Precipitation Processes. Precipitation Processes 2/24/11. Two Mechanisms that produce raindrops:

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1 Precipitation is any form of water that falls from a cloud and reaches the ground. How do cloud drops grow? Chapter 7 When air is saturated with respect to a flat surface it is unsaturated with respect to a curved droplet of water. Air must be super-saturated to keep growing if water is pure The solute effect lowers vapor pressure and lets clouds grow at lower RH Over land: more condensation nuclei More, smaller droplets than over water Normal condensation processes do not produce rain Would take several days to produce drops large enough to fall as rain 1 2 Two Mechanisms that produce raindrops: Collision & Coalescence Ice-Crystal (Bergeron) Process Collision & Coalescence Droplets of different sizes collide and coalesce into larger droplets; warm cloud process Largest drops reach ground first from the warm cloud process Fig. 7-3, p

2 Ice-Crystal (Bergeron) Process More common in middle or upper latitudes Cold clouds are a mixture of ice & water Ice crystals grow at the expense of surrounding water droplets Supercooled water drops can exist at temperatures below 0 o C At -10 o C, there are a million liquid droplets for every ice crystal Freezing is enhanced if there are ice nuclei Ice nuclei are more rare than condensation nuclei: clay minerals, bacteria Below -40 o C, the cloud is glaciated, only ice Cirrus clouds are generally high enough to be glaciated Saturation vapor pressure is greater over water as compared to ice. Table 7-1, p Ice-Crystal (Bergeron) Process Saturation vapor pressure is greater over water as compared to ice. The water vapor supplied by the liquid droplets diffuse toward the ice crystals and make them grow. Accretion Ice crystals collide with supercooled droplets and freeze them, making the crystal grow, forming graupel. Aggregation Ice crystals collide and stick together, forming snowflakes 9 10 Artificial Cloud Seeding Inject cloud with small particles that act as condensation nuclei, starting the precipitation process. There must already be clouds: seeding does not generate clouds Cold clouds with a low seed ration best Dry ice, silver iodide Fig. 7-10, p

3 Natural Cloud Seeding Cirriform clouds can drop ice onto lower cumulus clouds and seed the precipitation Precipitation in Clouds In cold, strongly convective clouds, precipitation may form quickly Most Precipitation is formed through accretion Rain usually starts as ice: it is therefore rare in stratus clouds which are too warm. Rimed crystals (ice crystals with attached frozen droplets) and graupel: Rain: falling drop of liquid water Drizzle: less than 0.5 mm (not rain) Most common from stratus clouds Can also be drops that fall through dry air and evaporate while falling Virga: Streaks of falling precipitation that evaporate before reaching the ground (see next picture)

4 Showers occur when downdrafts suddenly allow suspended rain to fall Excessively heavy showers: Cloudbursts Nimbostratus: continuous rain Cumulonimbus: lots of updrafts and downdrafts So there can be rain on only one side of a street. Raindrops are seldom larger than 6 mm because they tend to collide and break up. What conditions produce huge raindrops? Topic: Tear Drops Raindrops not tear shaped Shape is size dependent Less than 2 mm = sphere Greater than 2 mm = flattened sphere 21 Table 7-2, p Snow: frozen water falling from sky (crystal or flake) Most precipitation starts as snow The freezing level is generally about 3600 m (12,000 ft) in the summer. You can sometimes see the melting level. Snow & Cloud Appearance Looking up into precipitation, the snow looks more white than rain, so the shade of the region beneath the cloud lets you predict whether snow or rain will fall. Fall Streaks can be seen below cirriform clouds Similar to virga, they sublimate in the dryer air

5 2/24/11 Flurries Snow Squall Ground blizzards are characterized by blowing snow and drifts on the ground True blizzards have low temperatures and strong winds bearing large amounts of snow. Light, intermittent snow from developing cumulus clouds Brief heavy snow showers from cumiliform clouds A blanket of snow is a good insulator 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 (glaze): ground surface is freezing. As rain hits the surface it freezes. What s black about black ice? Small drops that freeze onto cold surfaces produce rime

6 2/24/ Snow Grains (<1 mm): solid equivalent of drizzle, no bounce or shatter Snow Pellets (<5 mm): bounce, break, crunch underfoot, tapioca snow Graupel: ice particle with a heavy coating of rime Hail: graupel acts as embryo in intense thunderstorm, grow through aggregation as pushed up by updraft. Killed two in the US in 20th century Golf ball-sized hail takes 5 to 10 minutes to grow

7 2/24/ Fig. 7-30a, p Table 7-5, p Fig. 7-30b, p

8 Measuring Precipitation Instruments Rain gauge: standard, tipping bucket (can under-measure during heavy rain), weighing Rain less than 0.01 = a trace Snow: average depth at 3 locations, 10:1 water equivalent Can vary from 6:1 to 30:1 Doppler Radar Transmitter sends microwaves toward target, the returned energy is measured and displayed Brightness of echo = amount/intensity of rain Doppler: measures speed of horizontal rain Radar sometimes produces images of rain that doesn t reach the ground

9 49 50 Measuring Precipitation Measuring from space Specific satellites designed to assess clouds, atmospheric moisture, and rain TRMM Tropical Rainfall Measuring Mission Infrared, microwave, visible 400 km high, 91 minute orbital period, Cyclone Susan CloudSat 700 km high Homework for Chapter 7 Chapter 7 Questions for Review, p. 190 #2, 7, 9, 12, 14, 18, 19 Chapter 7 Questions for Thought, p. 191 #3, 5 Chapter 7 Problems and Exercises, p. 191 #1,

10 Project for Chapter 7 None 55 10