5.6 ( I .6)
19 (25)
9 (3.7)
17 (12)
15 (20)
11 (5.2)
I4 (17)
7.1 (1.6)
11 (17)
5.6 (4)
11 (18)
3.9 (1.5)
17 (20)
7.2 (3.3)
11 (17)
17 (29)
13
3.5
0
4.2
7.8
8.5
22 (24)
4.3 (3.7)
14 (28)
4.4(1)
9.4 (14)
14.0(17.2)
10
13
9
9.9
7.3
7.9
4.8
6.1
3.7
14
7.9
9.4
16
-
44
5.2
15
0
3.9
0
0
10
5.4
15
18
19
0
0
7.7
2.4
-
41 (24)
13 (14)
25 (31)
18 (33)
5.1 (4.6)
20 (31)
8.5 (22)
23 (30)
5.9 (6.3)
19 (21)
20 (16)
29 (24)
25 (40)
14 (33)
6 (3.2)
9 (21)
102.7
13
105
155
154
62
155
16
155
26
154
155
13
154
31
144
13
155
42
144
145
42
74
155
155
8
I24
155
103
54
153
145
25
I55
155
155
27
145
-
-
-
-
-
11.6(4.2)11.7
-
12 (2.9)
10 (3.7)
-
-
13 (6.4)
-
1 1 (3.4)
10 ( 3 )
-
-
16 (4.4)
13 (6.2)
-
19 (5.4)
16 (4.9)
-
13 (6.6)
-
-
7 (2.1)
-
6 (4.1)
-
9 (2.3)
__
-
7 (2.4)
19.1
Effective cloud particle radius
Threshold diameter, LIT
(wm)
( w )
Average
Average
(standTrd
(standard
deviation)I 1Median Maximum deviation) Median Maximum
18 (29.3)
II
155
7.7 (3.8)
13
9.5
16 (8)
155
I6 (31)
2.6
8 (1.9)
6.1
IS (20)
66
SUMMARY
OF CLOUD PROPERTIES PARTITIONED BY CLOUD TYPE
Liquid-water content
Effective cloud droplet radius
(g m-3)
(wm)
Cumulative Droplet concentration ( ~ m - ~ )
sample
Average
Average
Average
duration (standard
(standard
(standard
(s)
deviation) Median Maximum deviation) Median Maximum deviation) Median Maximum
0.1 (0.4)
0
4
0
4260
30
70 (S5)
300
0.16
61
0
0 (0.2)
4
0.01
3331
25
50 (45)
0.01
170
48
0
0
0
218
0 (0)
1
0
0
1384
0 (0)
20
130
35 (38)
0.01
59
0
4
1866
0.3 (0.6)
0
0 (0.2)
0
1
0
1031
105 (65)
120
200
0.02
23
2
0
5468
0
0 ( 0 1)
0
0
0
1006
0 (0)
0
12970
0 (0.1)
4
0
8
6(1.1)
5
0.01
20
0
2
0
928
0.3 (0.4)
1
0
4
0.6 (0.5)
7839
16
(40)
7
0.04
220
40
0
0
0
677
0 (0)
0
0
0
1138
0 (0)
9
12 (16)
19
0
3
0
1
0.3 (0.5)
0
346
TABLE 2.
B
h,
m
ca
0
F
v)
Effective cloud droplet radius
(wm)
CONTINUED
(Wm)
Effective cloud particle radius
Threshold diameter, DT
( w )
~
j~
49 (30)
75 (22)
0.6 (0.6)
1.3 (0.8)
45 (35)
40 (23)
140 (1 11)
1.6(1.1)
I .6 ( I . I )
24 (21)
60 (32)
160 (75)
85 (58)
l(1.2)
1.6(1.1)
190 (99)
65 (28)
65 (64)
70 (48)
0.2 (0.7)
65 (20)
75 (40)
0.2 (0.8)
110 (45)
~
120
4
4
250
120
450
4
4
120
210
290
290
4
4
330
150
230
230
4
220
160
4
210
139
75
0
1
35
30
130
2
1
16
55
180
85
0
1
220
65
40
55
0
65
75
0
120
46
0.04
0
0
0.01
0
0.12
0
0
0.02
0.07
0.03
008
0
0
0.18
0.03
0
0.07
0
0.1
0.24
0
0
0.06(0.05) 0.04
0.04 (0.02)
0 (0)
0 (0.01)
0.07 (0.I I )
0.02 (0.03)
0.14 (0.1)
0 (0.01)
0 (0)
0.03 (0.03)
0.07 (0.06)
0.07 (0.09)
0.1 (0.08)
0 (0.02)
0 (0)
0.16 (0.11)
0.04 (0.03)
0.03 (0.04)
0.08 (0.06)
0 (0)
0.12 (0.08)
0.32 (0.27)
0 (0)
0.02 (0.07)
0.24
0.09
0
0.04
0.66
0.17
0.56
0.04
0.05
0.1
0.32
0.29
0.43
0.09
0.02
0.3
0.12
0.17
0.38
0.03
0.42
0.99
0.03
0.42
8.2
6.3
6.4 (0.8)
7.8(1.5)
-
7.8
-
12.0
-
-
11.4
11.8
8.4
8.8
-
8.4 (0.7)
8.8 ( I )
-
7.5
7
5.4
8.8
7.9
8.3
7
8.6
8.1
7.8
9.7
10.2
6.1
14.5
14.9
17.4
12
10.1
11.7
12.8
-
-
-
-
13.2
9.8
15.6
7.7
-
7.2 ( I .4)
7 (1.1)
5.3 (0.5)
8.9 (1.2)
8 (2.8)
8.7 (3.1)
7.6(1.2)
8.6 (0.6)
8.3 (1.2)
7.9 (1.6)
-
8.2
4.9
6.2
8 (1.5)
5.5 (2)
6.8 (1.7)
-
6.2
-
5.9 ( I .5)
11.7 (6.0)
4.8(1.l)
-
7.2 (3)
17 (14.6)
25 (23)
7.4 (2.3)
8.3 (1.5)
9 (4.4)
19 (11.8)
16 (9.8)
g(1.5)
7.8 (1.8)
5.4 (1.2)
8.3 (2.8)
I3 (4.6)
13 (3.2)
7.3 (1.4)
6.4 ( I .6)
9 (3.2)
8.2 (2.7)
8.7 (5.9)
10 (1.3)
11 (2.2)
10.9
4.6
-
6.5
15
14
7.9
8.2
7.7
16
15
8.2
7.6
5.4
8.2
14
14
7. I
6.6
8
7.9
9.2
11
12
48.4
8.6
-
II
16
15
21
13
13
11
20
121
84
15
12
74
135
85
11
19
7.9
46
70
21
20.1 (4.7)
13 (3)
-
-
25 (3.1)
28 (6)
21 (4.1)
16 (4.2)
16 (4.3)
20 (5.9)
-
17 (4.5)
18 (3.9)
15 (3.9)
20 (5.5)
-
22 (5.3)
19 (3.5)
22 (5.6)
20.6
13
-
26
30
30.0
23
32
34
-
-
27
25
28
34
-
-
22
17
17
20
-
26
27
22
33
-
33
29
36
20
-
-
17
19
16
20
-
23
19
21
-
-
15
-
15 (2.5)
Average
Average
Average
Average
Average
(standard
(standard
(standard
(standard
(standard
deviation) Median Maximum deviation) Median Maximum deviation) Median Maximum deviation) Median Maximum deviation) Median Maximum
lO(l0)
6
40
0.02(0.01) 0.02
0.04
15 (5.5)
16
27
21 (4.4)
23
25
1
4
O(O.006)
0
0.04
20(16)
16
154
l(O.8)
0.6 10.7)
0
4
0
0.03
21119)
I6
145
9.2 (1.6)
0.39
58
30 (25j
25
180
0.11 {0:07) 0.1
9.4
13.5
14 (6.2)
13
26 (5.1)
26
39
62
10.8 (2.5) 11.5
12
250
0.04 (0.05) 0.02
13.6
0.24
15 (10.4) 15
60 (77)
21 (8.6)
20
39
81
9.2 (1.6)
0.18 (0.12) 0.16
0.53
9.4
14.8
I40
28
38
40 (26)
30
14
14 (4.8)
27 ( 6 )
Droplet concentration ( ~ r n - ~ )
See text for definitions of the cloud categories.
Cumulative
Date
sample
(flight
Cloud
duration
(s)
number) category
3 June Ac/As/Ns
16
1243
(1675)
As/Ns
546
Ns
869
svsc
Sc (feeder)
79
5174
4 June
Sc
( 1676)
43
5 June Ac/As/Ns
1276
(1677)
As/Ns
27
Ns
1017
st/sc
Sc (feeder)
215
1842
6 June Ac/As/Ns
952
(1678)
As/Ns
190
Ns
svsc
208
Sc (feeder)
774
108
sc
11 June AciAsMs
2535
2496
(1682)
As/Ns
813
NS
117
svsc
298
Sc (feeder)
sc
480
1665
14 June Ac/As/Ns
(1684)
Ams
850
2635
st/sc
90
sc
15 June
Ns
61
931
(1685)
Sc
Summary for June
1995
950
TABLE 2.
Liquid-water content
(g m-3)
0
>
z
Q
z
7J
r
?
a
e;
E
m
0
z
5
P
03
P
N
0
BEAUFORT SEA CLOUDS
2049
AD
=I7
Ar
=8.3
~t =0.27
30
......................
(b) 4 June
*r =5.3
*,=p",
210
01&&
0
.............................
(d) 6 June
30t n
(e) 11 June
Droplet Size (pm)
(Q 14June
Legend:
....&.....
-~*.
Pristine arctic stratus and stratocumulus
clouds
Arctic stratus and stratocumulus clouds
at and near Deadhorse
- - Altocumulus clouds
.
3.5
9.5
15.5 21.5 27.5
Droplet Size (pm)
(g) 15 June
33.5
.
