Precipitation
p
precipitation processes, types, measuring
Chapter 5
September 15, 2009
1
How precipitation forms
• Through various mechanisms, moisture in
p
condenses and forms
the atmosphere
cloud,
• However,
However cloud is a cluster of condensed
liquid or ice, which do not automatically
produce precipitation
• Cloud needs certain mechanisms to
produce
d
precipitation
i it ti
2
How precipitation forms
•
•
•
•
Cloud
Cl
dd
droplets
l t are very tiny
ti
(~20 micrometers, 0.02 mm)
A cloud droplet’s diameter
must grow ~200 times (a
million times in volume) to
reach a raindrop’s diameter.
However, so many
condensation nuclei
(candidates) are present in
cloud,
l d competition
i i to growth
h
(by condensation) is very high
Therefore, there are some
special mechanisms which
enable a tiny cloud droplet to
grow effectively.
3
Precipitation processes
• Precipitation from Cold Clouds
– The Bergeron
g
process
p
• Precipitation from
f
Warm Clouds:
C
– The collision-coalescence process
p
4
Cold cloud (Bergeron) process
• The formation of precipitation in the cold
y ice crystal
y
growth
g
clouds by
• Called as “cold rain” or “ice crystal process”
• This
s process
p ocess is
sap
principal
c pa rain
a formation
o at o
mechanism in the middle and high
latitudes where cloud tops extend above
the freezing level (cold clouds)
5
Bergeron
process works
here
The typical distribution of ice and water in a cumulonimbus cloud.
6
Important properties of water droplets
1. Cloud droplets do not freeze at 0°C as expected. Pure
water suspended in air does not freeze until it reaches a
temperature of nearly -40
40°C
C.
2 Remains as so called ‘supercooled’
2.
supercooled water (water in the
liquid state below 0°C), which will freeze immediately if
it impact an object, on contact with solid particles that
have a crystal form closely resembling that of ice, called
‘freezing nuclei’.
–
Example: super cooled water contacts with ice
7
Freezing (ice) nuclei
• Ice-forming particles that exist in subfreezing air
• Clay materials, bacteria in decaying plant leaf material
and
d other
th iice crystals
t l
• A
Are relatively
l ti l sparse iin th
the atmosphere
t
h
and
dd
do nott
generally become active until -10°C
• -20°C ~ -10°C liquid droplets coexist with ice crystals
8
Important properties of water droplets
•
The saturation vapor pressure above ice crystals is
lower than above supercoolded liquid droplets.
•
In a saturated environment
(within clouds)
clouds), the water
droplet and the ice crystal are
in equilibrium, as the number
of molecules leaving the
surface of each droplet and ice
crystal equals the number
returning.
i
•
The greater number of vapor
molecules above the liquid
9
Bergeron Process – 1st step
•
•
•
When supercooled
Wh
l d water and
d
icy crystal coexist, the greater
number of water vapor
molecules around the liquid
droplets causes water
molecules to diffuse from the
liquid drops toward the ice
crystals and freeze on its sfc.
The ice crystals absorb the
water vapor and grow larger,
while the water droplets grow
smaller.
ll
Ice crystals grow at the
expense
p
of the surrounding
g
water droplets.
10
Bergeron Process – 2nd step
Growth by riming
• As crystals grows and
became bigger, they start
to fall.
fall Falling ice crystals
enlarge as they
additionallyy intercepts
p
supercooled cloud drops
that freeze on them
• This is called riming (or
accretion)
11
Bergeron Process – 2nd step
Generating
g secondary
y
ice particles
•
In colder clouds the ice
crystals may collide with other
i crystals
ice
t l
•
Ice crystals break up many
delicate particles (fragments),
which will serve as new
freezing nuclei for other liquid
droplets.
12
Bergeron Process – 2nd step
Growth by aggregation
•
•
•
A chain reaction develops:
riming – breaking up – riming
Additi
Additionally
ll th
they may collide
llid
and stick to one another
forming an aggregate of
crystals called a snowflake
snowflake.
