Why does it rain on us???

Review of last lecture 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

Satellite 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 precipitation

Precipitation 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

Mechanisms for cloud drops to grow larger Collision Coalescence (warm clouds, T > 0 C, form rain) Bergeron Process (cool/cold clouds, T < 0 C, form snow) Cold Clouds Cool Clouds

1. Collision Coalescence: Growth in Warm 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

Raindrop shape and maximum size 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

Video: Ice storm

Formation of snow and hails

2. Bergeron Process: Growth in Cool/Cold Clouds 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

Bergeron Process (cont.) 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

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.

Further growth: Riming and Aggregation 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

Change of snowflakes along the falling path leads 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 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

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

Summary 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 Riming/ Warm clouds Cool/cold clouds Collision- coalescence Bergeron Process Riming/ Aggregation Rain Snow (can change to rain, sleet, freezing rain, graupel, hail depending on underlying atmosphere

Works 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