Lofted stratocumulus clouds
39.5
Figure 6. Droplet spectra measured in ice-free or nearly ice-free regions of clouds on seven flights in June 1995 in
which stratus, stratocumulus and altocumulus clouds were sampled. Open triangles are stratus and stratocumulus
clouds sampled over the Beaufort Sea upwind of the northern Alaska coastline; filled triangles are stratus and
stratocumulus clouds sampled near the coastline or inland that exhibited continental influences; filled diamonds
are 'lofted' stratocumulus clouds with bases located above the boundary layer but below 2 km a.g.1.; and filled
squares are either altocumulus or embedded altocumulus in altostratus. The subscripts 'D',' r ' , and ' l ' , denote,
respectively, the average droplet concentration ( ~ m - ~the
) , effective cloud droplet radius (fim), and the liquid-water
content (g m-3) measured in each cloud type.
2050
P. V. HOBBS and A. L. RANGNO
drizzle drops. However, with very brief isolated exceptions, ice and drizzle drops did not
co-exist.
High ice particle concentrations (> 10 per litre over path lengths 2 1 km) were observed in stratocumulus clouds with top temperatures as high as -5 "C, both in the vicinity
of Deadhorse and in pristine conditions over the Beaufort Sea. On 3 June, ice particle concentrations of tens per litre were measured in stratocumulus with tops at -6 and -9 "C,
both near Deadhorse and over the Beaufort Sea. On 4 June, ice particle concentrations of
tens per litre were measured in clouds with tops at -4 to -6 "C. The maximum ice particle
concentrations measured were about 40 per litre averaged over distances of 1 km or more.
In all of these cases there was no hint from above cloud top of the considerable ice particle
concentrations lower down, since the tops of these clouds were comprised primarily of
liquid water.
Precipitation, in the form of either drizzle or ice particles, occurred in all of the
arctic stratus and stratocumulus clouds that were sampled in which the threshold drop
diameter exceeded 30 pm. The large tail of the droplet spectrum is surprising in view of
the low cloud-base temperatures, and hence limited moisture available to these clouds. For
example, extensive drizzle was encountered on 3 and 4 June, even though the cloud base
temperatures were -3.5 and -2 "C respectively. The maximum concentrations of drizzle
drops were often in the low tens per litre, and the maximum concentrations of embryonic
drizzle drops (50-200 p m diameter) were often in the low hundreds per litre.
Elsewhere on 4 June, a region of widespread stratus and stratocumulus clouds was
sampled that was shallower than the stratocumulus clouds that contained high ice particle
concentrations. The temperature at the top of this cloud layer was only -3 "C. This cloud
also contained extensive regions of drizzle and had a very broad droplet spectrum. Spikes
in the OIPC record indicated that a few of these drops (probably <0.001 per litre) were
frozen. This is the warmest cloud-top temperature at which ice has been reported. Ice was
not detected in clouds that had top temperatures warmer than -3 "C.
Various measures of LWC in stratus and stratocumulus clouds often varied erratically
with increasing height above cloud base (Fig. 7). Ninety per cent of the individual cloud
droplet layers profiled in June were less than 300 m thick, and no layer was even as much
as 650 m thick, although the overall depth of the moist layer in which the cloud layers were
embedded was often much greater. This fine layering of droplet clouds is reminiscent of
the layering of hazes in stable environments, where inter-layer mixing is suppressed and
each layer retains its own identity.
In contrast to June 1995, most of the clouds encountered in April 1992were colder than
- 15 "C at cloud top; they were either comprised completely of ice particles or very light
snow fell from them. The minimum low-level cloud temperature encountered in April was
-25 "C, and the warmest was -5 "C. The coldest droplet cloud encountered in horizontal
aircraft passes, however, was a mid-level altocumulus translucidus cloud at -31 "C near
3 km a.g.1. When droplets were present, they were small. The largest threshold diameter
encountered in April was 29 p m but more commonly it was t 2 0 p m (Table 2). As a result,
precipitation-sized drops did not form in these clouds by collision-coalescence.
Liquid-water contents were also very low in the cold April clouds, averaging only
0.01 g m-3. The highest LWC measured in April was 0.45 g m-3 in stratocumulus clouds
(Table 3 ) , which was encountered on the warmest day (19 April 1992).
A review of the video recordings showed that, in spite of the considerably lower
temperatures during the April flights than the June, regions of isolated to occasionally
overcast conditions comprised of thin droplet clouds (e.g. stratus fractus, stratus, stratocumulus perlucidus, altocumulus perlucidus) were observed in some portions of each
flight.
205 1
BEAUFORT SEA CLOUDS
Liquid Water Content (g rnS)
-
*
I.
'-"I
**
* &.:
-
..
*'
*
I
rn
9
.:*
*
.
'.
-
I .
300
.
loot
f
00
1
I..
J,
I
30
40
700
(c)
Js
600 t
*
500400300
2E,
200 -
***
a *
i.*
-
* a '*
. . :*.: * t o
:
u
8
ft ".. ,
't
* * s t
(u
-
.
*
*.
:
*.
*
***I*
* I . * '
*'
**.
*o*:i:
**
100 -
0
0
10
20
30
40
50
Figure 7. Three measures of cloud liquid water in the June 1995 arctic stratus and stratocumulus clouds versus
height above cloud base: (a) liquid-water content, (b) cloud particle effective radius, and (c) the threshold diameter
(DT)
for the droplet spectrum.
A
CloudProfile began- Lat. (N),
base
Profile orofile ended Long. (W) Cloud
height
Date
number . (LDT)
(deirees) situation? (km, &I.)
3April 1992
1
1342-1346 70.2,148.6 WTI
0
1417-1441 72.4. 144.3
G
1.47
2
0.23
1446-1449 72.4. 143.5 WTI
3
0
G
4
155CL1553 72.4, 143.9
7April 1992
1
0
G
1348-1352 72.8, 144.5
G
0
2
141CL1415 72.8, 144.6
G
0
1423-1427 72.8, 143.7
3
0
G
1435-1438 72.8, 143.9
4
0
9April 1992
1
1 103- 1 104 70.2, 148.4
G
1210-1212 72.2, 144.2 WTI
0.58
2
0.24
10Apnl 1992
1346-1354 70.3, 148.0 EDC
G
0
1433-1450 71.6, 150.3
L
0
G
3
1 653-1 701 72.0, 149.6
1.44
4
G
1704-1 708 71.9, 149.3
1.01
G
1718-1746 71.4, 149.6
5
0.07
1510-1549 70.8, 151.2 EDC
12April1992
1
0.21
2
1551-1619 70.2, 149.3 EDC
1.85
G
1541-1553 71.7, 147.6
13April 1992
1
WTI
0.03
1137-1 148 70.4, 148.2
18April1992
0.57
121CL1219 71.8, 147.6 WTI
i
0.03
1113-1218 71.4, 146.9 WTI
19April 1992
I
1.05
2
1313-1327 73.2, 150.0 EDC
0.22
1112-1140 70.6, 147.8 WTI
21 April 1992
1
0.01
2
1256-1315 72.0, 156.5 WTI
0.88
1743- 1744 71.9, 154.0
G
21April 1992
1
1.47
1328-1333 70.2, 148.0 WTI
23April 1992
1
7
0
1658-1704 72 S , 149.5 WTI
009
1 1 w 1 1 1 2 70.5, 148.3 WTI
24April1992
1
0
1437- I45 8 72.1, 155.3 WTI
2
0.08
1425-1430 71.1, 148.4 WTI
26 April 1992 1
5.59
G
1501-1510 70.2, 147.4
2
5.65
G
l51&1515 70.3, 147.0
3
4
0.14
17I 3-1 730 70.4, 148.5 WTI
114L1210 72.7. 152.9 WTI
0.04
27 April 1992 I
7
1227- 1259 72.7, 155.2 WTI
0.04
0.8
1302-1 302 73.0. 155.3 WTI
Summary for April 1992 (36 profiles)
0.66
~~
Liquid-water content
Average
Maximum Average effective Total
threshold ice particle cloud
liquiddiameter,
concenparticle
water
DT
tratiou
radius
path
Average Maximum
(g m-j)
(g m-3)
(pm)
(per litre)
(pm) (g-m-')
0.00
0.01
12
1.3
1 1.
n
.
