Open Access Article
Meteorologische Zeitschrift, Vol. 21, No. 4, 319-335 (August 2012)
c by Gebrüder Borntraeger 2012
Atmospheric boundary layer measurements at the 280 m high
Hamburg weather mast 1995–2011: mean annual and diurnal
cycles
BURGHARD B R ÜMMER∗ , I NGO L ANGE and H EIKE KONOW
Meteorological Institute, University of Hamburg, Germany
(Manuscript received August 9, 2011; in revised form March 20, 2012; accepted May 11, 2012)
Abstract
In this paper, the 280 m high Hamburg weather mast and its instrumentation are introduced. Digital data
recorded since 1995 are used to calculate the mean annual and diurnal cycles of the primary climate
variables (pressure, temperature, humidity, wind, short- and long-wave radiation, cloud coverage, cloud
base, precipitation). The annual average of 2 m temperature is 9.8 ◦ C indicating an increase compared to
the period 1971–2000 at the Hamburg airport climate station. Absolute humidity follows the temperature
cycle with a maximum in July/August. Relative humidity is highest in winter and lowest in April/May. The
fraction of received to clear-sky short-wave radiation is between 61 % in May and 34 % in December. Cloud
coverage classes of 0–1 octas and 7–8 octas occur most frequently, but have opposite annual cycles. Cloud
base distribution is narrow in winter and peaks around 300 m and is distributed over a wide height range
in summer. Average annual precipitation amounts to 716 mm and falls in 9.3 % of the time. Monthly mean
wind speed is highest (lowest) in January (August). Winds from west are most frequent followed by winds
from southeast. A channelling by the Elbe river valley is indicated. The diurnal temperature cycle is weak in
winter but strong in summer showing the evening generation and morning rise of the inversion. While relative
humidity has a single diurnal cycle, absolute humidity has a double cycle in summer, but not in winter. Shortwave radiation in summer shows a weak asymmetry between forenoon and afternoon. The diurnal cycles of
cloud cover and base are small in winter. In summer, cloud bases show a continuous increase from morning
to afternoon and a break afterwards simultaneously with the diurnal rain maximum. Wind speed has opposite
diurnal cycles at lower and upper levels. The upper-level cycle shows a temporal asymmetry in summer,
i.e. the upper-level wind minimum does not occur simultaneously with the lower-level wind maximum. The
reversal height between the opposite cycles is around 130 m in summer and 80 m in winter. The wind direction
difference (250 m–10 m) shows a strong diurnal variation between 15◦ (day) and 45◦ (night) in summer and
a small one between 23◦ and 35◦ in winter. The annual and diurnal cycles of all primary climate variables
together present an excellent basis for the validation of process, weather or climate models.
1 Introduction
The “Hamburg weather mast” is a meteorological measuring facility installed at the broadcasting tower of the
Norddeutscher Rundfunk (NDR) in Hamburg. The mast
is located at 53.5192◦ N, 10.1029◦ E at the easterly outskirts of Hamburg in about 8 km distance from the city
centre (Fig. 1a). The terrain at the facility has an altitude
of 0.2–0.5 m above mean sea level and belongs to the
Elbe river valley. The Elbe river valley is roughly SENW oriented. At several kilometers distance from the
mast the terrain slopes to 40–70 m towards north and
to up to 150 m (Harburger Berge) towards southwest.
The shortest distance to the Baltic Sea is about 62 km
and that to the North Sea about 81 km. The nearer surrounding of the mast is also rather flat but not homogeneous. There are shallow industrial buildings (<15 m)
in the west and north, allotment gardens in the south and
mainly rural landscape in the east (Fig. 1b). At nearer
distance in easterly direction an area is present where
∗ Corresponding
author: Burghard Brümmer, Meteorological Institute, University of Hamburg, Germany Bundesstraße 55, 20146 Hamburg, e-mail:
[email protected]
DOI 10.1127/0941-2948/2012/0338
gravel soil is dredged from the ground. The artificial
lake is about 350 m apart and gravel heaps with varying heights of 5–10 m are about 250 m apart. Except
for the gravel area which expanded with time there have
been no major topographical changes within a circle of
500 m around the mast during the last two decades.
Meteorological measurements at the tower have been
made since March 1963. At the beginning the facility
was installed and operated by the Technical University
of Darmstadt (e.g. M ARNIER, 1973). Later, in 1967, the
operation of the facility was handed over to the University of Hamburg. Until the early 1990s the meteorological data have been recorded analogously on paper charts.
Due to the time consuming data digitizing, scientific
analyses of the data were made only for special process
studies (e.g. WAMSER, 1976; K L ÖPPEL et al., 1978) or
case studies (e.g. B R ÜMMER, 1988) based on limited periods but could not be made for the entire time period.
During the early 1990s the old instrumentation was removed and new sensors together with a digital data registration were installed. Continuous digital recording of
the basic meteorological data started in April 1995. In
the course of the time the Hamburg weather mast facil-
0941-2948/2012/0338 $ 7.65
c Gebrüder Borntraeger, Stuttgart 2012
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B. Brümmer et al.: Atmospheric boundary layer measurements
a)
b)
Figure 1: a) Location of the Hamburg weather mast (x) and surrounding orography. Map from Paul List Verlag. b) Topographic map
of the area around the Hamburg weather mast.
ity was extended step by step by further meteorological
instrumentation (section 2). The Hamburg weather mast
is one of the highest comprehensively equipped meteorological masts in Europe.
This paper aims at two objectives. The first objective
is to present the Hamburg weather mast, its instrumentation, and the existence of a multi-year data set with
high temporal resolution to a larger scientific community. B R ÜMMER and L ANGE (2004) published the first
very short information on the mast which, however, was
available only to a limited meteorological community in
Germany. The second objective is to use the up to 16years long Hamburg weather mast data set for a climatological analysis of the mean annual and diurnal cycles
in the lower boundary layer. In this paper, we restrict the
analysis to the primary meteorological-climatological
variables, i.e. pressure, temperature, humidity, wind, radiation, clouds, and precipitation. An analysis of the tur-
Meteorol. Z., 21, 2012
bulent variables, e.g. turbulent fluxes and variances, will
be the focus of a subsequent paper for which, however,
the knowledge of the mean annual and diurnal cycles
of the primary meteorological variables is an important
prerequisite.
The special features of this paper compared to many
other climatological studies are the following: (a) We
analyse the conditions not only in the surface layer but
also up to 250 m height which is an essential part of the
boundary layer or can even be the total boundary layer
in case of stable stratification. (b) Many climatological studies of the lower boundary layer using data from
high towers have already been presented in the literature
(e.g. C RAWFORD and H UDSON, 1973; VAN U LDEN and
W IERINGA, 1996; B EYRICH and F OKEN, 2005). However, mostly these studies are limited to the analysis of
only one or a few variables like wind or temperature. In
this paper, all above-mentioned primary climatological
variables are jointly analysed which allows the detection
of and reference to mutual relations. (c) The analysis of
the annual and diurnal cycles is not based on only oneyear or a few-years data as it is often presented in the
literature (e.g. C RAWFORD and H UDSON, 1973), but is
based on up to 16-years long time series so that possible anomalies of one year do not bias the cycles. Nevertheless, the time periods used in this paper are below
the 30-years-length of an official climate reference period (WMO, 1989). (d) Due to the length and high temporal resolution of the Hamburg weather mast time series (see section 2) we calculate the annual cycle with a
daily instead of monthly time resolution and the diurnal
cycle with a 10-minute instead of 1-hour time resolution. This time resolution that is higher than common allows a better analysis and understanding of the boundary
layer processes e.g. during the most instationary phases
around sunrise or sunset.
