Why does it rain on us???

3 types of stability

Two factors limiting the height of clouds

3 cloud properties, 9 ISCCP cloud types

Why do clouds constitute a wildcard for climate change? Competition between greenhouse effect and albedo effect

Review of last lecture

Satellite observation of precipitationSatellite observation of precipitation

• Infrared-derived or visible-derived (GPI)• Microwave-derived (MSU, SSM/I, TMI)• Radar: Tropical Rainfall Measurement Mission (TRMM) • Merged with surface gauge measurements and model

forecast

Global distribution of precipitationGlobal distribution of precipitation

Precipitation formation - cloud drop growthPrecipitation formation - cloud drop growth

• Not all clouds precipitate due to their small sizes and slow fall rates• Balance between gravity and frictional

drag eventually become equal to achieve terminal velocity VT, which is proportional to the square root of cloud drop radius VT=c r0.5 ,where r is drop radius and c is a constant.

• For a cloud drop to fall, its terminal velocity must exceed the vertical velocity of the upward-moving air parcel. Otherwise it will be carried up.

• Cloud drop growth is required for precipitation to form

Fgravity

Fdrag

1. Collision Coalescence (warm clouds, T > 0 C, form rain)

2. Bergeron Process (cool/cold clouds, T < 0 C, form snow)

Mechanisms for cloud drops to grow larger

Cold Clouds Cool Clouds

• Process begins with larger collector drops which have higher terminal velocities

• Collector drops collide with smaller drops and merge with them (coalesce). Coalescence efficiency is generally very high, indicating that most collisions result in the two drops joining.

• If collector drop is too big: compressed air beneath falling drop forces small drops aside

• If collector drop is too small (same size as other drops) it will fall at same speed and no collision will occur

• So, collection efficiency is greatest when the size of collector drop is slightly larger than the size of the other drops

• After the collector drops become large, the larger one among them can serve as a “super-collector” to collide with other collector drops

1. Collision Coalescence: Growth in Warm Clouds

• Determined by competition between surface tension and frictional drag. Frictional drag is larger at the bottom than at the top

• Small drop (<0.08in): frictional drag << surface tension Sphere shape

• Medium-size drop (0.08in<size<0.25in): frictional drag approaches surface tension Parachute shape

• Large drop (>0.25in): frictional drag at bottom > surface tension Split (The surface tension at the top allows the raindrop to remain more spherical while the bottom gets more flattened out.)

• Maximum drop size of about 0.25in or 5 mm

Raindrop shape and maximum size

Video: Ice stormVideo: Ice storm

Formation of snow and hails

• Clouds are usually composed of: liquid water, super-cooled water, and/or ice (supercooled water exists down to T= -40C !!)• Supercooled water can exist at

T<0C because ice formation requires ice nuclei, which, unlike condensation nuclei, are rare unless the temp. is very cold

• Coexistence of ice and super-cooled water is critical to the creation of cool/cold cloud precipitation - the Bergeron Process

http://www.uwsp.edu

2. Bergeron Process: Growth in Cool/Cold Clouds

• Key: Saturation vapor pressure of ice < that of super-cooled water at the same temperature.

• When air is in saturation wrt super-cooled water, it’s over-saturated wrt ice - deposition of water vapor over ice.

• When air is in saturation wrt ice, it’s sub-saturated wrt super-cooled water - evaporation of super-cooled water into water vapor.

• In this way, ice crystals grow rapidly at the expense of super-cooled drops http://www.uwsp.edu

Bergeron Process (cont.)

Shape of snowflakes depend on formation conditions (humidity and temperature)

Dendrite ice crystals

Plate ice crystal

Wilson Bentley, a Vermont farmer, took photographs of snowflakes under a microscope as a hobby. These photographs were published in the "Monthly Weather Review" in 1902.

• Bergeron Process usually not enough to produce large enough crystals for preciptation

• Further growth is due to collisions between falling crystals and drops riming and aggregation

• Riming (or Accretion) = liquid water freezing onto ice crystals

• Aggregation = the joining of ice crystals through the bonding of surface water builds ice crystals, producing snowflakes

• Collision combined with riming and aggregation allow formation of crystals large enough to precipitate within 1/2 hour of initial formation

Further growth: Riming and Aggregation

Change of snowflakes along the falling path Change of snowflakes along the falling path leads to different precipitationleads to different precipitation

• Snow: When T is always lower than 0 oC

• Rain: When T is higher than 0 oC at low levels

Change of snowflakes along the falling path Change of snowflakes along the falling path leads to different precipitation (cont.)leads to different precipitation (cont.)

• Sleet: begins as ice crystals which melt into rain as they fall through the atmosphere. Before reaching the surface they solidify into a frozen state.

• Freezing Rain forms similarly to sleet, however, the drop does not completely solidify before striking the surface

• Graupel – ice crystals that undergo extensive riming

– Lose six sided shape and smooth out

– Either falls to the ground or provides a nucleus for hail

• Hail – concentric layers of ice build around graupel

– graupel carried aloft in updrafts high altitudes freezing temperatures

– water accreting to graupel freezes, forming a layer

– Hail begins to fall, carried aloft again by updrafts, process repeats

– Hailstones are very heavy – high density

– Capable of tremendous amounts of damage

– Great Plains = highest frequency of hail events

Change of snowflakes along the falling path Change of snowflakes along the falling path leads to different precipitation (cont.)leads to different precipitation (cont.)

SummarySummary• Forces acting on a cloud/rain droplet. Terminal velocity.

How does it change with cloud drop radius?• Growth mechanisms for rain and snow• Formation of rain: coalescence process (the collector is

larger than the cloud droplets but not too large)• Bergeron process: happens with coexistence of ice and

super-cooled water. Key: Saturation vapor pressure of ice < that of super-cooled water at the same temperature.

• Further growth of ice crystals (riming and aggregation)• Change of falling ice crystals: depends on atmospheric

temperature and winds (snow, rain, sleet, freezing rain, graupel, hail)

Summary

Condensation

Collision-coalescence

Bergeron Process

Warm

clouds

Cool/cold clouds

Rain Snow(can change to rain, sleet, freezing rain, graupel, hail depending on underlying atmosphere

Riming/Aggregation

Works citedWorks cited

• http://www.edudemic.com/study-finds-most-people-think-cloud-computing-is-run-on-actual-clouds/

• http://hyperphysics.phy-astr.gsu.edu/hbase/electric/diph2o.html

• http://nyffetyff.deviantart.com/art/Raindrop-189805290 • http://www.its.caltech.edu/~atomic/snowcrystals/photos/

photos.htm • http://www.crh.noaa.gov/unr/?n=06-04-99_pg1 • http://www.clker.com/clipart-cartoon-sun.html • http://pmm.nasa.gov/node/145