Large snowflakes may consist
of 10 to 30 individual crystals
•
Snowflakes fall and melt as
entering level where
temperature is above freezing
level
•
Animation
13
How the Bergeron process was
discovered
A health resort at an altitude
of 430 meters on a hill near
Oslo
Swedish meteorologist, Tor
Bergeron (August 15,
15 1891 –
June 13, 1977)
Supercooled
He noticed that in very cold
day there was no fog along a
road in the forest
Some of supercooled cloud
droplets (fog) are crystallized
as they contacts branch of
trees, and then grow rapidly
14
Warm Cloud Process
• Rainfall associated with clouds located below
the freezing level (warm clouds), especially in
the tropics
• In such warm clouds, collisions between droplets
can play a significant role in producing
precipitation.
• The collision-coalescence process
15
Collision and Coalescence
• IIn a warm cloud
l d
composed only of small
cloud droplets
p
of uniform
size, the droplets are less
likely to collide as they all
fall very slowly at about
the same speed.
ose d
droplets
op e s that
a do
• Those
collide, frequently do not
coalesce because of the
strong surface tension
that holds together each
tiny droplet.
16
Collision and Coalsence
•
•
•
Some llarge d
S
drops fform on
large condensation nuclei or
through random collisions of
droplets
Larger droplets fall faster than
smaller droplets
droplets.
Although some tiny droplets
are swept aside, some collect
on the larger droplet's forward
edge,
g , while others (captured
( p
in
the wake of the larger droplet)
coalesce on the droplet's
backside.
backside
17
Warm Clouds
• Often there is updraft in
the convective clouds
• A cloud droplet rising
th falling
then
f lli th
through
ha
warm cumulus cloud can
grow by collision and
coalescence, and emerge
from the cloud as a large
raindrop.
18
Factors in raindrop production
by warm clouds
• The cloud’s liquid water content
• The cloud thickness
– heaviest precipitation occurs in those clouds with
most vertical development
• The updrafts of the cloud
• The range of droplets sizes
19
Cloud Seeding
• To inject (or seed) a cloud with small particles
that will act as nuclei, so that the cloud particles
will grow large enough to fall to the surface as
precipitation.
• Natural
N t l seeding
di
– When cirrus clouds lie directly above a lower cloud
deck ice crystals
deck,
cr stals descend into lo
lower
er clo
clouds.
ds
• Artificial seeding
– First experiments in late 1940s using dry ice.
– Silver Iodide is also used today because it’s structure
i similar
is
i il tto th
thatt off iice crystals.
t l
20
Natural Cloud Seeding
• N
Natural
t l seeding
di – cirrus
i
fform clouds
l d lilie di
directly
tl
above a lower cloud deck, ice crystals descend
into lower clouds
clouds.
21
Artificial seeding
Seeding
S
di ffreezing
i nuclei
l i to trigger
i
precipitation
i i i
Silver iodide, dry ice are often used
22
Artificial seeding
• In order for cloud seeding to trigger precipitation,
conditions must be just right.
• Clouds
Cl d ((moisture)
i t ) mustt b
be present;
t seeding
di cannott
create clouds.
• A portion of the clouds must contain supercooled water
water.
• This method assumes that the clouds are lacking in
freezing nuclei and adding them will stimulate
precipitation by the Bergeron process.
• One must be careful not to overseed as this will produce
too many,
y, too small ice crystals.
y
23
Weather control
•
•
To keep rain away from the opening or closing ceremony
Beijing officials set up several banks of rocket launchers outside the
city to seed threatening clouds and cause them to release their rain
before it reaches the capital
24
Break?!
25
Precipitation types and their
temperature profiles
• Rain
• Snow
• Sleet/glaze
• Depends on many factors, vertical
temperature profile is the most important
one.
26
Rain
• F
Falling
lli d
drop off liliquid
id water
t th
thatt h
has a di
diameter
t
equal to or greater than ~0.5 mm
• Drizzle
D i l – drops
d
ttoo smallll tto qualify
lif as rain
i
• Virga – raindrops that fall from a cloud but
evaporate
t before
b f
reaching
hi th
the ground
d
• Shower – intermittent precipitation from a
cumuliform
lif
cloud
l d usually
ll off short
h td
duration
ti b
butt
often heavy intensity
• Acid
A id rain
i – rain
i th
thatt iis mixed
i d with
ith gaseous
pollutants (sulfur, nitrogen) and becomes acidic
27
Temperature Profile for Rain
28
Virga
29
Snow
• A solid form of precipitation composed of ice crystals in
complex hexagonal form
• Much of the p
precipitation
p
reaching
g the g
ground actually
y
begins as snow.
• Fallstreaks – Ice crystals and snowflakes falling from
high cirrus clouds. Behave similar to Virga – fall into
drier air and disappear before reaching the ground.
