1. Why does it rain on us???

2. 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

3. 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

4. Global distribution of precipitation

5. 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

6. 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

7. 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

8. 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

9. Video: Ice storm

10. Formation of snow and hails

11. 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

12. 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

13. 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.

14. 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

15. 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

16. 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

17. 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

18. 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)

19. 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

20. Works cited