Transcript MOISTURE - University of Maryland, College Park

AOSC 200
Lesson 8
Precipitation Growth in Warm Clouds
• Cloud droplets are typically 10 microns in
size. small raindrops are typically 1000
microns (almost one million droplets)
• How does one go from one to the other?
• Simplest explanation is that the droplet
grows by condensation of water vapor on
its surface. But this would take several
days.
• One way for raindrops to form is by a
processes known as collisioncoalescence.
• This process requires lots of droplets high absolute humidity - seen only in the
tropics.
Collision-Coalescence
• Warm cloud because process because it only
occurs in the tropics.
• As the cloud droplet is formed it is pulled down
by gravity but then is moved upward by the
rising air within the cloud. Hence within a cloud
one has droplets moving in all directions and
they can collide and form larger droplets by
coalescing.
• Once a drop grows to a size when the force of
gravity exceeds the uplift from the rising air, the
drop moves downward through the cloud picking
up other droplets as it falls.
Collision Coalescence
Collisioncoalescence
process
Fig. 4-31, p. 114
Precipitation Growth in Cold Clouds
• Outside of the tropics there are just not
enough cloud droplets to form rain drops by
the collision-coalescence process.
• A Swedish atmospheric physicist, Dr.
Bergeron, would vacation in the fall in the
mountains. He often took an early morning
walk along a path that led through a pine
forest and encountered uplift fogs.
• He noted that on the days when the
temperature was above 0 C, the fog went all
the way to the ground, whereas if the
temperature fell below -10 C, the fog lay
above the tops of the trees.
Bergeron walk
Why?
• He further noted when the temperature fell
below -10 C, that the pine needles were
covered with ice.
• The reason is that the vapor pressure of
water above ice is less than the vapor
above water.
Attraction of
water vapor to
ice versus water
Fig. 4-33, p. 116
Saturation vapor pressure over ice and over water
Fig. 4-35, p. 117
Fig. 4-34, p. 116
Accretion and Aggregation
• It should be noted that the ice crystal growth
gives a ‘snow flake’ which eventually will also
begin to fall and then rise in the updraft.
• These ice crystals then begin to aggregate into
larger snowflakes.
• The ice crystal can also collide with a
supercooled water droplet which instantly
freezes - accretion.
• As the ice crystals descend at some point the
temperature can rise above 0 C, and the crystal
melts to form a raindrop.
Process of aggregation
Fig. 4-32, p. 115
Aggregation
Fig. 4.27
These picture shows fall-streaks. They consist of ice
particles that have fallen out of a cloud and
evaporate before they reach the ground.
Fig. 4.33
Steps in the formation
of the precipitation
types
Fig. 4.38
Warm Front
Fig. 9.13
Fig. 4-37, p. 118
FORMS OF PRECIPITATION
•
RAIN - DROPLETS OF WATER GREATER THAN 0.5 MM IN
DIAMETER. WHEN DROPLETS SMALLER THAN 0.5 MM CALLED
DRIZZLE.
•
MUCH RAIN STARTS OUT ALOFT AS ICE CRYSTALS
•
SNOW - ICE CRYSTALS. IF AIR IS COLD (LOW HUMIDITY), WE
GET LIGHT AND FLUFFY SNOW (POWDER). IF AIR IS WARM
THAN ABOUT -5 CELSIUS, THEN WE GET WET SNOW (GOOD
FOR SNOWBALLS).
•
SLEET - SMALL PARTICLES OF ICE. RAINDROPS ENCOUNTER
FREEZING AIR ON DESCENT. IF FREEZING NOT COMPLETE FREEZING RAIN.
•
HAIL - LAYERS OF ICE FORM AS THE HAILSTORM TRAVELS YUP
NAND DOWN IN A STRONG CONVECTIVE CLOUD
•
RIME - FORMED BY FREEZING OF SUPERCOOLED FOG ON
OBJECTS.
The effects of airflow over a mountain
Fig. 4-42, p. 122
Clouds and Precipitation near
Mountains
• As air ascends mountain it cools adiabatically,
clouds form, and precipitation occurs.
• Above this altitude the relative humidity stays at
100%
• At the peak of the mountain the absolute
humidity is determined by the saturation vapor
pressure at -12C.
• As the air descends its absolute humidity
remains the same as at the peak
Clouds and Precipitation near
Mountains
• As the air descends it is compressed, so it
warms
• Hence the saturation vapor pressure will
increase, and the relative humidity will decrease
• The net effect of the air ascending and
descending the mountain is that the air becomes
drier and warmer.
• On the island of Hawaii, the west side of the
coast (westerly winds) has rain forests, the
eastern side has deserts.
Table 4-4, p. 123