0.00
0.00
0
0.26
15
0
0.04
0.17
28
0.36
18
12
0.00
0.00
0
8
0.4
12
0.00
0
0.13
24
0
0.00
0.00
0.12
0.00
29
0
0
0.38
0.00
0.00
0
45
0
0.34
0.00
0.00
0
40
0
0.00
0
22
0
1.1
0.00
0.00
12
0.5
22
0
0.03
1.1
0.00
0.00
5
10
0
0.4
0.00
0.00
0
18
0
0.00
3.9
0.00
26
0
0
0.15
0.00
0.00
0
4.3
0
0.21
0.00
0
7
0
0.00
0.64
0.01
14
18
0.1
0.00
0.7 1
16
0.3
0.00
0.06
20
0.00
0
0
0.8
25
0.00
0.09
24
4.6
0
1.3
0.00
2.2
16
0
0.03
I1
0.00
3.3
0.02
20
76
0.45
26
0.57
0.03
9
58
0.30
24
I .3
10
7
24
0.00
0.06
13
16
0.55
0.14
24
0.01
0.00
0
0.07
0
0.00
0.06
0.00
12
2
0
0.01
0.01
0.88
14
3
8
0.02
0.00
0.69
14
6
26
0.07
0.00
21
0.33
0.04
5
5.5
0.00
11
0.36
0.01
10
0.7
0.00
1.7
18
0.00
0
0
1.8
0.00
0.00
0
24
0
0.00
16
0.49
0.03
9
0
0.00
18
0.08
7
0
0.06
0.01
0.03
17
0.06
4
9
0.00
0.05
4
0.0 I
0
10
~0.01
0.05
10.3
0.79
15.6
5.5
OBTAINED IN VERTICAL PROFILES
Cloud- CloudCloudDepth
base
top Droplet concentration
top
of
temper- temperheight
profile
ature ature Average Maximum
(km, &.I.) (km, a.g.1.) ("C)
("C) ( ~ r n - ~ ) (cme3)
1.05
1.05
-12.5 -19
9
300
1.87
0.40
-19
-21.5
0
0
0.54
0.31
11
-21
-19.5
55
0.39
0.39
0
5
-22.5 -20
0.05
0.05
0
<5
-26.5 -27.5
0
0
0.04
0.04
-26.5 -27
-27.5
0
0.03
0.03
0
-26
-27
0.11
0.11
0
-25
0
0.32
0.32
0
-21
-23
C5
25
0.87
0.29
200
-21
-20
8
1.64
1.40
0.3
-25
-25
0
0
-25.5
-25
1.60
1.60
0.70
0.70
0
<5
-22.5
-25
2.22
0
0
0.78
-24.5 -28.5
4.79
3.78
0
0
-23.5 -36.5
0.6
220
4.93
4.86
-20.5 -40
11
0.8
4.86
4.65
-22.5 -39.5
0
0
2.99
1.14
-21.5 -28
- 10
270
50
0.49
0.46
- 10
0
-9.5
9
0.77
0.20
-6
3.84
3.81
30
-8.5 -18.5
490
2.98
1.93
13
-6.5 -15
100
11
-9.5 -14.5
2.00
1.78
100
-10.5 -13
35
1.57
1.56
20
0
0
-11.5 -13
1.06
0.18
13
85
-14.5 - 16
1.64
0.17
0.32
0.32
- 14
30
240
-11
114
1.05
14
-13
-16.5
230
-13.5 -17
0.76
0.76
40
210
0.05
0.13
240
-21
- 18
65
5.80
0.21
0
<5
-34.5 -36.5
5.85
0.20
0
C5
-35.5 -36.5
4
310
0.63
0.49
-20.5 -13.5
- 16.5 -20
0.62
0.58
90
I90
280
-16.5 - 18.5 95
0.96
0.92
55
100
-18.5 -18.5
0.85
0.05
1.68
1.02
-19.1 -22.0
16
121
TABLE 3. SUMMARY
OF CLOUD DATA
tso
r
2
?
3.2
-
1
1
0
0.7
0.7
0.7
-
2
0
4 ' 9
4
0.7
0.3
4
-
-
0.6
5
20
62
5
1
0.3
3
-
2
0.4
2
R
Total
icewater
path
(g-m-')
11 June 1995
6 June 1995
5 June 1995
6
2
3
4
5
1
6
7
8
9
1
2
3
4
5
6
1
2
3
4
5
6
7
5
Profile
Date
number
3 June 1995
1
2
3
4
5
4 June 1995
1
2
3
4
1042-1 101
1130-1200
1250-1256
1326-1332
1412-1 41 9
1055-1057
1116-1126
1127-1137
1145-1205
1212-1213
1302-1 3 I9
131 8-1 33 1
1335-1336
141G-1431
144-1442
1445-1501
1511-1512
1717-1 7 18
1719-1 72 1
1722-1724
1412-1414
1429-1435
1529-1532
16541657
1658-1 701
1700-1731
1738-1750
1142-1143
1158-1205
1241-1 300
1301-1348
1351-1432
1453-1455
(LDT)
Profile beganprofile ended
~
Average
Maximum Average effective Total
Cloud- CloudCloudCloudDepth
base
top Droplet concentration Liquid-water content threshold ice particle cloud
liquidLat. (N),
base
top
of
temper- temperdiameter,
concenpecle
water
Long. (W) Cloud
height
height
profile
ature ature Average Maximum Average Maximum
DT
tration
radius
path
(degrees) situation? (km, a.g.1.)(km, a.g.1.) (km, a.g.1.) ("C) ("C) ( ~ m - ~ ) ( ~ m - ~ ) (g mP3) (g m-3)
(pm)
(per litre)
(pm)
(g m-')
70.4,147.5 WTI
0
2.77
2.77
-2
-18.5
16
250
0.01
0.24
39
0.72
15
28
71.8, 151.6 WTIL
0.73
-3.5
0.59
5
40
0.14
-9
0.11
35
5.9
20
65
0.33
71.8. 150.9 WTIL
0.08
~-8.5
0.69
-4
0.61
13
30
0.14
0.42
39
16
25
85
71.1: 150.3 WTI
2
2.39
2.92
-10.5 -11.5
0.53
17
40
1
0.00
25
2
0.04
70.2, 148.6 WTI
0.21
0.47
-3.5
0.26
13
40
0.09
180
-5.5
34
0.32
0.32
23
70.2, 148.3 WTI
0.62
20
-5.5
0
-1.5
31
0.62
0.48
43
0.07
16
140
30
71.0, 148.2 WTI
-6
70
0.14
0.63
-2.5
0.49
0.51
132
110
10
0.27
11
35
71.0, 148.3 WTI
-2
0.67
0.55
25
100
11
-6
0.12
30
0.32
10
50
0.09
70.6, 147.9 WTI
0.62
0.53
-2
9
20
95
-6
0.09
31
0.30
21
6.3
0.04
70.4, 146.8 WTI
0.46
0.32
-5.5
7
-2.5
25
35
0.14
36
13
61
0.33
0.19
-1
L
-3
71.5, 146.0
0.07
0.33
0.26
19
0
0.48
0.24
30
55
38
62
L
0
71.1, 145.8
0.59
0.52
0
19
-3.5
35
100
0.24
0.07
37
125
0.52
71.2, 144.5
0.25
L
0.10
-1.5
-3
35
0.15
32
60
0
20
10
0.49
0.20
0.53
70.2, 148.4 WTI
0.40
-1.5
-4.5
30
0.13
32
140
28
16
26
0.32
0.04
70.4, 148.3
0
0.05
0
-0.5
0.08
0.03
10
55
0.05
0
10
0.12
32
3
70.7, 147.3 0, F
0.50
0.28
-1.5
6.5
35
I20
0
0.22
29
0.02
6
6
0.16
71.1, 146.9 0, F
2.11
0.15
0
-0.5
0.5
75
130
1.96
20
0.09
5.5
5
0.03
70.8, 145.6
0
5.5
0.47
0.19
0.5
0.27
0.66
0
7.5
95
140
0.28
31
51
70.8, 145.7
-0.5
-1
0
0.14
0.05
0
5
0.09
0.16
25
45
0.09
21
5
70.7, 145.7
0
4
0.46
0.21
0.5
9
42
0
0.20
0.48
65
120
0.25
33
0.42
0.35
5
25
70.2, 148.3 0, F
-1
60
0.07
7
46
0.13
0.3 1
0
120
0.63
-4
31
70.6, 145.6 WTIS
-1.5
3.06
2.43
25
0.10
6.7
17
7
0.01
180
0.07
0
-1
70.5, 144.5
0.22
25
0
8
-1.5
22
40
0.15
0.09
3
0.04
70.8, 144.9
2.24
3.5
0
0.33
2.5 230
22
0
460
1.91
3
0.29
23
0.07
0
1
0
I80
0.15
70.8, 145.0
2.61
2.46
0.24
20
0
0.11
370
3.5
17
1
-2
70
30
1.04
70.7, 145.8 0, F
2.60
1.56
7
0.43
99
0.10
0
280
-0.5
6
65
26
0.56
70.2, 148.9 0, F
0.69
0.13
0
5.5
0.43
34
0.06
210
70.2, 148.4
0.14
0
0
3.5
0
7
11
6.5 160
27
0.14
320
0.16
0.08
70.9, 147.7 0, F
0.06
0.03
6
4.5
0
65
25
0.03
150
0.04
I
6.5
0.13
3.07
-7
-13.5
21
1.07
71.6, 146.8 WTI
4.14
0.03
3
6
0.38
33
240
32
3.18
-7.5 -13
0.88
71.4, 147.0 WTI
4.06
30
0.04
4.2
8.5
0.31
30
270
35
1.48
30
1.40
71.2, 148.5 WTIS
2.88
1.5 -6
42
0.03
23
13
0.20
18
200
n 22
I
4
19n
zzn
0
n
70.2. 148.5
0
26
33
0.15
7.5
0.30
TABLE 3. CONTINUED
0
0
0.8
2
3.7
0
-
0
0
0
0
0
0
0.9
0.6
1
2
0. I
0
0
0
0.6
0
0
0
0
0
0
-
0. I
3
3
-
Total
icewater
path
(g m-')
R
0
C
r
0
F
IA
2s
F
m
*In vertical profiles the aircraft climbed through most of the depths of a cloud from near cloud base to near top.
I'WTI = liquid-water-topped, ice-producing layer; WTIS = liquid-water layer containing secondary ice particles due to a settling of snowflakes from above; WTIL = liqud-watertopped, ice-producing layer, which also contained drizzle; G = glaciated cloud; EDC = embedded droplet cloud layers; L = contained drizzle: 0 = no precipitation; F = precipitation,
liquid or solid, fell into clouds at above freezing temperatures (feeder cloud).
LDT = Local daylight time (GMT - 9 hours).