The data of the Hamburg weather mast are only to
a minor part specific for its location. This holds for
the near-surface conditions such as surface temperature
and roughness. Most observations and, thus, the annual
and diurnal boundary layer processes behind them are
representative for the region of Northern Germany or
even beyond. This region is mainly characterized by
agricultural flatlands and gentle elevations. The climate
in this region is influenced by the near-by North Sea
and Baltic Sea and by the large-scale atmospheric circulation connected with the North Atlantic Oscillation
(NAO) and its variability (e.g. JAHNKE -B ORNEMANN
and B R ÜMMER, 2009). This is manifested by frequent
changes of the synoptic conditions caused by passing
cyclones and anticyclones. The frequent occurrence of
atmospheric fronts is also a typical feature of the region
of Northern Germany (H ENNEMUTH and B R ÜMMER,
1990).
This paper is organized as follows: In section 2 the
Hamburg weather mast, i.e. the platforms, the meteorological equipment, and the available data set are de-
Meteorol. Z., 21, 2012
B. Brümmer et al.: Atmospheric boundary layer measurements
scribed. The results of the climatological analyses are
presented in the following sections, with respect to the
annual cycle in section 3 and with respect to the daily cycle in section 4. In section 5 the results are discussed in
relation to other known climatologies. A summary and
conclusions are given in section 6.
2 Hamburg weather mast: platform,
instrumentation, data coverage
The facility “Hamburg weather mast” consists of three
major components: the main mast, a 12 m mast, and a
central data reception hut close to the main mast (Fig. 2).
a. The main mast
The main mast is in total 305 m high, has a diameter of 2.0 m and an elevator inside. Platforms (about 1
m wide) are mounted around the mast at 50, 70, 110,
175, 250, and 280 m height. The platforms at 50, 110,
175 and 250 m height have 5 m long foldable booms
which extend into 190◦ direction. They carry (at 7 m
distance from the mast centre) the instruments for the
wind measurement: cup anemometer, wind vane (both
are self-produced instruments), and three-dimensional
sonics (METEK USA-1). The foldable boom at the 280
m platform is 3 m long and is equipped with a sonic
only. There is no wind measurement at 70 m height.
Temperature sensor (Pt-100) and relative humidity sensor (Vaisala humicap HMP-45) are installed in radiationshielded and ventilated tubes mounted at the outside
of the platform railing. In addition, a dew-point mirror
(Meteo Swiss VTP-6) is installed on the 110 m platform
and a webcam is installed on the 50 m platform. Thermometer, humicap, cup, and vane data are sampled with
1 Hertz and averaged and stored as 1 minute means. The
sonic instruments sample the three wind components
and the virtual temperature with 20 Hertz. From these
data 5 minute means, variances and co-variances (turbulent fluxes) are calculated and stored. Since 2004 also
1 minute means and the highest gust (highest 3 seconds
wind average per minute) are stored. Since August 2009
the complete 20 Hertz original sonic data are stored
additionally. An infra-red open path sensor (Licor LI7500) for water vapour and carbon dioxide is installed
at 50 m height since 2010. The installation of Licor sensors at 10, 110, 175 and 250 m height will follow in near
future. The combination of the 20 Hertz Licor and sonic
data delivers the turbulent vertical fluxes of water vapour
and carbon dioxide.
The mast’s influence on the wind measurement has
been determined by L INK (1966). He used cup and vane
wind measurements on the 110 m platform which have
been made for a one year period at three identical booms
extending into 70◦ , 190◦ and 320◦ direction. The results
of the study show a symmetric influence with respect
to the 190◦ boom direction. In the sector 140◦ –240◦ ,
321
the wind is reduced by –2 % on the average with a
minimum of –4 % at 190◦ . Enhanced winds occur in
the sectors 35◦ –140◦ (240◦ –345◦ ) with an average of
+7 % and a maximum of +11 % at 60◦ (320◦ ). In the
sector 350◦ –30◦ the wind is reduced by more than –
10 % with a minimum of –50 % at 10◦ . Winds from
this sector occur in about 5 % of the time (see Fig.
10). No correction for the mast effect was applied to
the data in this study because we believe that there is
still uncertainty in the exact corrections to apply and
that even with corrected winds the results of the analyses
below on the annual and diurnal cycles will be affected
only marginally. It is mentioned that a SODAR and a
LIDAR (Wind Cube) are actually installed near the mast
to derive a further estimate of the mast‘s influence on the
wind measurement.
b. The 12 m mast
Since the main mast is surrounded by trees and flat
buildings of the NDR broadcasting corporation, measurements in the surface layer are not meaningful there.
Instead they are made on a meadow at a 12 m mast in
about 170 m distance towards 60◦ direction from the
main mast. At the 12 m mast the wind is measured at 10
m height with cup anemometer, wind vane and a sonic
as at the main mast. Temperature and relative humidity
sensors are installed at 2 and 10 m height in the same
way as on the main mast (same sensors, same radiationshielded and ventilated tubes). The surface temperature
of the meadow is measured by an infra-red radiometer
(Heimann KT-19) which is installed at 2 m height. At
the top of the 12 m mast a pyranometer (Kipp and Zonen
CM11) and a pyrgeometer (Eppley PIR) are mounted
to measure the down-welling short-wave and long-wave
radiation flux. Except for the sonic, the data from all
other instruments are sampled with 1 Hertz and averaged and stored as 1 minute means. The sonic data are
handled in the same way as those from the main mast.
c. Central data reception hut
All data from the main mast and the 12 m mast are transmitted via optical fibre cables to a wooden hut at about
15 m distance from the main mast. The hut houses the
central computer installation which handles the data reception, storage, and control and the communication to
the platforms and sensors and the external data access.
The hut also contains an air pressure sensor (Vaisala
PTB200A). Further instruments are placed outside near
the hut. These are a tipping rain bucket (Lambrecht)
with a reservoir of 0.1 mm, a precipitation (rain, snow,
hail) detection sensor (RLS IRSS-88) and an infra-red
ceilometer (Vaisala CT25K) for the detection of cloud
base. The rain data are stored with 1 minute time resolution and the ceilometer data with 15 seconds resolution. Furthermore, a rain RADAR (METEK MRR-2) to
measure the vertical rain rate profile at 35 m intervals
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B. Brümmer et al.: Atmospheric boundary layer measurements
Meteorol. Z., 21, 2012
Figure 2: Photographs of the Hamburg weather mast facility: main mast with a close-up of the 50 m platform and the wind instrumentation
at the end of the boom, 12 m mast with the gravel heaps in the background, and instruments near the central data reception hut.
Table 1: Instruments at the six platforms at the main mast with measuring period (month/year), data coverage (percentage of time with
recorded data), sampling frequency and recording interval, resolution and accuracy (status 6/2011). Cup and vane maintenance ended 2001,
but some of them still recorded for several further years.
Instrument
50 m
70 m
110 m
175 m
250 m
280 m
Thermometer
Pt-100
since
4/1995
95 %
since
4/2004
99 %
since
4/1995
96 %
since
4/1995
93 %
since
12/2000
97 %
since
4/2004
90 %
since
4/1995
90 %
since
4/2004
90 %
since
5/1995
92 %
since
8/2004
93 %
since
7/2010
100 %
since
7/2010
100 %
Humidity
sensor
HMP 45
Dewpoint
mirror
VTP 6
H2O/CO2Analysor
LI-7500
Sonic USA-1
5 min data
since
3/2010
85 %
since
10/2000
97 %
since
12/2003
99 %
since
8/2009
since
10/2000
97 %
since
5/2004
99 %
since
8/2009
since
10/2000
96 %
since
5/2004
99 %
since
8/2009
since
10/2000
94 %
since
9/2004
98 %
since
8/2009
4/1995 to
9/2002
4/1995 to
9/2002
Wind vane
4/1995 to
11/2009
4/1995 to
3/2006
4/1995
to
4/2010
4/1995
to
5/2011
4/1996
to
2/2000
5/1995
to
5/2009
Webcam
since
10/2010
99 %
Sonic USA-1
1 min data
and gusts
Sonic USA-1
20 Hz raw
data
Cup
anemometer
and a three-dimensional SODAR (METEK) to measure
the vertical wind profile at 25 m steps are installed near
the central data hut. Ceilometer and SODAR backscatter
profiles can also be used to retrieve the boundary layer
top and/or inversion base, respectively (e.g. M ÜNKEL et
al, 2007; OTTERSTEN et al., 1974).
