Chapter 7

Global Precipitation Climatology Project (GPCP)

http://www.gewex.org/gpcp.html

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.

Gravity

Air resistanceA 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.

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.

Table 7-1, p. 166

Stepped Art

Fig. 7-5, p. 169

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.

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 DropletsSpontaneous or homogeneous freezingIce embryo

Precipitation Types

Rain: falling drop of liquid waterDrizzle less than 0.5 mmVirgaCloudburst

Snow: frozen water falling from sky (crystal or flake)Most precipitation starts as snowFreezing level, snow & cloud appearance, fall

streaks, drifting snow, blizzardA blanket of snow is a good insulator

Precipitation Types

Topic: Tear DropsRaindrops not tear shaped.Shape is size dependent

○ Less than 2 mm = sphere○ Greater than 2 mm = flattened sphere

Small drops less than 2 mmLarger drops

Precipitation Types

Topics: Sounds and snowA blanket of snow will act like an acoustic

tile and absorb sound waves.

Topics: Snow with Temperature above FreezingUnsaturated 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 IcingAviation 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

InstrumentsRain gauge: standard, tipping bucket,

weighing○ Snow: average depth at 3 locations, 10:1

water equivalent

Doppler RadarTransmitter 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 spaceSpecific satellites designed to assess

clouds, atmospheric moisture, and rain○ TRMM○ CloudSat