Average
Total
Cloud- CloudMaximum Average effective Total
CloudCloudDepth
base
top Droplet concentration Liquid-water content threshold ice particle cloud
liquidicebase
top
of
temper- temperdiameter,
concenparticle
water
water
Profile began- Lat. (N),
Profile profile ended Long. (W) Cloud
height
height
profile
ature ature Average Maximum Average Maximum
OI.
tration
radius
path
path
(LDT)
Date
number
(degrees) situationt (km, a.g.1.) (km. a.g.1.)(km, a.g.1.) ("C) ('C) (cm-')
(cm-')
(g m-3)
(Wm)
(per litre)
(Wm) (g m-*) (g m-2)
(g m-3)
14 June 1995
1
1147-1147 70.2. 148.4
0
0.2
0.23
0.03
1.5
3.5
0
0
50
140
0.00
12
0
2
0.01
2
0.04
5.5
5
60
110
0.09
4
0
24
1148-1 I48 70.2, 148.3
0
0
9
0.5
0.54
0.19
0
0.04
0.56
0.58
0.02
5.5
5.5
13
18
0.03
0
0
3
21
1148-1 148 70.2. 148.3
0
9.5
4
0
0.06
1 1 50-1 15 1 70.2, 148.0
0.04
0
5
2
0
0.91
0.95
0.04
4
3.5 100
160
17
0
50
5
1153-1 153 70.2, 147.7
0.04
1
1
0.05
0.10
0
8
2
0
1.35
1.39
85
27
1153-1 153 70.1, 147.7
0
0
6
0
1.42
1.43
0.01
1
0.5 110
0
5
0.02
160
15
0.04
1154-1 155 70. I , 147.6
L
0.5 - 1
0
7
1.49
1.67
0.18
75
1.00
0
12
0.38
I60
33
1 155-1 155 70.1, 147.5
0
1.71
1.76
0.05
8
-1
-1.5
60
0.42
0
12
13
0
0.25
110
32
47
0.6
0
1200-1207 70.2, 146.3
9
17
2.59
3.37
0.78
0.32
0.9
10
-5
-8.5
0.06
I30
34
3.5
4.41
0.91
10
46
3
-8.5 -14.5
1208-1 219 70.3, 144.7 WTIL
140
20
11
0.28
1.5
0.05
32
2.6
11
1225- 1240 70.5, 142.9 WTIL
3.78
4.13
0.35
I00
I .6
11
0.03
-8
-13
13
0.16
26
12
19
130
0.27
13
-10.5 -15
1242-1 250 70.8, 143.9 WTI
3.77
4.35
0.58
35
1.4
0.06
31
13
-15
-19.5
12
0.36
200
1250--1300 71.1, 144.3 WTI
4.35
4.89
0.54
0
0.9
14
0.00
0.03
14
4.74
4.89
-18.5 -19
0.06
5.5
9
0.1
95
180
0.10
0.2
1303-1309 71.0, 143.9 WTI
0.15
18
15
L
1336-1340 72.3, 146.5
0.18
0.33
0.15
0.5 -3
100
0
11
0.17
26
0
55
0.42
32
0.31
0.19
25
16
1 340- 1 348 72.5, 146.9
L
0
0.12
-2
-2.5
50
75
0
11
0.13
0.32
31
L
79
0.14
0.29
-1.5
-2.5
0.15
26
0
0.17
17
1401-1403 72.8, 147.0
50
0
10
0.33
31
L
70
0.18
0.28
0.10
-1.5
-2
0
7.5
9
0
0.09
18
1408-1 410 73.0, 147.0
35
0.25
27
55
1410-1416 72.9, 147.2
L
95
0
7.5
22
0
0.08
0.26
0.18
-1
-2.5
0.12
19
0.29
28
1416-1
429
L
0.11
029
0.18
1
-2.5
60
0
7.5
25
0
85
0.14
72.5, 147.5
20
0.29
31
L
0.07
0.23
0.16
-1
-2.5
65
11
35
0
0
1432-1435 72.0, 147.7
85
0.22
21
0.37
31
0.07
-1.5
-2.5
L
0.19
0.12
0
II
17
0
65
1435-1435 71.9, 147.8
70
0.14
22
0.20
32
0
0.77
0
0
1511-1511 70.5, 148.5
23
0.01
0.78
3.5
3.5
0.00
0.00
0
3.5
30
50
10
0
6
0.32
5
0
0
1527-1528 70.2, 148.6
220
24
20
0.06
5.5
3.5 170
0.38
0.08
0.15
0
0.22
0.34
0.12
8
0
200
1136-1136 70.2, 148.4
20
0
0.5
15 June 1995
I
0.19
0
6
0.07
110
2
0.09
1
0.5
0
7.5
0
0.27
0.30
0.03
42
0
1152-1152 70.6, 147.6
21
55
0.06
35
0
1
0.5 100
0.12
0.22
0.10
190
0.04
0.08
0
3.5
3
1152-1 156 70.6. 147.7
17
4
1205-1206 70.6; 148.0
0
0.12
0.26
0.14
13
0
0.5
0.1 110
0.09
0
6.5
170
0.18
22
5
0
5
1.62
1217-I218 70.6, 148.2
0.09
I
0.5
21
0
4.5
0
1.53
70
120
0.05
0.12
6
8
n
1359-1400 70.2, 148.6
0
0.23
0.33
23
0
4.5
0.10
2.2
180
0.08
0.16
0.8 120
9.5
26.1
0.4
Summary for June 1995 (63 profiles)
0.92
1.27
0.35
-1.2
-2.9
58
140
0.10
0.26
27.3
2.8
TABLE 3. CONTINUED
BEAUFORT SEA CLOUDS
2055
Cirrus and thin altostratus (translucidus) clouds, comprised solely of ice crystals,
were also observed. Occasionally, weak sun dogs and 22" haloes were produced by ice
crystal clouds.
Ice crystal clouds or hazes that reduced visibility were common. These clouds generally had much greater depths than the droplet clouds, which were often only a few tens
of metres in depth. The ice crystal clouds encountered were almost always within 1 km of
the surface, but several ice crystal hazes extended to above 2 km (Table 3).
The WMO definition of altocumulus cloud requires that it has a base at or above
2 km a.g.1.; however, these clouds are similar in their physical structure to stratocumulus.
Altocumulus clouds must contain droplets. Altostratus clouds are also defined by cloudbase height, although in other respects they are the same as nimbostratus clouds (i.e. a
precipitating stratiform cloud composed largely of cloud ice particles and snowflakes)
thus producing the 'ground glass' appearance associated with these clouds. Occasionally,
layers of embedded droplet clouds were encountered in altostratus, and the tops of some
altostratus layers consisted of altocumulus-like droplet clouds (cf. Cunningham 1957;
Hobbs and Rangno 1985; Rauber and Tokay 1991).
( e ) Altocumulus clouds
It can be seen from Fig. 4 that the thicknesses of the altocumulus clouds (-30 to
800 m) were generally similar to those of stratus and stratocumulus clouds, although they
were at lower temperatures (1 to -3 1 "C).Altocumulus clouds were also usually present in
very fine layers. Droplet concentrations in the altocumulus ranged from -105 to 450 ~ m - ~ ,
which was generally higher than for the stratus and stratocumulus. Droplets larger than
25 p m diameter were rarely encountered. Also, unless they were colder than - 10 "C, the
altocumulus clouds did not precipitate. However, in some cases, secondary ice particles
were apparently created by the fall of ice crystals and snow into altocumulus layers having
a broad droplet spectrum. The average effective cloud droplet radius for the altocumulus
was 10 pm. An exception to this picture was observed on 14 June when a few drizzle
drops formed in altocumulus clouds at temperatures below - 13 "C.
As in the case of the lower clouds, there was no systematic trend in any of the liquidwater parameters with height above cloud base for the altocumulus clouds (Fig. 8). We
attribute this to the frequent lack of pseudo-adiabatic lapse rates, resulting in poorly mixed
conditions. Also, while some clouds appeared to be contiguous over substantial depths
(e.g. Figs. 4(f) and (g)), large changes in droplet concentrations indicated otherwise.
( f ) Glaciated clouds: altostratus, nimbostratus, and ice crystal hazes
Glaciated altostratus clouds, and the glaciated upper portions of nimbostratus clouds,
could not be distinguished in these studies. This is because those upper regions of nimbostratus clouds that produced widespread precipitation at the ground were virtually identical
with altostratus clouds, which, by definition, are above 2 km a.g.1. Portions of nimbostratus
clouds below 2 km were composed of larger particles than aloft. Hence, the effective cloud
particle radius for nimbostratus clouds below 2 km was larger than for Ns/As clouds above
2 km (Table 2).
Ice crystal hazes, which were colder than the Ns/As clouds, contained fewer large
(> 100 p m maximum dimension) ice particles than Ns/As, but otherwise their microstructures were similar to Ns/As clouds. They had the smallest effective cloud particle radius
of the ice clouds sampled (Table 2). Frequently, these hazes seemed to be the residuals of
briefly lived, very thin, droplet layer clouds, such as stratocumulus perlucidus or stratus
fractus clouds, or droplets from open leads during very cold periods.
2056
P. V. HOBBS and A. L. RANGNO
Effective Cloud Particle Radius (pm)
Threshold Diameter, DT (pm)
Figure 8. As for Fig. 7 but for the June 1995 arctic altocumulus clouds, separate or embedded in or at the top of
altostratus clouds.
BEAUFORT SEA CLOUDS
2057
6. Two ILLUSTRATIVE CASE-STUDIES
In this section we describe two case-studies that illustrate in more detail some of the
characteristics of arctic stratus clouds that have been summarized in the previous section.
( a ) Pristine low-level clouds that produced considerable ice and drizzle
On 3 June 1995 a large region of low pressure was situated over the interior of
Alaska. This brought vigorous, but shallow, north-easterly onshore flow to Deadhorse,
Alaska, with stratocumulus clouds over the north shore of Alaska and the Beaufort Sea.
The top of this cloud layer varied in height from 0.5 to 1 km a.g.1. (e.g. Figs. 3(a) and 4(a)).
Back trajectories suggest that the surface flow originated over the polar ice cap to the
north and north-east of Deadhorse. Above 1 km the flow was southerly to south-westerly
around a quasi-stationary upper trough, the axis of which lay near Barrow, Alaska. Thick
altocumulus and altostratus clouds, with bases above 2.5 km a.g.l., were embedded in
south-westerly flow above the stratocumulus clouds. These higher clouds produced spotty
areas of light precipitation in the form of single ice crystals and ice aggregates and their
fragments, which fell into the stratocumulus clouds sampled early during the flight on this
day. However, the measurements in the stratocumulus to be discussed here were obtained
in a region where there was no cloud overhead. Nor were any ice crystals detected in the
clear air just above the stratocumulus cloud layer.