A summary of all instruments belonging to the
“Hamburg weather mast” facility is given in Tables 1–3
together with the respective measuring period, data cov-
since
7/2010
100 %
since
7/2010
100 %
since
7/2010
Sampl.Freq.
Av./Rec.
Interval
1 Hz
1 min
Resolution
Accuracy
1 Hz
1 min
0.1 %
2-3 %
few samples/
10 min
10 min
20 Hz
1 min
0.01 K
0.1 K
20 Hz
5 min
0.01 m/s
0.1 m/s
20 Hz
1 min
0.01 m/s
0.1 m/s
20 Hz
0.05 s
0.01 m/s
0.1 m/s
1 Hz
1 min
0.01 m/s
0.2 m/s
1 Hz
1 min
1°
1-2°
0.01 K
0.1 K
0.001 g/m3
0.05 g/m3
Videostream
1 picture/
min
erage (i.e. the percentage of time with recorded data),
sampling frequency, averaging and recording intervals,
resolution and absolute accuracy. The calibration of the
instruments was taken from the manufacturers. Erroneous measurements of temperature and relative humidity could be detected by comparison with the other instruments of the vertical profile. In such a case the defective instrument was replaced and the faulty data were
deleted in the data set. The wind instruments (cup, vane,
B. Brümmer et al.: Atmospheric boundary layer measurements
Meteorol. Z., 21, 2012
323
Table 2: Instruments at the 12 m mast with measuring period, data coverage, sampling and recording frequency, resolution and accuracy
(status 6/2011). Cup and vane maintenance ended 2001, but both are still recording.
Instrument
Height
Measuring
period
Thermometer
Pt-100
Thermometer
Pt-100
Humidity sensor
HMP 45
Humidity sensor
HMP 45
Cup anemometer
2m
since 4/1995
10 m
since 4/1995
2m
since 4/2004
10 m
since 4/2004
10 m
since 4/1995
Wind vane
10 m
since 4/1995
Sonic USA-1
5 min data
Sonic USA-1
1 min data/gusts
Sonic USA-1
20 Hz raw data
Pyranometer S↓
Kipp+Zonen
Pyrgeometer L↓
Eppley
IR-Radiometer
(surface), KT-19
10 m
since 10/2000
10 m
since 5/2004
10 m
since 8/2009
12 m
since 4/1996
12 m
since 4/1996
2m
since 1/1997
Data
Sampl. Freq.
coverage Av./Rec.
Interval
93 %
1 Hz
1 min
93 %
1 Hz
1 min
93 %
1 Hz
1 min
93 %
1 Hz
1 min
1 Hz
1 min
1 Hz
1 min
95 %
20 Hz
5 min
99 %
20 Hz
1 min
20 Hz
0.05 s
94 %
1 Hz
1 min
91 %
1 Hz
1 min
81 %
1 Hz
1 min
Resolution
Accuracy
0.01 K
0.1 K
0.01 K
0.1 K
0.1 %
2-3 %
0.1 %
2-3 %
0.01 m/s
0.2 m/s
1°
1-2 °
0.01 m/s
0.1 m/s
0.01 m/s
0.1 m/s
0.01 m/s
0.1 m/s
0.1 W/m2
3 W/m2
0.1 W/m2
3 W/m2
0.01 K
~1 K
Table 3: Instruments near the central hut with measuring period, data coverage, sampling and recording frequency, and resolution and
accuracy (status 06/2011).
Instrument
Pressure sensor
PTB 200 A
Tipping bucket
rain gauge
Rain indicator
IRSS-88
Rain Radar
MRR-2
Ceilometer
CT25K
3-D Sodar
Measuring
period
since 4/1995
Data
coverage
96 %
since 6/1997
95 %
since 7/2006
100 %
since 5/2008
94 %
since 11/2003
(backscatter
profiles since
4/2004)
since 10/2010
99 %
(100 %)
81 %
sonic) were checked and calibrated at irregular times in
the wind tunnel of the Meteorological Institute of the
University of Hamburg. Especially the sonics showed no
remarkable errors. Data gaps occurred mostly due to a
defective sensor or an electricity outage (often caused by
lightning). Data gaps are not systematically distributed
with time. Nevertheless, in order to avoid biases caused
by longer data gaps, we used only days (months) with
completely available data for the calculation of diurnal
and annual cycles. The “Hamburg weather mast” facility
has been described by B R ÜMMER and L ANGE (2004).
A comparison with Tables 1–3 shows the grown instrumental and technical volume of the facility since then.
The actual instrumental status of the mast facility and
Sample Freq.
Av./Rec. Interval
1 Hz
1 min
each 0.1 mm
1 min
1 min
1 min
10 s
10 s and 1 min
5.57 kHz
15 s and 5 min
Resolution
Accuracy
0.01 hPa
0.1 hPa
0.1 mm
~ 15 s
10 min
0.1 m/s
triggered by
5 drops/min/12 cm2
0.01 mm/10 s
Cloud base: 100 ft
Cloud cover: 1-2/8
actual meteorological data can be found in the internet
via http://wettermast-hamburg.zmaw.de.
The boundary layer climatology presented below is
based on data from the 16 years long period from April
1995 until May 2011. We note that the time series
for individual meteorological quantities have different
lengths. Data from the 280 m platform at the main mast,
from the rain RADAR, the SODAR, and the Licor instrument are not considered here because these time series are not yet long enough for a meaningful calculation
of mean annual and diurnal cycles. Even the longest (16
years) time series of temperature is still too short according to the climate definition of the World Meteorological
Organization (WMO, 1989) to determine climate trends,
324
B. Brümmer et al.: Atmospheric boundary layer measurements
Figure 3: Annual cycles of temperature, absolute and relative humidity at 2 m height together with corresponding standard deviation
and number of available data in daily time resolution. The temperature results are based on 5346 days within the 16 years from 6/1995
to 5/2011. The measurement of (relative) humidity in all height levels began in 2004, so the results are based on 2428 days for the absolute humidity and 2448 days for the relative humidity.
but this is not the intention of this paper. However, we
can assume that the basic characteristics of the mean annual and daily cycles presented below are – because of
their large amplitudes compared to the standard deviations (see e.g. Fig. 3) – close to those which would have
been calculated from 30 years long time series.
3 Mean annual cycles
3.1 Temperature and moisture
The annual cycles of temperature and moisture measured at 2 m height are shown in Fig. 3. Instead of
monthly means we present mean values for each day of
Meteorol. Z., 21, 2012
the year together with the corresponding standard deviation (STD) and the number of years with available data
for the respective day. In this way, not only the year to
year variability but also the variability and trend within
a month can be demonstrated better.
For the annual temperature cycle the period from
6/1995 to 5/2011 is regarded. The annual temperature
cycle is not symmetric. It shows a 7 months long raise
and a 5 months long decrease. January is the coldest
month with 1.1 ◦ C, however, the lowest temperatures
are already reached at the beginning of the month. The
warmest month is July with 18.6 ◦ C, but the warmest
period, colloquial known as “Hundstage”, occurs at the
end of July and beginning of August.