Change from ice to vapor (sublimation)
• Blizzard – severe weather condition. Low temperatures
and strong
g winds (g
(greater than 30 kts)) bearing
gag
great
amount of falling or blowing snow.
30
Temperature Profile for Snow
31
32
Fallstreaks
33
Sleet and Glaze
• Sl
Sleett – type
t
off precipitation
i it ti consisting
i ti off
transparent pellets of ice 5 mm or less in
diameter (ice pellets)
• Glaze – rain/drizzle that falls in liquid form and
then freezes upon striking a cold object or
ground. (Freezing Rain/drizzle )
34
Sleet and glaze
•
•
•
Sleett iis a wintertime
Sl
i t ti
phenomenon
h
that
th t refers
f
to
t the
th fall
f ll off smallll particles
ti l off
ice that are clear to translucent. Sleet forms when rain passes through a
cold layer of air and freezes into ice pellets.
This occurs most often in the winter when warm air is forced over a layer of
cold air.
In case that the subfreezing air near the ground is not thick, the raindrops
do not freeze but became supercooled. Super cooled raindrop turn to ice
immediately as fall down to surface.
35
Sleet
36
Glaze
A heavy coating of freezing rain during this ice storm
37
Rime
Rime iis a d
Ri
deposit
it off ice
i crystals
t l formed
f
d by
b the
th freezing
f
i off super cooled
l d fog
f or
cloud droplets on objects whose surface temperature is below freezing.
When rime forms on trees, it covers them with ice feathers; in windy
conditions only
onl the windward
ind ard surfaces
s rfaces will
ill accumulate
acc m late the layer
la er of rime.
rime
38
Bergeron process at surface, taking place in very cold, humid, and pure air.
Hailstones
Hail is precipitation in the form of hard
hard, rounded pellets or
irregular lumps of ice. The layers of ice accumulate as the
hailstone travels up and down in a strong convective cloud.
Hailstones begin
g as small
ice pellets that grow by adding
supercooled water droplets as
they move through the cloud.
As the ice crystal cycles up and down
in the cloud the hailstones increase
in size until they are forced out by
a downdraft or become heavy
enough to fall out.
39
The giant Coffeyville hailstone first cut then photographed under regular light...
September 1970 weighed 760g
40
Measuring Precipitation
• Rain gauge – instrument used to collect and
measure rainfall
– Standard gauge, tipping bucket rain gauge
• Weather radar
– Doppler
pp radar
– TRMM satellite
41
Standard Rain Gauge
• A standard rain gauge
with an 20cm
diameter collector
funnel and a tube that
amplifies rainfall by
ten.
• If amount of rain is
less than 0.025cm,
reported as being a
trace of precipitation
42
Tipping Bucket Rain Gauge
• The tipping bucket rain
gauge
• Consists
C
i t off two
t
compartments (buckets),
each one capable of
holding a certain volume
of water
• Each time the bucket fills
rain, it tips, sending an
electric signal to the
remote recorder.
43
Measuring precipitation
• T
Trace – an amountt off precipitation
i it ti less
l
than
th
0.025 cm (0.01 inch), reported as being a trace
of precipitation
– Trace, no rainfall, and missing data are separately
marked.
• Snow
–D
Depth:
th d
determined
t
i db
by measuring
i iin th
three or more
representative areas and taking an average.
– Water equivalent:
q
g
generally
y about 10 cm of snow will
melt down to about 1 cm of water. Varies greatly and
depends on texture and packing of snow.
44
Weather radar
45
Radar image now
Weather radar
• D
During
i WWII
WWII, military
ilit
radar
d operators
t
noticed
ti d noise
i iin
returned echoes due to the rainfall, snow …
• Sending radar pulses (microwave) and listening for
return signals
g
((echoes))
• Modern types are pulse-doppler radar,
• Capable of detecting motion of rain droplet in addition to
intensity of precipitation
• Useful to determine the structure of storms & their
potential to cause severe weather
46
Doppler radar
Falling speed is proportional to the
47
size of droplets
Spaceborne radar
Precipitation Measuring Missions (PMM)
Satellite radar is able to measure the vertical distribution of
precipitation. This enhances our understanding of rainfall processes,
its inherent physics/dynamics,
/
and influences
f
on climate
48
Thanks!
Tomorrow, seminar will be started at 10:15
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