The stratocumulus cloud layer was sampled over the Beaufort Sea about 100 km
north-west of Deadhorse. The moist layer between cloud base and cloud top was about
600 m thick, and sometimes the cloud itself approached this depth. This was the deepest,
contiguous low cloud layer sampled during the field programme. There were signs of weak
internal convection, evidenced by weakly rounded, cumuliform-type tops. Despite their
shallow depth, precipitation from these clouds was widespread and, although the precipitation was very light, it reduced visibility considerably in some areas. In some locations,
such as in the region where the highest ice particle concentrations were encountered, the
clouds had highly localized and dense precipitation shafts*, which resembled those of
deep convective clouds and which obscured the horizon beyond the base of the cloud.
Total droplet concentrations were only about 20 cmP3.The maximum LWC measured
was 0.4 g mP3, and the maximum ice particle concentration measured over at least 1 km
pathlength was 28 per litre. Needle-like or columnar crystals dominated, although frozen
drops and their fragments were briefly present. Microphysical measurements obtained in
one of the deeper portions of this stratocumulus cloud layer are shown in Fig. 9.
The lapse rate from the surface to cloud top was nearly moist adiabatic (Fig. 3(a)).
Cloud-base temperature was -3.5 "C, cloud-top temperatures were -7.5 "C in the shallower, chaotic portions of the cloud layer and -9 "C in the solid, deeper portions of the
cloud.
Supercooled drizzle drops (200-500 p m in diameter) were common (Fig. 9). The
concentrations of these drops averaged over a path length of 1 km were as high as 18 per
litre. Heavy clear icing (indicative of large drops) occurred on the windscreen and on the
frame of the aircraft.
On the following day the synoptic situation was virtually the same as on 3 June. However, no upper clouds were present during any period of the flight. The low stratiform cloud
layer was particularly interesting because it exhibited very high ice particle concentrations
in clouds with tops that were even warmer than those on 3 June. The lowest cloud-top
* The degree to which arctic stratus and stratocumulus clouds precipitate, or are affected by precipitation, has
received little consideration. However, in a 10-year period, precipitation occurred at Deadhorse, Alaska, on 55% of
the days in June, July, and August.
P. V. HOBBS and A. L. RANGNO
2058
T
O
40
30
20
10
Horizontal Distance (km)
I
I
1141LDT
I
LDT
11
11
11
11
11
li
11
11
11
11
11
11
11
11
1:
/
1142
*,
1143
1144
I
1145
Particle
Concentrations
(per litre)
21.4
19.7
17.1
15.4
21.0
22.0
21.5
28.0
19.1
24.5
17.9
13.3
18.1
20.0
17.1
18.8
28.1
I
I
1146
1147
Particle
Concentrations
LDT
11 : 4 2 : 4 0
11: 4 2 : 4 0
11:42:41
11:42:41
11: 4 2 : 4 1
11: 4 2 : 4 2
11: 4 2 : 4 2
11 :42: 4 2
1 1 : 4 2 : 42
11 : 4 2 : 43
11: 4 2 : 4 3
11: 4 2 : 4 4
11 : 4 2 : 44
11: 4 2 : 44
11: 4 2 : 45
11 4 2 ~ 4 5
11 : 4 2 : 4 5
(per litre)
15.4
13.9
18.8
20.8
13.7
14.9
16.8
20.0
16.5
22.8
7.7
12.1
19.5
16.6
11.0
25.4
18.8
Figure 9. Depictions of the microstructure of a region of the stratocumulus cloud on 3 June 1995: (a) droplet
concentration; (b) liquid-water content measured by the FSSP (solid line) and the King liquid-water probe (heavy
dashed line), and ice particle concentration measured by the Optical Ice Particle Counter (none observed in this
case); ( c ) aircraft flight path (solid line) through the cloud (shaded regions); and (d) PMS 2-D cloud-probe imagery.
BEAUFORT SEA CLOUDS
2059
temperature measured was only -6 "C, but ice particle concentrations of tens per litre
over path lengths of many kilometres were measured (Fig. 10). Back trajectories suggest
that the boundary-layer air originated well over the polar ice cap. The stratus cloud was
about 550 m thick at it maximum depth and had a base temperature of -2 "C. The almost
perfectly flat top of the stratus, which was capped by a strong and partially cloud-filled
temperature inversion (Figs. 3(b) and 4(b)), consisted mostly of droplets that had a broad
size spectrum, with the tail of the spectrum 2 3 0 p m diameter.
Figure 3(b), which shows the sounding through 700 hPa on 4 June derived from
the airborne measurements, reveals an interesting characteristic that was seen on other
flights. The very highest portion of the stratiform cloud layer was not a boundary-layer
cloud, rather it appeared to be a thin lifted layer associated with a different air trajectory
(and equivalent potential-temperature value) than the boundary-layer portion of the cloud
(which had a moist adiabatic lapse rate).
Laboratory studies of the rate of ice splinters produced by riming of ice particles
in the temperature range from -2.5 to -8 "C indicate that one splinter is shed for about
every 200 cloud droplets accreted with diameter >23 p m when the ice particle is falling
at 2 1 m s-' (Hallet and Mossop 1974; Mossop 1985b). In the stratocumulus cloud on
4 June there was an unusually rich supply of cloud drops with diameters >23 p m from
the base all the way to the top of the cloud. The concentrations of such drops averaged
30 cmP3,but in some regions they exceeded 50 cmP3,The concentrations of columnar ice
particles >300 p m in maximum dimension (where riming begins) frequently exceeded
20 per litre. These ingredients provided a prolific environment for ice particle production
by riming-splintering. The rate of ice splinter production by this mechanism is given by
(e.g. Willis and Hallett 1991):
N gNd V,D E
800
where N g is the concentration of riming ice particles (in this case those columnar ice particles and their aggregates >300 p m diameter), Nd the concentration of droplets >23 p m
diameter, Vg the terminal velocity of the riming ice particles (in this case 0.5 m s-I), D the
diameter (in mm) of the riming ice particles (0.8 mm in this case), and E the collection efficiency of droplets by the ice particles (we use 0.75). Assuming these conditions extended
through most of the cloud layer (note that most of the stratocumulus layer was at or near
-4.5 "C where riming-splintering is a maximum), a splinter production rate of about 0.3
splinters per second per litre is obtained. Thus, 1 litre of cloudy air would contain more
than 200 ice particles after being subject to these conditions for only 13 min. The fact
that the measured maximum ice particle concentration was considerably less than 200 per
litre could have been due to the depletion of droplets by evaporation and riming after the
measurements in the water-rich initial phase of the riming-splintering process described
above were obtained.
The above calculation suggests that riming-splintering was responsible for the high
ice particle concentrations observed on this occasion. But how did this process get started?
As in the case of maritime clouds in coastal waters off Washington State (Hobbs and
Rangno 1985; Rangno and Hobbs 1991), we suggest that the first ice particles in these
slightly supercooled clouds were frozen precipitation-sized drops. Spotty regions of drizzle
were encountered, with concentrations of drizzle drops sometimes as high as 50 per litre
over path lengths of just tens of metres, in a region of the shallower stratocumulus clouds
well removed from regions of high ice particle concentrations. In spite of the relatively high
cloud-top temperature of - 3 "C, the OlPC indicated frozen particles in several regions.
These particles could not be distinguished from drizzle drops in the 2-D cloud-probe
2060
P. V. HOBBS and A. L. RANGNO
0.6
h
0.5
04
v
0.4
Y
5
0.3
g
0.2
g
U
'3
.e
4
0.1
0
3 0.8I
d
I
I
l
11 16 LDT
LDT
\
I
10
20
Honzontal Distance
1120
Particle
Concentrations
(per litre)
LDT
I
30
40
1124
Particle
Concentrations
(per litre)
Figure 10. As for Fig. 9 but for a portion of the flight on 4 June 1995 where the stratus/stratocumulus clouds had
tops of -6 "C.
BEAUFORT SEA CLOUDS
206 1
imagery (not shown), presumably because they were insufficiently rimed to produce the
knarled appearance that sometimes distinguishes frozen drops from supercooled drizzle
drops. We estimate from the OIPC that these frozen drizzle drops could have attained
concentrations as high as 0.1 per litre in localized regions 4 0 0 m across, although,
overall, the concentrations were probably less than 0.001 per litre.
While the region of shallower cloud just described was largely outside the temperature
range for effective riming-splintering, the measurements in this region suggest that a
relatively few frozen drizzle drops in the deeper stratocumulus clouds on this day might
have started the riming-splintering process. For example, if the concentration of frozen
drizzle drops (S0.5 mm in diameter) was 0.01 per litre at -6 "C, and they fell at a speed
of 1 m SKI,the concentration of ice splinters produced in about 7 min (the time for the
frozen drizzle drop to fall 400 m through the riming-splintering zone) would have been
about 0.1 per litre. That is, the concentration of ice particles would have increased by a
factor of ten in only 7 min. Since the lifetime of water-rich regions of stratocumulus cloud
layers is probably many tens of minutes, ice particle production by riming-splintering
in the presence of copious concentrations of large drops can easily explain the high ice
particle concentrations observed on this occasion.
There is, however, another possible mechanism for the initiation of riming-splintering
in this stratocumulus cloud. As noted earlier, precipitation (both frozen and liquid) fell from
deep overrunning clouds into the stratocumulus clouds below during the earlier portion of
the flight on 3 June. In view of the strong directional wind shear with increasing height,
the ice in the stratocumulus clouds could have been initiated by ice particles that fell from
higher-level clouds that were located far from the stratocumulus clouds at the time of our
measurements.