For the annual moisture cycle only the period from
4/2004 to 5/2011 with Humicap measurements could
be used because the moisture measurements before this
period were made with dew-point mirrors (manufacturer Kroneis) which showed non-correctable long-term
drifts. The moisture is presented in two ways, as absolute (water vapour density) and relative humidity. The
absolute humidity shows a similar annual cycle as the
temperature with a minimum in January and a maximum at the beginning of August. The annual cycle of
relative humidity is roughly opposite, but not in detail.
The maximum with 90 % occurs in December. However,
the minimum does not occur at the time of the temperature and absolute humidity maximum in July/August but
during the spring months April/May. This may be linked
to cold-air advection from north over the still cold North
Sea and Baltic Sea. According to their origin, air masses
from north have still little absolute moisture at this time
of the year (like in winter) but in spring they are warmed
over the comparatively warmer land surface so that the
relative humidity goes down. The STD of temperature is
larger in winter than in summer and reflects the stronger
synoptic variability in winter. The STD of relative humidity has an opposite cycle with small values in winter
when the general level of relative humidity is closer to
saturation.
3.2 Down-welling short- and long-wave
radiation flux
The annual cycle of shortwave radiation is based on data
from 6/1996 to 5/2011 (2005 is omitted due to data gaps)
and is presented in Fig. 4 as daily and monthly means.
The daily means of shortwave radiation flux reach values of up to 255 W/m2 in summer. Interestingly there is
a weak minimum in the middle of the summer season.
This occurs simultaneously with a weak maximum of
total cloud cover (see section 3.3). The STD of radiation
flux is clearly larger in summer than in winter, while the
relative STD (STD/mean value) is almost the same. The
annual mean of the down-welling shortwave radiation
is about 120 W/m2 . The monthly received radiation energy sums are highest in May, June, and July with values
Meteorol. Z., 21, 2012
B. Brümmer et al.: Atmospheric boundary layer measurements
325
Figure 4: Mean annual cycle of down-welling short-wave radiation
flux with standard deviation and number of available data in daily
time resolution (top) and annual cycle of received shortwave radiation energy with corresponding standard deviation in monthly time
resolution (bottom). The upper curve gives the maximum of energy
reception in case of clear sky. The measurement includes 14 years
from 6/1996 to 5/2011 (2005 is omitted due to data gaps).
Figure 6: Annual cycle of the freqency distribution of total cloud
coverage (a) and cloud base (b) from ceilometer measurements
during the period 1/2004 to 12/2010. The frequency distribution of
cloud base covers a height range up to 7500 m, only the lowest 3000
m are shown.
Figure 5: Annual cycle of down-welling long-wave radiation flux
with standard deviation and number of available data in daily time
resolution. This measurement includes 13 years from 6/1996 to
5/2011 (2001 and 2005 are omitted due to data gaps). The annual
average is 321 W/m2 .
between 155 and 159 kWh/m2 . The radiation energy integrated over the whole year amounts to 1013 kWh/m2 .
This is 56 % of the maximum possible amount of 1818
kWh/m2 in case of a cloudless sky. The fraction of received to maximum possible energy is clearly higher in
summer (May 61 %) than in winter (December 34 %).
The annual cycle of down-welling long-wave radiation is based on data from 6/1996 to 5/2011 (2001 and
2005 are omitted due to longer data gaps) and is presented in Fig. 5 as daily means. Down-welling longwave radiation depends primarily on air temperature, absolute humidity and cloud coverage. The annual cycle
of long-wave radiation follows the cycle of air temperature over wide parts of the year. Both maxima occur
almost at the same time (end of July/beginning of August). However, there is an essential difference between
both cycles in winter. Whereas air temperature exhibits
a clear minimum at the beginning of January, the minimum of down-welling long-wave radiation occurs over
a long period from December to April with daily mean
values around 300 W/m2 . The increase of air temperature from January (around 1.5 ◦ C) to April (around 8 ◦ C)
is not reflected in the long-wave radiation curve. The effect of temperature increase on the long-wave radiation
flux appears to be compensated by the decrease of total cloud cover from January (80 %) to April (46 %)
(cf. Fig. 6) since the absolute humidity remains almost
constant during this time period. Thus, the shape of the
long-wave radiation curve follows more or less the absolute humidity curve.
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Meteorol. Z., 21, 2012
3.3 Clouds and precipitation
Cloud base and cloud coverage were calculated from
the ceilometer data using the “sky condition algorithm”
from the manufacturer Vaisala. The main features of the
algorithm are briefly reported. The 5570 individual laser
shoots per second are averaged to a mean backscatter
profile over 15 s. From the mean profile up to three
cloud bases (hits) are derived. The hits are arranged in 80
height intervals (bins) with a width of 100 ft (500, 1000
ft) until 5000 ft (15000, 25000 ft) height and multiplied
with weights of (5,0,0), (3,2,0) or (3,1,1) depending
on the number of 1, 2 or 3 hits found in the mean
backscatter profile. Afterwards a statistical evaluation
of all 15 s measurements during the last 30 minutes is
performed in which the weights for the hits during the
latest 10 minutes are doubled. For each bin the counts
(count = weight x hit) are summed up. The bins where
the sum exceeds 1/33, 3/8, 5/8, 7/8 of the maximum
number of counts are recorded as layer heights with a
corresponding cloud cover of 1, 3, 5, 7 octas. Layer
heights which are close to each other are combined to
one layer (clustering) with the height of the lower layer
and the cloud cover of the upper layer. The minimum
layer distance for clustering increases with height and
is 100 ft (200, 300, 400, 500, 1000, 5000 ft) for heights
up to 1000 ft (2000, 3000, 4000, 5000, 15000, 25000 ft).
Finally, up to four clustered layers with height and cloud
cover are given by the algorithm. The cloud coverage
is 8 octas if all 15 s profiles during the last 30 minutes
have at least one hit. This is a very restrictive criterion
which causes to lower the frequency of the 8 octa class in
favour to the 7 octa class (see results below). If the sum
of counts does nowhere exceed 1/33, the cloud cover is
zero octas. The results of the statistical evaluations for
the last 30 minutes are reported every 5 minutes and
are the basis for our calculations of annual and diurnal
cycles.
Fig. 6 displays the annual cycle of the frequency distributions of total cloud coverage and cloud base for the
period 1/2004 to 12/2010. For cloud coverage we use
the six classes (0, 1+2, 3+4, 5+6, 7, 8 octas) as given by
the “sky condition algorithm”. For cloud base we combine the above-mentioned 80 height bins to regular 150
m height intervals. Low (0 octa) and high values (7 and
8 octas) of cloud coverage occur most frequently. The
intermediate classes (1+2, 3+4 and 5+6 octas) are much
rarer. Low and high cloud coverage values have an opposite annual cycle. Cloudless conditions are most frequent in spring and summer. The maximum with 37 %
occurs in April. High cloud coverage conditions are frequent in winter (the maximum of 7+8 octas is 75 % in
December and January) and seldom in spring and summer (the minimum of 7+8 octas is 39 % in April and
July). The arithmetic mean of the cloud coverage varies
between 4 and 6 octas and, thus, in a cloud coverage
range which itself is seldom observed. It should be mentioned that the retrieval of total cloud coverage from the
Figure 7: Mean monthly amount (top) and duration (bottom) of precipitation. In the “short period” from 8/2006 to 7/2011 both, amount
and duration, are measured simultaneously and all 60 months are
included. The “long period” starts in 7/1997 when only the measurement of amount was available. These results are based on 146 of 169
months.
