Transcript Stability & Cloud development

Chapter 7
http://www.gewex.org/gpcp.html
Global Precipitation Climatology Project (GPCP)
Most rain falls near the equator. Notice how dry it is on the west coast of
the US and South America. The Eastern US is relatively wet.
A fallen drop stops speeding up
when gravity is balanced by air
resistance. The speed that a drop
must fall so air resistance is large
enough to balance gravity is the
terminal velocity.
Air resistance
Large drop have much larger
terminal velocities than small
drops.
In fact the reason cloud drops
don’t fall from the sky is that their
terminal velocities are so slow (like
1 inch per minute) that any small
updraft can easily keep them
suspended.
Gravity
Table 7-1, p. 166
Conditionally unstable atmosphere. The atmosphere is conditionally unstable when
unsaturated, stable air is lifted to a level where it becomes saturated and warmer
than the air surrounding it. If the atmosphere remains unstable, vertical developing
cumulus clouds can build to great heights. The rain drop falls out of the cloud when
its terminal velocity exceeds the mean updraft speed.
Stepped Art
Fig. 7-5, p. 169
Super cooled liquid water drops are droplets that are present at temperature
between -40 oC and 0 oC. The freeze on contact with either another particle, tree
branch, airplanewing, and make rime ice.
Fig. 5, p. 183
Precipitation Processes

Topic: Freezing of Cloud Droplets
 Spontaneous or homogeneous freezing
 Ice embryo
Precipitation Types

Rain: falling drop of liquid water
 Drizzle less than 0.5 mm
 Virga
 Cloudburst

Snow: frozen water falling from sky
(crystal or flake)
 Most precipitation starts as snow
 Freezing level, snow & cloud appearance,
fall streaks, drifting snow, blizzard
 A blanket of snow is a good insulator
Precipitation Types

Topic: Tear Drops
 Raindrops not tear shaped.
 Shape is size dependent
○ Less than 2 mm = sphere
○ Greater than 2 mm = flattened sphere
Small drops less than 2 mm
Larger drops
Precipitation Types

Topics: Sounds and snow
 A blanket of snow will act like an acoustic tile
and absorb sound waves.

Topics: Snow with Temperature above
Freezing
 Unsaturated wet bulb temperature below or
equal to 0°C, rain cooled by evaporation
forms snow despite environmental
temperature above freezing.
Warm air typical has more moisture than cold air giving larger snow flakes.
Precipitation Types
Sleet: air below freezing, then travels
through a layer of air above freezing,
begins to melt and then falls through a
layer of air below freezing just above the
ground surface.
 Freezing Rain: ground surface is
freezing as rain hits the surface it
freezes.

Precipitation Types

Observation: Aircraft Icing
 Aviation hazard is created by the increase in
weight as ice forms on the body of the
airplane.
 Spray plane with anti-freeze.
Precipitation Types
Snow Grains: solid equivalent of drizzle,
no bounce or shatter
 Snow Pellets: larger than grains,
bounce, break, crunch underfoot
 Graupel: ice particle accumulation with
rime
 Hail: graupel act as embryo in intense
thunderstorm, grow through aggregation
as pushed up by updraft.

Stepped Art
Fig. 7-29, p. 185
Measuring Precipitation

Instruments
 Rain gauge: standard, tipping bucket,
weighing
○ Snow: average depth at 3 locations, 10:1
water equivalent

Doppler Radar
 Transmitter generates energy toward target,
returned energy measured and displayed
○ Brightness of echo = amount/intensity of rain
 Doppler: measures speed of horizontal rain
Stepped Art
Fig. 7-33, p. 188
Somewhat like police radar,
Doppler radar can also
determine the storm velocity
away from or towards the
radar station by measuring
the frequency difference
between the transmitted and
reflected signal.
Here m/s stands for meters
per second.
60 m/s =130 mph
The figure shows a tornado.
The purple region is moving
rapidly towards the station
and then 200 meters to the
right the pink is moving
rapidly away from the station.
Measuring Precipitation

Measuring from space
 Specific satellites designed to assess
clouds, atmospheric moisture, and rain
○ TRMM
○ CloudSat