Figure ll(a) shows the extremely narrow regions in which drizzle drops were encountered during this flight. In an environment primed for riming-splintering, a few frozen
drizzle drops would produce an effect analogous to 'dry ice' seeding, namely, an outward
spread of high concentrations of ice particles. In this case it is to be expected that the highest ice particle concentrations would be found in narrow regions, and lower concentrations
over broader regions. There is some evidence that this was the case. Figures 1l(b) and (c)
show measurements of ice particle concentrations obtained in extensive horizontal legs
near cloud top. High concentrations of ice particles were measured over extremely short
path lengths that were relatively isolated (similar to the distributions of drizzle drops found
in other regions of the cloud), while lower concentrations of ice particles were present over
broad regions.
One unusual aspect that may have enhanced the riming-splintering mechanism on 4
June was that the temperature zone in which riming-splintering occurs (-2.5 to -8 "C,
Mossop 1985b) was greater in depth in the complex cloud layers than would ordinarily be
observed in well-mixed clouds with a strictly pseudo-adiabatic lapse rate. This was due to
the inversion and isothermal regions centred around -5 "C.
Figure 12 shows the relationship between several riming-splintering parameters and
the concentrations of the smallest ice particles measured during a few minutes of the
first aircraft ascent through this cloud. The ice particles encountered in this pass were
almost exclusively needles, which form between -4 and -6 "C. However, some irregularly
shaped, perhaps fractured, frozen drops were also present. Ice particles existed singly and
in aggregates, with some aggregates exceeding 3 mm in diameter. (Feathery ice aggregates
consisting of about 10-1 5 needle-like crystals were observed on the ground in Deadhorse;
these aggregates appeared to be unrimed or only lightly rimed.) Peak concentrations of
ice particles over path lengths of at least 1 km exceeded 40 per litre on several occasions.
Figure 12 shows a trend between two of the riming-splintering parameters (large droplets
2062
P. V. HOBBS and A. L. RANGNO
-8
B
70
aa
60
E
50
E
"v40
g&
k30
u-v
-B
.-g
20
g
10
0
s o
1335
2
.-
1340
1345
LDT
1350
1, )O
135;
25 km
40
24
.
e
Y
30
G
%
%n.
20
10
0
3
0
LDT
u
25 km
120 1
I125
LDT
1135
t130
~
25 km
Figure 11. Horizontal distributions of (a) drizzle (>200 W r n diameter) drops, and (b) and (c) ice particle concentrations in regions where the stratocumulus clouds on 4 June 1995 were deeper (top temperature -6 "C).
2063
BEAUFORT SEA CLOUDS
-
5
Concentration of Droplets > 23 pm Diameter (cm-3)
Concenfration of Droplets < 13 p n Diameter (cm-3)
Figure 12. Relationships between the concentrations of small ice particles in stratus/stratocumulus clouds and
several parameters considered to be important for ice particle production by riming-splintering. (a) Concentrations
of cloud droplets 223 p m diameter, (b) concentration of ice particles a 3 0 0 p m diameter (when droplets with
diameters r 2 3 p m are present), and (c) concentration of cloud droplets <13 p m diameter. Measurements were
obtained from the surface to cloud top on 4 June 1995.
2064
P. V. HOBBS and A. L. RANGNO
and large snowflakes), and the concentrations of the smallest ice particles. However, no
relationship was observed between the concentrations of small droplets and small ice
particles (Goldsmith et al. 1976).
Later, when entering the top of the cloud, ice particles were found in successive
bursts of 5-30 per litre, that lasted for only 1 or 2 s and were separated by ice-free regions
(Fig. 1 l(b)). However, broad regions of high ice particle concentrations were found where
the LWC was comparable with the highest measured on this day (0.5 to 0.6 gm-3);
this suggests that the high ice particle concentrations were of recent origin. However,
extreme uniformity in the sizes of the ice particles, which would have indicated their
nearly spontaneous formation (e.g. Hobbs and Rangno 1990), was not evident in any of
the PMS imagery on either 3 or 4 June. Strong riming of the crystals was also not evident.
Aggregates were generally rare or absent in many of the regions of high ice particle
concentrations, which also indicates that they were of relatively recent origin.
( b ) Stratus and stratocumulus clouds with stable lapse rates
By 5 June 1995 another pulse of deep clouds at middle and high levels had moved in
from the south and overspread the boundary-layer stratus and stratocumulus clouds that
were moving from the north-east. Rain from these higher clouds fell into scattered regions
of the lower clouds. The tops of the stratus and stratocumulus clouds were at 946 hPa
(0.55 km) and 5.2 "C, and the bases at 978 hPa (0.3 km a.g.1.) and 0.4 "C (Figs. 3(c) and
4(e)). A separate stratudfog top was located at 997 hPa (0.13 km a.g.1. and 0.7 "C). The
base of the stratudfog layer was too close to the ice surface for the aircraft to sample safely;
presumably, it was on or near the surface and at approximately 0 "C.
A low-pressure centre (1005 hPa) was located 300 km south of Deadhorse, and a
1028 hPa high-pressure centre was located over Victoria Island, Northwest Territories,
Canada. A ridge from the latter centre extended north-westward across the Beaufort Sea.
Surface winds were east-north-east along the north-east coast of Alaska. Air trajectories
suggested that the air in this region was transported across north-west Canada before turning south-westward toward the coast of Alaska. There was a high-level blocking high over
north-western Canada. Long fetch, southerly flow, from south-east Alaska and beyond,
was present at pressure levels around 850 hPa and higher.
In the first portion of the flight on 5 June the stratudfog deck, which extended to the
surface of the ice pack, was sampled near its midpoint. Above this deck was a scattered
to broken layer of thin stratocumulus clouds which, later in the flight and at a different
locale, became a solid stratocumulus overcast.
Figure 13 shows two profiles through the higher cloud layer, and a short horizontal
sample of what was probably the top of a fog; these are illustrative of clouds with stable
lapse rates. The profiles of LWC, measured during the descent into and the ascent out
of the higher cloud layer, show it increasing with height above cloud base (Fig. 13(a)).
However, the temperature structure (Fig. 13(b)) is quite complicated, with an isothermal
layer in the top half of the cloud layer and an inverted lapse rate in the lower half. During
the aircraft ascent two isothermal layers were encountered. Both of the upper isothermal
layers were relatively turbulent compared with the lower half of the cloud (Fig. 13(c)).
The cloud droplet effective radius actually increased slightly as cloud base was approached
during the descent (Fig. 13(a)). There was little turbulence in the top of the stratudfog layer
and the lapse rate in it was nearly isothermal (assuming the ice surface was at about 0 "C).
This thin (< 100 m thick) cloud contained droplets t 2 0 p m in diameter at and near cloud
top. Drizzle drops (from 200 to 500 p m in diameter) were not observed.
BEAUFORT SEA CLOUDS
I
I
I
I
I
I
2065
I
I
I
0.6
0.4
0.2
I
01
0
I
I
I
I
I
I
I
25
I
I
I
50
Horizontal Distance (km)
Figure 13. Vertical profiles through two stratus/stratocumulus layers on 5 June 1995 over the Beaufort Sea. (a)
Aircraft fight altitude (solid lines), liquid-water content (dashed lines), and cloud droplet effective radius (dotted
line): shading denotes clouds. (b) Temperature (solid line) and droplet concentrations (dashed line). (c) Turbulence.
7.
COMPARISONS OF DATA WITH THE ARCTICSTRATUS EXPERIMENT
The results reported in this paper on drizzle and ice particle concentrations in summertime stratus and stratocumulus clouds are quite different from those found in the Arctic
Stratus Experiment in which neither drizzle nor high ice particle concentrations were encountered (Tsay and Jayaweera 1984; Herman and Curry 1984). Possible reasons for this
are revealed by comparing average 500 hPa surface heights and temperatures for the days
on which the various flights were made with a 30-year record for Barrow, Alaska. The results are shown in Fig. 14. In Fig. 14(a) it can be seen that the flights we conducted in April
1992 were carried out under fairly normal 500 hPa conditions for the time of year. Our
flights in June 1995 occurred on days that had somewhat lower 500 hPa heights and temperatures than average. In contrast, for the days on which flights occurred in the ASE in June
I980 the 500 hPa heights and temperatures were substantially above normal (Fig. 14(b)).
This was because synoptic conditions on the days of the ASE flights were dominated by
anticyclonic flows and general subsidence. Consequently, the clouds would have tended
P. V. HOBBS and A. L. RANGNO
2066
530 -
Q
525
AA
AoA
A
A
A
A
A
zj
A
A
9
A
f 520-
2
52
%
A
A
A
A
A
515-
IA
51?&
A
-36
-34
500 hPa Temperature ("C)
-38
-32
1
-3(
Figure 14. (a) Average geopotentialheight at 500 hPa versus 500 hPa temperaturefor the 16 flights in April 1992
(open circle) and for April in each of the 30 years from 1961-91 for Barrow, Alaska (triangles). (b) As for (a) but
for the average of the seven flights in June 1992 (open circle), the average for the Arctic Stratus Experiment (filled
circle) and for June in each of the 30 years from 1961-91 for Barrow, Alaska (diamonds).
to be thinner and less likely to drizzle. Also, because of the warmer-than-average conditions and the relatively thin clouds encountered in the ASE, only slightly supercooled
clouds were present (the lowest cloud-top temperature encountered was -7.5 "C, but the
next coldest cloud top was -1 "C (Tsay and Jayaweera 1984)). Thus, it was unlikely that
high ice particle concentrations would have been generated by either primary or secondary
mechanisms.
To summarize, the ASE measurements are more representative of relatively warm,
anticyclonic conditions aloft in the Arctic, while our results for June 1995 are for colder
and more cyclonic conditions. Our measurements in April 1992 were carried out under
fairly normal conditions for that time of the year; they resemble the measurements for
April reported by Jayaweera and Ohtake (1973), Curry et al. (1990) and Pinto and Curry
(1997).