CT25K ceilometer might be biased towards lower values due to height limitation and under-detection of ice
clouds (e.g. C REWELL et al., 2008). A detailed comparison of a one-year data set from five ground-based remote
sensing techniques (including a ceilometer; but not the
type used in this study) is given in B OERS et al. (2010).
Beside the pros and cons of the individual techniques
compared to a human observer all techniques deliver a
concurring form of the cloud cover frequency distribution with high frequencies for both the low (0–1 octas)
and high (7–8 octas) classes. There is some preference
of a vertically looking instrument (like the ceilometer)
for these classes.
The height distribution of cloud base (Fig. 6) shows
a distinct variation in the course of the year. In winter,
cloud bases are low and distributed only over a narrow
height range. The frequency maximum lies in the 150300 m height range. Low cloud bases are observed most
frequently in December. In summer, there is a broad
distribution of cloud bases up to more than 2500 m
height without a distinct frequency maximum. Thus, the
arithmetic average of cloud base is low in winter and
high in summer. The annual cycles of cloud cover (low
in summer and high in winter) and cloud base (high in
summer and low in winter) are in accordance with the
annual cycle of relative humidity (low in summer and
high in winter).
The amount of precipitation (tipping bucket) was
measured for a much longer period from 7/1997 to
6/2011 than the duration of precipitation (rain detector) from 8/2006 to 7/2011. For a better comparison of
precipitation amount and duration, the annual cycle of
Meteorol. Z., 21, 2012
B. Brümmer et al.: Atmospheric boundary layer measurements
327
Figure 8: Mean annual cycle of air pressure from 6/1995 to 5/2011
(16 years) on a daily base with the standard deviation and number of
available data for each individual day in the year.
Figure 9: Mean annual cycle of wind speed based on 122 months
from 10/2000 to 6/2011.
precipitation amount was additionally calculated only
for the period with available precipitation duration data
(Fig. 7). The main features of the annual cycle are the
same for the long and short time series. The mean annual sum of precipitation is 715 mm for the long and
714 mm for the short period. The main monthly precipitation falls in summer during July and August with 106
mm and 83 mm, respectively, however with a high STD
caused by the fact that few heavy convective rain events
dominate the summertime precipitation maximum. On
the average the modest precipitation falls in April (28
mm) and September (48 mm). Winter months have more
precipitation (around 60 mm) without a distinct peak in
a certain month. Precipitation duration sums up 815 h/a
on the average, i.e. it rains in 9.3 % of the time. Winter months from November to March have clearly more
precipitation hours (between 81 and 103 h) than spring
and summer months from April to September (between
24 and 62 h). As for the cycle of precipitation amount
the precipitation duration is lowest in April with 24 h
corresponding to 3.4 % of the time and it is highest in
January with 103 h corresponding to 13.8 % of the time.
3.4 Air pressure and wind
Fig. 8 shows the annual course of air pressure during
the period from 6/1995 to 5/2011. The pressure amounts
Figure 10: Frequency distribution of wind direction at 10 and 250
m height for summer and winter (32 segments, radial axis gives
percentage of frequency) based on averages over 10 minutes from
10/2000 to 6/2011.
to 1014.7 hPa on the average and shows almost no annual cycle. However, a clear annual cycle holds for the
pressure STD (note that the STD is calculated from the
daily pressure means; thus the sub-daily time scale is
excluded). STD is 8–18 hPa during the winter months
from November to March and thus twice as large as during the summer months from May to August with 4–8
hPa. The smallest STD occurs in August. The annual cycle of STD reflects the annual course of the amplitude of
the synoptic activity. Highs and especially lows are more
extreme in winter than in summer, which is related to a
stronger North Atlantic Oscillation (NAO) as the result
of a deeper Icelandic low and a higher Azores high in
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B. Brümmer et al.: Atmospheric boundary layer measurements
winter (JAHNKE -B ORNEMANN and B R ÜMMER, 2009).
The annual course of pressure STD is thus not Hamburgspecific but holds for all regions in Europe which are
influenced by the NAO. This statement, however, does
not hold for the mean annual pressure cycle because its
amplitude increases with decreasing distance to the Icelandic low.
Along with the annual course of the synoptic activity as represented by the pressure STD, the wind
speed shows a clear annual cycle, too (Fig.9). The high
wind season is from October to March and the low
wind season from May to August. The maximum occurs in January and the minimum in August. The vertical wind shear (difference between 10 m and 250 m
mean wind) is in winter (around 6 m/s) almost 50 %
larger than in summer (slightly above 4 m/s). The prevailing wind direction at 10 m height is WSW and a
secondary maximum occurs for SE (Fig. 10). SE winds
are observed more frequently in winter than in summer.
Due to decreasing frictional force with height, the dominating wind directions at the 250 m level are turned to
the right. Surprisingly, this does not hold for the 10 m
level SE maximum in winter which has no correspondingly shifted maximum at higher levels. The reason is
not known. It may be that this feature is related to the,
though flat, orography around Hamburg (Fig.1). The air
flow might be channelled by the SE-NW oriented Elbe
river valley, especially in winter with frequent stable
stratification (see section 4.1) so that SE winds at 10 m
height occur relatively frequent and the usual wind veering with height does not hold.
4 Mean diurnal cycles
Mean diurnal cycles are presented for summer (JJA) and
winter (DJF) separately. The calculations are based on
10 minute averages and the figures below are given with
10 minute time resolution. To compare the various levels
we used only days which have complete data (24 h) at
all levels.
4.1 Temperature and moisture
Fig. 11 shows the mean daily temperature cycle for all
levels between 2 m and 250 m height for summer and
winter. With the above-mentioned data restriction we
have 1058 days for JJA and 1288 days for DJF. The
smaller number of days in JJA is caused by several
instrument damages due to thunderstorms.
The amplitude of the daily temperature cycle diminishes with height and from summer to winter. In summer
(winter) the amplitude is 7.9 K (2.3 K) at 2 m height
and 4.2 K (1.0 K) at 250 m height. In summer the 2
m temperature minimum occurs at sunrise and simultaneously with the strongest inversion (+1.2 K from 2 to
250 m). The minimum at 250 m occurs almost 1 h later.
The boundary layer warming starts from the surface and
Meteorol. Z., 21, 2012
leads to an inversion rise. Between 07 and 08 CET the
inversion base leaves the height range of the mast. Between about 07 and 18 CET the temperature stratification is super-adiabatic especially between 2 and 10 m
but to a large portion of time also between 10 and 50 m.
From 09 CET on the stratification above 50 m height is
nearly adiabatic. The temperature maximum is reached
almost simultaneously at all levels between 15 and 16
CET. After 18 CET the stratification becomes stable between 2 and 10 m and the development of a surfacebased inversion begins. Simultaneously with a continuous cooling at all levels the depth of the inversion grows
and extends up to the 250 m level at 04 CET. The temperature cycle is not symmetric with time; the period of
temperature rise lasts 10–11 h and that of temperature
decrease 13–14 h. In winter, the minimum of the mean
temperature cycle is reached at 08 CET and the maximum at 15 CET, in both cases about 0.5 h later at 250 m
than at 2 m height. The temperature cycle is even more
asymmetric than in summer; the period of increase lasts
about 6 h and that of decrease about 18 h. The mean
temperature stratification is stable during the whole day.
The temperature difference 250 m minus 2 m varies between –1.8 K and –0.5 K. The corresponding values for
summer are –2.6 K and +1.2 K.
