1. Ch 7: Precipitation Processes: Why does it rain on us???

2. 4 different lifting mechanisms 3 types of stability Two factors limiting the height of clouds 3 types of inversions 3 cloud properties. Types of clouds Review of last lecture

3. Environment Parcel Environment Parcel Environment Parcel The three types of stability Absolutely Unstable Conditionally Unstable Absolutely Stable

4. Create classification scheme based on height and form: high clouds – cirrus, cirrostratus, cirrocumulus middle clouds – altostratus, altocumulus low clouds – stratus, stratocumulus, nimbostratus extensive vertical development – cumulus, cumulonimbus Cloud classification in the textbook

5. NASA’s International Satellite Cloud Climatology Project (ISCCP) Cloud Classification - commonly used in climate research

6. Satellite observation of clouds - Projects NASA’s International Satellite Cloud Climatology Project (ISCCP) Combine the measurements of 5 geostationary and 1-2 polar orbiting satellites. 1983-Now, cloud top height and optical depth. NASA’s Earth Observation System including a set of polar orbiting satellites (A-Train), especially CloudSat (with a cloud radar) and CALIPSO (with a cloud lidar). Ongoing, cloud particle information, detailed vertical structure.

7. The 3rd in-class assignment

8. 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

9. Global distribution of precipitation

10. 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 V T, which is proportional to the square root of cloud drop radius V T =c r 0.5,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 F gravity F drag

11. 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 CloudsCool Clouds

12. Process begins with large collector drops which have high terminal velocities Collector drops collide with smaller drops If drops too small: compressed air beneath falling drop forces small drops aside If drop is too big (same size as collector) it will fall at same speed and no collision will occur So, collection efficiency is greatest for drop sizes that are intermediate with respect to the size of the collector When collisions occur, drops either bounce apart or coalesce into one larger drop. Coalescence efficiency is generally very high, indicating that most collisions result in the two drops joining. 1. Collision Coalescence: Growth in Warm Clouds

13. spherical initially – as frictional drag increases  parachute shape eventually flatten out – bar shape split when frictional drag > surface tension of water maximum drop size of about 5 mm Raindrop Shape

14. 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 Saturation vapor pressure of ice < super- cooled water. When ice and water are present, water will be deposited directly onto ice. Ice crystals grow rapidly at the expense of super- cooled drops http://www.uwsp.edu 2. Bergeron Process: Growth in Cool/Cold Clouds

15. 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 = liquid water freezing onto ice crystals Aggregation = the joining of ice crystals through the bonding of surface water builds ice crystals 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

16. Variable shapes/sizes  depends on moisture content and temperature Shape of snowflakes 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.

18. 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 Concentric layers of ice in hail indicate the cyclical hailstone formation process

19. Sleet begins as ice crystals which melt into rain through a mid-level inversion before solidifying in colder near surface air Freezing Rain forms similarly to sleet, however, the drop does not completely solidify before striking the surface

20. Induce precipitation  injection of dry ice or silver iodide into clouds convert super-cooled droplets to ice initiate Bergeron process Dry ice (frozen CO2) – lowers temperature to -40C, no ice nuclei are required for water droplets to freeze Silver iodide – acts as ice nuclei at warmer temp (-5C) Effectiveness is debated Cloud Seeding

21. Summary of Precipitation processes: Condensation Collision- coalescence Bergeron Process Warm clouds Cool/cold clouds RainSnow (can change to rain, sleet, or any other type of precipitation depending on underlying atmosphere Riming/ Aggregation

22. Forces acting on a cloud/rain droplet. Terminal velocity. How does it change with cloud drop radius? Growth mechanisms for rain and snow Concepts

23. Next week we will discuss about different cyclones and storms (Part Four in the textbook) Monday: Tropical storms and hurricanes (Ch 12)