8. CONCLUSIONS
In this paper we have described extensive airborne measurements obtained in April
1992 and June 1995 of the microstructures of several cloud regimes in the Arctic: stratus
and stratocumulus in clear air over the Beaufort Sea; stratus and stratocumulus clouds
affected by local sources of pollution from Deadhorse, Alaska; middle-level altocumulus
and altostratus clouds; nimbostratus clouds; layers associated with large-scale lifting; and
ice crystal 'hazes'.
We have shown that the definition of a cloud in terms of particle concentrations can
have appreciable effects on cloud dimensions and derived average cloud microphysical
properties. For example, changing the definition of a cloud from 3 10 to 3 5 droplets per
cubic centimetre increased the coverage of the low and middle-level clouds we studied
in the Arctic by about 10%. Portions of the arctic atmosphere are often filled with ice
particles 3 100 p m in maximum dimensions in concentrations of -0.1 to 1 per litre. Such
regions can contribute significantly to total cloud cover and precipitation ice. Cloud depths
and areal coverage increased by about 40% when the definition of a cloud, in terms of ice
particle concentrations, was changed from a 1 to aO.1 per litre.
BEAUFORT SEA CLOUDS
2067
A pseudo-adiabatic lapse rate does not ensure that stratiform cloud layers in the
Arctic are either vertically or horizontally homogeneous over large regions. This study has
revealed extreme fine layering of arctic stratiform clouds, even in the presence of a pseudoadiabatic lapse rate throughout much or all of the saturated layer. Such layering occurs
in regions where clouds are forming or dissipating. The vertical profiles of liquid-water
parameters were generally erratic in such cloud layers.
Changes in the vertical profiles of temperature that we measured in arctic stratus and
stratocumulus clouds over horizontal distances of only a few hundred kilometres are larger
by a factor of 10 to 100 than those expected from radiational effects alone (e.g. compare
the changes in model output lapse rates in Smith and Kao's (1996) Fig. 4,for days 3
and 5 , with the changes in lapse rates observed during one of our flights as depicted in
Fig. 3 of the present paper). Since the Arctic Basin is quite active synoptically, large-scale
dynamical factors are likely to far outweigh radiational effects in the mutation of stratus
and stratocumulus clouds in the Arctic.
In well-mixed stratiform clouds in mid-latitudes, cloud LWC and droplet sizes increase with height above cloud base (e.g. Neiburger 1949). However, about one-half of
the stratiform clouds we studied in the Arctic did not show this tendency; instead, they
exhibited more erratic behaviour. In these cases the droplet concentrations peaked at the
base, middle or top of a cloud layer. In most situations of this type the temperature lapse
rates were not saturated adiabatic, and in some cases even strong temperature inversions
were present (e.g. Figs. 3(b) and (e)). We attribute the erratic variations of cloud-water
parameters to (i) complex lapse rates inhibiting vertical transport of droplets, and (ii) different levels of the cloud being fed by different aerosol. Another unusual feature of arctic
stratiform clouds is that their tops often extend into the region of a temperature inversion, instead of being capped by the base of the inversion. The presence of cloud within
temperature inversions at the tops of arctic stratus and stratocumulus clouds has not been
explained satisfactorily.
Most of the clouds sampled during the very cold first half of April 1992were glaciated,
or were very thin semi-transparent (perlucidus-type) stratocumulus or altocumulus droplet
clouds from which low concentrations of ice crystals fell. The disk of the sun could always
be seen through these clouds. Following the sudden warming that took place after 14 April,
droplet clouds, such as stratus fractus, and stratocumulus, were most frequent. These clouds
exhibited the highest LWC and the broadest cloud droplet spectra observed in April. The
tops of some of the stratocumulus clouds were as warm as - 10 "C. In contrast to the clouds
in the first half of April, the clouds during the second and warmer period in April were
thick enough occasionally to obscure the disk of the sun (optical depths >4).
In June 1995, flights generally took place on days that were cooler aloft than normal,
and under general cyclonic conditions at the surface or aloft, especially at the beginning of
our field project. Perhaps due to these disturbed conditions, the stratus and stratocumulus
clouds sampled in June exhibited very low droplet concentrations, larger drops, drizzle
and high ice particle concentrations.
For our April and June data set as a whole, ice particle concentrations were poorly
correlated with temperature, showing, if anything, a decrease with decreasing temperature
(Fig. 5(a)). Ice particle concentrations correlated better with the sizes of the largest droplets
in the cloud (Fig. 5(b)). This is consistent with the observations of Hobbs and Rangno
(198S), and is strong evidence that secondary ice particle processes dominated ice particle
concentrations at the higher temperatures.
The concentrations of ice particles in altocumulus clouds were in approximate agreement (or somewhat higher) than the ice nucleus concentrations given by Meyers et d.
(1992), which indicates that ice should start at around - 10 to -15 "Cin concentrations of
2068
P. V. HOBBS and A. L. RANGNO
about 1 per litre. However, this was not always true. In one case the droplet spectrum in an
altocumulus cloud at -6 "C had broadened enough to allow riming-splintering to occur
when snow fell into it.
The concentrations of cloud droplets in the stratus and stratocumulus clouds far from
local anthropogenic sources were comparable with those in clean maritime clouds (e.g.
Mossop et al. 1970; Hobbs and Rangno 1985; Boers et al. 1996). Droplet concentrations
and droplet spectra in stratus and stratocumulus clouds in the presence of local air pollution
around Deadhorse in June were two to three times higher than for the corresponding clean
clouds (e.g. Fig. 6). The main effect of the pollution was to increase the concentration
of cloud droplets with diameters t 1 5 pm, rather than to narrow the droplet spectrum
significantly. This was probably due to the recent ingestion of anthropogenic CCN into
clouds that already had broad droplet spectra as they moved inland over the pollution
sources.
Our measurements of altocumulus clouds, and the droplet regions of altostratus and
nimbostratus clouds, indicate that many of them were affected by anthropogenic aerosols
from long-range sources, as indicated by their relatively high droplet concentrations (see
Tables 2 and 3) and air chemistry measurements (Hegg et al. 1996).
Because stratus and stratocumulus clouds in the Arctic generally contain low droplet
concentrations they are particularly vulnerable to modification by CCN (e.g. Hegg et al.
1996). Increases in CCN (e.g. from anthropogenic sources) would likely produce the
following changes in the structures of these clouds. Droplet concentrations would increase,
which would result in more solar radiation being reflected back into space. Droplet sizes
would decrease, which would reduce drizzle formation and the formation of high ice
particle concentrations by both primary and secondary processes. Reductions in drizzle
and ice particle concentrations would likely affect cloud coverages and lifetimes.
There are two main avenues whereby aerosols might enter clouds: through their bases
and through their tops. The stable stratification of air within the stratus and stratocumulus
clouds that we encountered in the Arctic greatly limited cloud-top entrainment of overlying
aerosols. For example, the correlation coefficient between cloud-top droplet concentrations
and aerosol concentrations within 100 m above cloud top was only 0.3. On the other
hand the correlation coefficient between aerosol concentrations below cloud base and
cloud droplet concentrations was about 0.7. Thus, under stable stratified conditions, arctic
boundary-layer clouds appear to be little affected by overlying aerosols (which can be
transported to the Arctic over large distances).
Drizzle formation, by the collision-coalescence process, and high ice particle concentrations were documented in arctic stratus and stratocumulus clouds. These two processes
are likely to be commonplace during the warmest few months in the Arctic. However, before either process can get underway, large cloud droplets must be present in appreciable
concentrations. In this paper we have documented two cases where, before the development of high ice particle concentrations, droplets >23 Fm diameter were present in
concentrations > 15 cmP3at temperatures between -2.5 and -8 "C (the Hallett-Mossop
riming-splintering zone). Drizzle drops (diameters 200-500 pm), a few of which were
frozen, were also present on these two days throughout most of the deeper cloud regions.
The cloud microstructural measurements reported here suggest that liquid water in the
tops of clouds may be more prevalent at low temperatures than previously thought. In one
case droplets were measured at a cloud-top temperature of -3 1 "C. These findings, when
combined with the fact that all rawinsonde stations in the Arctic have average temperatures
greater than -30 "C from 850 to 700 hPa levels during the coldest months of October
through March, suggest that at these pressure levels the tops of stratiform clouds are
often very thin liquid layers, particularly when warm air advection is occurring and/or
BEAUFORT SEA CLOUDS
2069
the temperature itself is warmer than average. The existence of liquid-phase clouds at low
temperatures has potential ramifications for the arctic radiation budget (Curry and Herman
1985; Curry and Ebert 1992). Also, the optical extinction and long-wave emissivity of
arctic clouds during spring, summer, and autumn are likely to be greater than presently
estimated for clouds with tops colder than -22 “C, because it is generally assumed that
only ice crystals are present ( e g Curry and Ebert 1992).
Low and middle clouds in the Arctic often interact with each other. For example, on
several flights we observed a ‘seeder-feeder’ mechanism operating, in which precipitation
particles from a thick cloud at middle levels fell into stratus and stratocumulus clouds below.
Sometimes the particles survived 2 km or more of fall before reaching the lower clouds.
The removal of cloud water, and the localized clearings of stratus and stratocumulus, was
observed in some regions where the seeder-feeder mechanism was particularly active,
suggesting that, at times, accretional riming and/or downdraughts produced by the drag of
precipitation can dissipate arctic stratiform clouds. In the Arctic the air above low clouds
is relatively moist. Hence, the feeder-seeder phenomenon is likely to be common.
Altostratus clouds with top temperatures below - 15 ‘C shed ice particles over great
depths. The ice crystals originated in liquid (altocumulus-like) tops. When viewed from
above these clouds often appeared to be all water, although they were composed mainly
of cloud and precipitation ice for extensive depths immediately below their tops. Because
of the important role that droplet size plays in the production of ice, ice concentrations in
altostratus clouds (with altocumulus-like tops) in the Arctic that have tops warmer than
about -35 “C (where droplets can still occur) are susceptible to modification by pollution.