The diurnal cycle of relative humidity is displayed
in Fig.12. Relative humidity has a single diurnal cycle in summer and winter. This cycle is around a lower
mean value and has a larger amplitude in summer than
in winter. Maxima and minima occur simultaneously
with those of temperature. This shows that, since relative humidity is the ratio of water vapour pressure to
the temperature-dependent saturation water vapour pressure, the diurnal cycle of temperature dominates the cycle of relative humidity. The daily changes of the absolute moisture content (see Fig.13) are not apparent. Furthermore, it is interesting to note that the vertical gradient of relative humidity reverses in the course of the
day in summer: relative humidity decreases with height
during the night and increases during the day. This is
mainly due to the change of the vertical temperature gradient and the different degrees of vertical turbulent mixing during day and night.
The diurnal cycle of absolute humidity is presented
in Fig.13 and allows further conclusions with respect
to the moisture-impacting processes. The vertical gradient of absolute humidity has the same sign (negative)
in summer and winter, but its magnitude is stronger in
summer. The most striking feature when comparing the
diurnal cycles during both seasons is that the absolute
humidity has a single cycle in winter but two cycles in
summer. At the time of the daily temperature maximum
(around 15 CET) the absolute humidity has a maximum
in winter, but a minimum in summer. Since the daily
cycles in Fig.13 have been averaged over several hundred days we can neglect moisture advection and also
rain evaporation as significant processes effecting the
Meteorol. Z., 21, 2012
B. Brümmer et al.: Atmospheric boundary layer measurements
Figure 11: Mean diurnal cycles of air temperature at 2, 10, 50, 70,
110, 175, and 250 m height based on 10 minutes averages on 1058
days in summer (top) and 1288 days in winter (bottom) from 6/1995
to 5/2011. Bars at the abscissa mark sunrise and sunset.
Figure 12: Mean diurnal cycles of relative humidity at 2, 10, 50, 70,
110, 175, and 250 m height based on 10 minutes averages on 460
days in summer (top) and 550 days in winter (bottom) from 9/2004
to 6/2011. Bars at the abscissa mark sunrise and sunset.
daily cycle. If so, the absolute moisture is then primarily determined by the following processes: evaporation
from the surface, condensation at the surface (dew formation), and degree of vertical mixing and, thus, depth
of the boundary layer. The impact of these processes
can be demonstrated at the daily absolute humidity cycle
in summer. The cooling of the surface during the night
causes dew formation and, thus, extracts moisture from
the air. The nocturnal negative moisture tendency due to
dew formation is largest at levels near the ground and decreases with height. This vertical distribution of drying
was also observed at the meteorological mast at Cabauw,
The Netherlands, and occurs predominantly in nights
with clear sky and weak winds (D E ROODE et al., 2010).
After sunrise, at first the dew is evaporated at times when
the boundary layer is still shallow. This leads to an increase of absolute humidity. When the dew is gone the
evaporation from the vegetation and the ground still continues. Simultaneously, the boundary layer grows, the
329
Figure 13: Mean diurnal cycles of absolute humidity at 2, 10, 50,
70, 110, 175, and 250 m height based on 460 days in summer (top)
and 539 days in winter (bottom) from 9/2004 to 6/2011. Absolute
humidity is calculated from air temperature, relative humidity and air
pressure averages over 10 minutes. Bars at the abscissa mark sunrise
and sunset.
moisture is distributed by turbulent mixing over a deeper
layer, and drier air is entrained from the inversion layer,
altogether with the result that the absolute humidity decreases in the lower part of the boundary layer. This is
underlined by the fact that the first moisture maximum
at 250 m coincides with the temperature minimum (see
Fig. 11). The temperature minimum marks the end of
the night-time regime (no vertical mixing, only radiative
cooling) and the upward passage of the top of the boundary layer. In the afternoon at the time of maximum temperature, the boundary layer has reached its largest depth
and also the absolute humidity at the lower layers has
reached its lowest values. Together with the reduction of
the vertical turbulent moisture exchange in the late afternoon and early evening the absolute humidity increases
again in the lower layers. Dew formation begins gradually. For a certain period (21–24 CET) moisture loss by
dew formation and moisture supply by evaporation balance each other. Later in the night and until sunrise the
dew formation dominates. Of course, all these processes
occur also in winter, however, with smaller amplitudes
of evaporation, dew formation, vertical turbulent mixing
and, thus, variation of boundary layer depth in the course
of the day. The latter process (boundary layer depth variation) is the main reason for the formation of the daylight absolute humidity minimum in summer.
4.2 Wind speed and direction
The mean diurnal cycles of wind speed in summer and
winter are displayed in Fig. 14. Wind speed increases
more with height in winter than in summer. The daily
cycles at the lower levels (10 and 50 m) with the maximum during daytime are opposite to those at the upper
levels (>110 m) with the maximum at night. The wind
speed cycles have larger amplitudes in summer than in
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B. Brümmer et al.: Atmospheric boundary layer measurements
Figure 14: Mean diurnal cycles of wind speed at 10, 50, 110, 175,
and 250 m height based on 756 days in summer (top) and 935 days in
winter (bottom) from 10/2000 to 6/2011 and derived from averages
over 10 minutes of the two horizontal components u and v. Bars at
the abscissa mark sunrise and sunset.
Figure 15: Mean diurnal cycles of wind direction difference between 250 m and 10, 50, 110, 175 m height based on 756 days
in summer (top) and 935 days in winter (bottom) from 10/2000 to
6/2011 and derived from averages over 10 minutes of the two horizontal components u and v. Bars at the abscissa mark sunrise and
sunset.
winter and show an asymmetry in summer with respect
to the temporal development of the daytime wind minimum at upper levels.
The opposite cycle of wind speed at lower and upper
levels is a result of the changing stability and thus vertical mixing in the course of the day. At lower levels (10
and 50 m) the wind speed starts to increase after sunrise. Due to the increasing turbulence, higher momen-
Meteorol. Z., 21, 2012
tum from upper levels is mixed down and lower momentum from lower levels is transported upwards so that the
wind speed decreases at upper levels. With the weakening turbulence in the afternoon and thus the reduced vertical mixing, the wind speed decreases again. This tendency continues during late afternoon and night when
the low-level stability develops. Later, with the formation of a surface-based inversion, lower and upper levels are almost decoupled so that the impact of surface
friction is restricted to a shallow near-surface layer and
the air flow at upper levels is almost frictionless. This is
mainly the cycle in winter when stability and boundary
layer height do not change strongly in the course of the
day.
This is different in summer and the strong changes in
boundary layer depth cause the above-mentioned asymmetry at upper levels. After sunrise, the decrease of wind
speed is strongest when the rising inversion passes the
respective level (e.g. 250 m at 07.30 CET). Wind decrease continues for some time as long as the upward
mixing of lower momentum dominates the downward
mixing of higher momentum. As the boundary layer
grows higher above the mast height, there is a time when
the effects of both momentum transports balance each
other so that the wind minimum is reached (between 09
and 10 CET). This occurs at the same time when the
absolute humidity reaches its maximum (see Fig. 13).
Since the boundary layer growth continues in summer
(almost until the temperature maximum is reached; see
Fig.11) so that even higher momentum is mixed down,
the wind speed now increases at the upper mast levels like it does at the lower mast levels. This phase
of momentum balance continues for a few hours after
the temperature maximum. With the development of a
stable layer or inversion from below, the influence of
surface friction is reduced at the upper levels or even
stopped. The absence of friction leads to an imbalance
of forces and thus to the generation of an inertial oscillation which causes a further increase of wind speed and
can lead to the formation of nocturnal low-level jets (e.g.
T HORPE and G UYMER, 1977; A NDREAS et al., 2000).
BAAS et al. (2009) found a maximum frequency of nocturnal low-level jet formation at Cabauw, The Netherlands, in July and August with more than 35 %. Due to
the short summertime night (6-8 hours) the inertial circle (15 hours) cannot be completed so that only a small
part of the oscillation which leads to lower winds comes
to pass before sunrise.
Since averaging the wind direction or taking the wind
direction from the average wind vector does not lead to
a meaningful daily cycle of wind direction, we restrict
here to the vertical wind direction differences taking the
250 m wind direction as reference. The results are shown
in Fig. 15. The daily mean of wind direction difference
between 10 and 250 m is 29◦ in summer and 31◦ in winter. The direction difference exhibits a single daily cycle with high differences during night and small differ-
Meteorol. Z., 21, 2012
B. Brümmer et al.: Atmospheric boundary layer measurements
331
Figure 17: Diurnal cycle of the frequency distribution of cloud
coverage in octas from ceilometer measurements during (a) summer
(JJA) from 2004 to 2010 and (b) winter (DJF) from 2003 to 2011.
Bars at the abscissa mark sunrise and sunset.
Figure 16: Mean diurnal cycles of down-welling radiation fluxes,
short-wave (top) and long-wave (bottom) for summer and winter
based on 10 minutes averages. The measured short-wave radiation
is supplemented by the calculated values for clear sky conditions.
Short-wave data are based on 1236 days in summer and 1260 days
in winter, long-wave data on 1179 days in summer and 1260 days in
winter, both from 6/1996 to 5/2011.
ences during daytime. The amplitude of the daily cycle
is larger in summer (minimum 15◦ and maximum 45◦ )
than in winter (23◦ and 35◦ , respectively). With increasing vertical mixing after sunrise, the directional shear diminishes. In winter, the smallest difference is reached at
the time when the temperature reaches its maximum. In
summer, the reduction of the wind direction difference
in the forenoon hours occurs much faster and reaches
small values already at the time (09 CET) when the absolute humidity maximum and the wind speed minimum
occur at 250 m. These small values of the directional
shear remain almost constant until and even two hours
after the temperature maximum. With the following reduced turbulent mixing, the wind direction difference
begins to increase. This tendency continues until sunrise with the further increase of stability and inversion
depth during the night.
4.3 Radiation, clouds, and precipitation
The mean diurnal cycles of down-welling short- and
long-wave radiation during summer and winter are presented in Fig. 16. The average maximum of short-wave
radiation flux amounts to 543 (134) W/m2 in summer
(winter) which is 64 (41) % of the maximum flux measured under clear sky conditions. The short-wave radiation flux curve for summer is not symmetric around
the maximum. The flux is 6–8 % smaller in the afternoon than before noon for the same sun elevation. This
is caused by the diurnal cycle of cloudiness (cf. Fig. 17).
There is no asymmetry in winter.
The mean long-wave radiation flux in winter has almost no diurnal variation (maximum 299 W/m2 , minimum 295 W/m2 ). In summer, a clear diurnal cycle is
present. The maximum with 365 W/m2 occurs near 16
CET when the air temperature has its maximum. The
minimum is not clearly expressed. It occurs with 344
W/m2 around 02 CET and does not coincide with the air
temperature minimum which occurs between 04 and 06
CET depending on height (cf. Fig. 11). The reason for
the time difference is the increase of cloudiness in the
morning (cf. Fig. 17) which leads to more down-welling
long-wave radiation.
Fig. 17 shows the mean diurnal cycle of total cloud
cover broken down to the six coverage classes (0, 1+2,
3+4, 5+6, 7 and 8 octas). The cycles are based on seven
years of ceilometer data during the period 01/2004–
12/2010. In total, 631 (643) complete days entered into
the winter (summer) averages. In winter, cloud cover
shows no marked diurnal cycle. The most frequent class
is 7 octas with more than 40 % of the time followed
by the 8 octas class. Cloudless sky is observed in only
12 % of the time. In summer, the total cloud cover
is distributed over more classes and shows a distinct
diurnal cycle. Cloudless sky occurs in 28 % of the total
time, more frequent during the night with up to 36 %
than during the day with down to 15 %. The 7 octas
class is the most frequent class with 36 % of the total
time and is lower during night and higher during the day.
The classes of 1–6 octas occur altogether in 32 % of the
time and have their maxima during the day. This may
partly reflect the diurnal cycle of fair weather cumulus
clouds. The frequent occurrence of low and high cloud
cover indicates that the diurnal cycle of the mean total
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B. Brümmer et al.: Atmospheric boundary layer measurements
Meteorol. Z., 21, 2012
Figure 19: Mean diurnal cycle of amount (top) and duration (bottom) of precipitation for summer and winter. Results for amount are
based on 1218 days in summer and 1209 days in winter from 7/1997
to 7/2011, result for duration are based on 457 days in summer and
449 in winter from 8/2006 to 7/2011. Values are mean sums over one
hour. Bars at the abscissa mark sunrise and sunset.
Figure 18: Diurnal cycle of the frequency distribution of cloud base
from ceilometer measurements during (a) summer (JJA) from 2004
to 2010 and (b) winter (DJF) from 2003 to 2011. Bars at the abscissa
mark sunrise and sunset.
cloud cover would not be very informative. This would
result in a curve around 50 % cloud coverage with little
variation during the day.
Fig. 18 shows the diurnal cycle of cloud base. To
this end cloud bases were subdivided into 150 m height
intervals. The relative frequencies of the height intervals were normalized with the above-mentioned numbers of days, i.e. 643 (631) for summer (winter). In winter, cloud base is rather low during the entire day. The
frequency maximum occurs for the 150–300 m height
range. Even lower cloud bases occur rather frequently,
more at the end of the night and less in the afternoon.
Cloud bases above 750 m are observed very rarely. In
summer, the cloud base frequency distribution shows a
distinct diurnal cycle. In the course of the night the frequency of low-level clouds (150–300 m) increases. Also
even lower clouds occur with a maximum at the end
of the night. After sunrise and later, together with the
growth of the boundary layer, the cloud base rises and
the frequency distribution becomes broader with height.
In the early afternoon, at the time of maximum air temperature (cf. Fig. 11) and minimum relative humidity (cf.
Fig. 12), cloud bases reach their highest levels in a broad
interval between 900 m and 1800 m. When the turbu-
lence decreases in the afternoon and later ceases in the
evening, the moisture supply to cloud base also ceases
so that many clouds dissolve (cf. Fig. 17). The bases of
the remaining clouds are scattered over a wide height
range up to 2500 m. Low-level cooling and stable stratification lead to the formation of a new boundary layer
with an increased frequency of low-level cloud bases.
The diurnal cycle of cloud base frequency in Fig.18
demonstrates impressively the continuous cloud development from morning to afternoon and the break during
the early evening. This break would not have been obvious from the diurnal cycle of the mean cloud base.
Fig. 19 displays the mean diurnal cycle of precipitation amount and duration. In winter, precipitation exhibits only a weak diurnal cycle. Precipitation amount
and duration are slightly higher in the afternoon. As already shown in Fig. 7 for the annual cycle, amount of
precipitation is higher and duration is shorter in summer
compared to winter. This implies that convective precipitation occurs more frequent in summer. It is mainly the
convective rain events in the afternoon and early evening
which contribute to the summertime rain maximum.
5 Discussion
In this paper the 280 m high Hamburg weather mast
and its instrumentation were introduced. The digital data
recorded with high temporal resolution since 1995 were
used to calculate the mean annual and diurnal cycles
of the primary climate variables (pressure, temperature,
humidity, wind, short- and long-wave radiation, cloud
coverage, cloud base, precipitation) for differently long
time intervals during the period 1995–2011.