ACKNOWLEDGEMENTS
We thank the University of Washington flight crew for help in collecting data. This
research was supported by grants OPP-9414172, ATM-9015189 and ATM 9408941 from
the US National Science Foundation and grant NOOO14-95-1-0932from the US Office of
Naval Research.
REFERENCES
Abelson. P. A.
1989
Ackerman, T. P., Stenback, I. M.
and Valero. F. P. J.
1986
Baumgardner, D.
1987
Boers, R., Jensen, J. B.,
Krummel, P. B. and Gerber, H.
1996
Cunningham, R. M.
1957
Curry, J . A.
1986
Curry, J. A. and Ebert, E. E.
1992
Curry, J. A. and Herman, G. F.
1985
Editorial on ‘The Arctic: a key to world climate’. Science, 243,
873
‘The importance of arctic haze for the energy budget of the Arctic’. In Arctic air pollution. Ed. B. Stonehouse. Cambridge
University Press, New York
‘Corrections for the response time of particle measuring probes’.
Pp. 148-151 in Preprints 6th Symposium on meteorological
observations and instrumentation. American Meteorological
Society, New Orleans, USA
Microphysical and short-wave radiative structure of wintertime
stratocumulus clouds over the Southern Ocean. Q. J. R.
Meteorol. SOC.,122, 1307-1339
‘A discussion of generating cell observations with respect to the
existence of freezing or sublimation nuclei’. In Artificial stimulation of ruin. Ed. H. Weickmann. Pergamon Press, New
York
Interactions among turbulence, radiation and microphysics in arctic stratus clouds. J. Atmos. Sci., 43,90-106
Annual cycle of radiation fluxes over the Arctic Ocean: sensitivity
to cloud optical properties. J. Climate, 5, 1267-1280
Infrared radiative properties of summertime Arctic stratus clouds.
J. Clim. Appl. Meteorol., 24,526-538
2070
Curry, J. A., Meyer, F. G.,
Radke, L. F., Brock, C. A. and
Ebert, E. E.
Curry, J. A., Rossow, W. B.,
Randall, D. and Schramm, J. L.
Gardiner, B. A. and Hallett, J.
P. V. HOBBS and A. L. RANGNO
I990
Occurrence and characteristics of lower tropospheric ice crystals
in the Arctic. Int. J. Climatol., 10,749-764
Overview of arctic cloud and radiation characteristics.
J. Climate, 9, 1731-1764
1985 Degradation of in-cloud forward-scattering spectrometer probe
measurements in the presence of ice particles. J. Atmos.
Oceanic Techno/.,2,171-180
Gerber, H., Arends, B. G. and
1994 New microphysics sensor for aircraft use. Atmos. Res., 37, 235252
Ackerman, A. S.
1976 ‘The ice phase in clouds’. In Preprints of the International conGoldsmith, P., Gloster, J. and
ference on cloud physics. American Meteorological Society,
Hume. C.
Boulder, USA
1980 Polar processes and world climate (a brief overview). Mon.
Goody, R.
Weather Rev., 108,1935
1974 Production of secondary ice particles during the riming process.
Hallet, J. and Mossop, S. C.
Nature, 249,2628
Hegg, D. A., Ferek, R. J. and
1995 Cloud condensation nuclei over the Arctic Ocean in early spring.
J. Appl. Meteorol., 34,20762082
Hobbs, P. V.
Hegg, D. A., Hobbs, P. V., Gass6, S., 1996 Aerosol measurements in the Arctic relevant to direct and indirect
radiative forcing. J. Geophys. Rex, 101,23349-23363
Nance, J. D. and Rangno, A. L.
Herman, G. F.and Curry, J. A.
1984 Observational and theoretical studies of solar radiation in arctic
stratus clouds. J. Clim. Appl. Meteorol., 23,5-24
Hobbs, P. V. and Rangno, A. L.
1985 Ice particle concentrations in clouds. J. Atmos. Sci., 42,2523-2549
1990 Rapid development of ice particle concentrations in small, polar
maritime cumulifom clouds. J. Atmos. Sci., 47,2710-2722
Hobbs, P. V., Politovich, M. K. and 1980 Structures of summer convective clouds in eastern Montana. I.
Natural clouds. J. Appl. Mefeorol., 19,645-663
Radke, L. F.
Hobbs, P. V., Radke, L. F.,
1991 Airborne measurements of particle and gas emissions from the
Lyons, J. H., Ferek, R. J. and
1990 volcanic eruptions of Mt. Redoubt. J. Geophys. Res.,
96,1873551 8752
Coffman, D. 1.
Huschke, R. E. (Ed.)
1989 Glossary of meteorology. P. 179. American Meteorological Society, Boston, MA 02108, USA
Jayaweera, K. 0. L. F. and
1973 Concentration of ice crystals in arctic stratus clouds. J. Res. Atmos.,
Ohtake, T.
7,199-207
Jiusto, J. E.
1981 ‘Fog structure’. In Clouds, their optical properties, and effects.
Eds. P. V. Hobbs and A. Deepak. Academic Press, New York
King, M. D.
1993 ‘Radiativeproperties of clouds’. F’p. 123-149 in Aerosol-cloudclimate interactions. Ed. P. V. Hobbs. Academic Press
King, W. D., Parkin, D. A. and
1978 A hot-wire liquid water device having fully calculable response
characteristics. J. Appl. Meteorol., 17, 1809-1 8 13
Handsworth, R. J.
Lewis, W.
‘Meteorologicalaspects of aircraft icing’. Pp. 1197-1203 in Com195 I
pendium of meteorology. American Meteorological Society
Locatelli, J. D. and Hobbs, P. V.
1974 Fall speeds and masses of solid precipitation particles.
J. Geophys. Res., 79,2185-2197
Meyers, M. P., DeMott, P. J. and
1992 New primary ice-nucleation parameterizations in an explicit cloud
model. J. Appl. Meteorol., 31,708-721
Cotton, W. R.
Mossop, S . C.
1985a The origin and concentrations of ice crystals in clouds. Bull. Am.
Meteorol. Soc., 66,264-273
1985b Secondary ice particle production during rime growth: the effect of
droplet size distribution and rimer velocity. Q. J. R. Meteorol.
SOC., 11,1113-1124
1970 Ice particles in maritime clouds near Tasmania. Q. J. R. Meteorol.
Mossop, S . C., Ono, A. and
Wishart, E. R.
Sac., 96,487-508
Neiburger, M.
1949 Reflection, absorbing, and transmission of insolation by stratus
cloud. J. Meteorol., 6,98-I04
Pinto, J. 0. and Curry, J. A.
1997 Radiative characteristics of Arctic atmosphere during spring as
inferred from ground-based measurements. J. Geophys. Res.,
102,6941-6952
Plank, V. G., Atlas, D. and
1955 The nature and detectability of clouds and precipitation as determined by 1.25 cm radar. J. Meteorol., 12,358-378
Paulsen, W. H.
Rangno, A. L. and Hobbs, P. V.
1991 Ice particle concentrations and precipitation development in small
polar maritime cumuliform clouds. Q. J. R. Meteorol. Soc.,
117,207-241
1996
BEAUFORT SEA CLOUDS
Rangno, A. L. and Hobbs, P. V.
1994
Rauber, R. M. and Tokay, A.
1991
Schlesinger, M. E.
1986
Smith, W. S. and Kao, C.-Y.
1996
Tsay, S .-C. and
1984
Jayaweera, K. 0. L. E
Turner, F. M. and Radke, L. F.
1973
Turner, F. M., Radke, L. F. and
Hobbs, P. V.
Walsh, J. E. and Crane, R. G.
1976
I992
Warner, J. and Newnham, T. D.
1952
Warren, S. G., Hahn, C. J.,
London, J., Chervin, R. M. and
Jenne, R. L.
1988
Willis, P. T. and Hallet, J.
1991
Witte, H. J.
1968
World Meteorological Organization
I956
1969
1975
207 1
Ice particle concentrations and precipitation development in small
continental cumuliform clouds. Q. J. R. Meteorol. Soc., 120,
573-601
An explanation for the existence of supercooled liquid water at the
top of cold clouds. J. Amos. Sci., 48, 1005-1023
Equilibrium and transient climate warming induced by increased
atmospheric COz. Climate Dyn., 1,35-51
Numerical simulations of observed arctic stratus clouds using a
second-order turbulence closure model. J. Appl. Meteorol.,
35,47-59
Physical characteristics of arctic stratus clouds. J . Clirn. Appl.
Meteorol., 23,584-596
The design and evaluation of an airborne optical ice particle
counter. J. Appl. Meteorol., 12, 1309-1318
Optical techniques for counting ice particles in mixed phase clouds.
Atmos. Technol., 8,25-31
A comparison of GCM simulations of arctic climate. Geophys.
Res. Lett., 19,29-32
A new method of measurement of cloud-water content. Tellus, 4,
46-52
‘Global distribution of total cloud cover and cloud type amounts
over the ocean’. National Center for Atmospheric Research
Tech. Note NCARRN-317SSTR. (Available from the National Center for Atmospheric Research, Boulder, Colorado,
USA, 80302)
Microphysical measurements from an aircraft ascending with a
growing isolated maritime cumulus tower. J. Atmos. Sci., 48,
283-300
‘Airborne observations of cloud particles and infrared flux density (8-14 p n ) in the Arctic’. M.Sc. Thesis. University of
Washington
Unabridged international cloud atlas. World Meteorological Organization, 41 Ave. Guiseppe Motta, Geneva 2, Switzerland
International cloud atlas. Abridged atlas. World Meteorological Organization, 41 Ave. Guiseppe Motta, Geneva 2,
Switzerland
International cloud atlus, Vol. I. World Meteorological Organization, 41 Ave. Guiseppe Motta, Geneva 2, Switzerland