The annual cycles at the Hamburg weather mast coincide to a large extent with those for the 30 years
Meteorol. Z., 21, 2012
B. Brümmer et al.: Atmospheric boundary layer measurements
time period 1971–2000 published by the Deutscher Wetterdienst (e.g. L EFEBVRE and ROSENHAGEN, 2008;
R IECKE and ROSENHAGEN, 2010; ROSENHAGEN and
S CHATZMANN, 2011). This holds for the times of the
maxima and minima in the course of the year. However, there are some differences concerning the annual means which are outlined below. The annual 2 mtemperature mean is 9.81 ◦ C at the weather mast (1995–
2011) and 9.0 ◦ C at the Hamburg-Fuhlsbüttel airport station (1971–2000) which is located 15 km northwest of
the weather mast (R IECKE and ROSENHAGEN, 2010).
The three separate decade means for Fuhlsbüttel show a
tendency from 8.7 ◦ C (1971–1980) over 9.0 ◦ C (1981–
1990) to 9.4 ◦ C (1991–2000). The weather mast mean
of 9.81 ◦ C indicates a continuation of this trend. The
positive difference (16-years mean of Hamburg weather
mast minus 30-years mean Fuhlsbüttel) applies to all
months except December and is largest for the summer
months with the maximum of +1.24 K for August.
The difference in precipitation amount (Hamburg
weather mast (1997–2011) minus Fuhlsbüttel (1971–
2000)) is –56 mm, corresponding to –7 %. This fits
into the general west-east gradient of precipitation in the
Hamburg region as the maps by L EFEBVRE and ROSEN HAGEN (2008) for the period 1971–2000 and by R EIDAT
(1971) for the period 1931–1960 show. Precipitation
is a very variable quantity. Nevertheless, the measured
716 mm mean value for the weather mast fits well between the two values taken at the position of the weather
mast from the 30-years maps from Reidat (670 mm) and
Lefrebvre and Rosenhagen (750 mm). The annual precipitation cycle at the weather mast with a minimum in
April and a maximum in July agrees with that measured
at Fuhlsbüttel for the period 1971–2000 (R IECKE and
ROSENHAGEN, 2010). However, the amplitude of the
cycle at Fuhlsbüttel is smaller with less precipitation in
summer and more in winter.
The annual 10 m wind mean at the Hamburg weather
mast (2000–2011) amounts to 2.96 m/s and is clearly
smaller than the Fuhlsbüttel mean (1971–2000) which
amounts to 3.9 m/s (R IECKE and ROSENHAGEN, 2010).
The difference (weather mast minus Fuhlsbüttel) is negative for each month and is larger in the wintertime
high-wind season (–1.6 m/s in December) than in the
summertime low-wind season (–0.5 m/s in June). One
reason for the systematic difference is probably the
higher roughness at the weather mast compared to the
Fuhlsbüttel airfield (especially for westerly winds from
the city of Hamburg). Another reason may be the lower
location of the weather mast in the Elbe river valley.
The annual cycles of the other above-mentioned climate variables cannot be compared directly with other
existing climatologies. Often sunshine hours are presented instead of short-wave radiation flux. A climatology for moisture and long-wave radiation flux is not
available. The same is true for the frequency distributions of cloud base and cloud coverage. Concerning
333
cloud coverage, our distribution shows that a comparison with mean coverage values is not meaningful because low and high cloud coverage classes occur as frequent.
6 Conclusions
In this paper, the diurnal cycles of the most important
climate variables were jointly determined with high temporal resolution so that many temporal relations between
the individual boundary layer processes could be detected. Again, the most important results are summarized below. In summer, the evolution of the temperature
stratification with the generation and decay of the lowlevel inversion could be documented in detail. While
the relative humidity has a single cycle, the absolute
humidity has a double diurnal cycle. This is related to
the varying strength of evaporation but also to the distinct increase of the boundary layer depth in the course
of the day. The latter is also the reason for the opposite phase of the diurnal wind speed at the lower and
higher levels and for the temporal asymmetry of the
upper-level wind speed in the morning. The strong diurnal change of stability also cares for the large differences of the directional wind shear between day and
night. The down-welling short-wave radiation is 6–8 %
smaller in the afternoon than in the morning due to the
uneven cycle of cloud coverage. Cloud base shows a distinct rise from the morning to the afternoon followed
by a break in the evolution and distribution over a wide
height range. Most precipitation falls in the afternoon
and early evening hours.
In winter, when the boundary layer depth varies only
little, many characteristics of the summertime boundary
layer are missing. There is no double diurnal cycle of
absolute humidity and no temporal asymmetry of the
upper-level wind speed. Due to missing convection the
cloudiness shows little diurnal variation and is mostly
stratiform with low bases. The ratio of actually received
to maximum possible short-wave radiation at noon is
only 41 % compared to 63 % in summer. Precipitation
is almost equally distributed over the day.
The above-presented results on the diurnal cycles
are not totally new. For example, some can be found
in other publications which are also based on measurements at high towers as in Cabauw (Netherlands) up
to 213 m (VAN U LDEN and W IERINGA, 1996), in Oklahoma (USA) up to 450 m (C RAWFORD and H UD SON , 1973) and in Lindenberg (Germany) up to 99 m
(B EYRICH and F OKEN, 2005). However, these publications are mostly restricted either to only single climate variables, shorter time periods, or individual cases
so that the interrelations between the various climate
variables were not detected. For example, the study of
C RAWFORD and H UDSON (1973) which is based on
one year of data shows also the temporal asymmetry of
the upper-level wind speed in the morning but does not
334
B. Brümmer et al.: Atmospheric boundary layer measurements
discuss this phenomenon. Concerning the diurnal cycle
of humidity, mostly relative humidity is considered although absolute humidity with its double diurnal cycle
in summer allows much more conclusions with respect
to the on-going processes in the boundary layer. The
double cycle of absolute humidity was already described
in the textbook of G EIGER et al. (1995).
In recent time, particularly in the wind energy community, the vertical profiles of the standard deviation of wind speed and the shape parameter k of the
Weibull wind frequency distribution from long time series (month or year) are discussed. The profiles show
a minimum and a maximum, respectively, at intermediate height levels around 100 m (e.g. W IERINGA, 1989;
P ETERSEN et al., 1998; E MEIS, 2001). Our annual and
diurnal cycles of wind speed show that this level is
just the turn-over height of the gradient of the vertical
wind profile (cf. Figs. 9 and 14). The turn-over height is
around 130 m in summer and 80 m in winter.
The vertical average (10–250 m) of the wind speed
has its maximum in January when the short-wave radiation is low (Fig. 9). In the course of the day, the vertical
wind speed average is largest around 15 CET and smallest around 07 CET in summer. In winter, there are no
significant diurnal variations of the vertical wind speed
average.
The annual and diurnal cycles of temperature, humidity, wind, radiation, cloudiness, and precipitation
presented in this paper are a good reference for model
validation (process, weather, or climate models). The
documented cycles are the result of boundary layer processes which are not specific for the site of the Hamburg
weather mast but occur wide-spread over mid-latitude
flatlands. In particular, the models should be tested if
they are able to reproduce the most important characteristics of the summertime boundary layer such as the
double diurnal cycle of absolute humidity, the opposite
cycles of the upper and lower wind speed, the temporal asymmetry of the upper-level wind speed, the bimodal distribution and opposite cycles of cloud coverage classes, the forenoon increase of cloud base and the
cloud collapse with the rain maximum in the afternoon
and evening.
Acknowledgments
The Hamburg weather mast instrumentation and operation is funded by the University of Hamburg and
the Cluster of Excellence EXC-177 “Integrated Climate
System Analysis and Prediction (CLISAP)” at the University of Hamburg. We thank the Norddeutscher Rundfunk for the allowance to use the broadcasting tower
and surrounding areas. We especially acknowledge the
always careful and reliable work of our colleague,
Michael Offermann, during the many years with the
installation and maintenance of the Hamburg weather
mast. We thank two anonymous reviewers for their constructive comments.
Meteorol. Z., 21, 2012
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