GEU 0047: Meteorology Lecture 7 Precipitation

1. GEU 0047: MeteorologyLecture 7Precipitation

2. Precipitation Types • Precipitation: The deposit of water (solid or liquid) falling from the clouds and reaching the ground. It includes the following types: • drizzle • rain • snow • sleet (雨夾雪) • hail (冰雹) • Glaze (also verglas; 雨淞)

3. Water Droplet Sizes Rates of condensation and evaporation control the increase and decrease in water droplet sizes.

4. Why “droplet” ? Why not “balls” ? • Saturation (equilibrium) vapor pressure varies with surface area because the evaporation does also. • Water molecules are less attached to a curved surface and therefore evaporate more readily. • More water vapor is needed to reach equilibrium over a curved surface. • This Curvature effect lessens when the diameter is larger than 10 μm.

5. Curvature Effect Greater curvature means a higher Relative Humidity (R.H.) is needed to achieve equilibrium over the larger surface area. Smaller droplets evaporate more easily. Only large droplets can survive at lower R.H.

6. Condensation Droplets • Typical scenario: adiabatic cooling of ascending air parcel. • It passes the condensation level (i.e. LCL) and water vapor begins to condense on condensation nuclei, especially hydroscopic. • Soon, all of the condensation nuclei have seeded tiny water droplets and condensation slows down. (Wet Haze) • Condensation alone can produce water droplets on the order of ~ 20 mm in radius. Further growth requires droplet-droplet interactions.

7. Solute Effect of Condensation Nuclei Hydroscopic condensation nuclei allow droplet formation and growth with less than 100% R.H. (as low as 75% R.H.) They increase the likelihood of encounters between water vapor and liquid droplets. A solution (water and nuclei) also keeps water vapor from evaporating so easily as it lowers the saturation vapor pressure (i.e., solute effect).

8. Concept of Air Friction (review) • Friction is dependent upon (1) droplet surface area, and (2) velocity. • Frictional force ~ r2v2 Ff = k 4 p r2 v2 As the velocity increases, so does the frictional force. When the acceleration due to the frictional force is equal to the gravity, no acceleration takes place. When velocity stops changing, this is terminal velocity.

9. Physics of Terminal Velocity FORCE = Weight - Air resistance ma = mg - Ff Ff = k4pr2v2 a = g - Ff/m Acceleration occurs as long as the acceleration due to gravity is greater than the acceleration due to the frictional force. Droplet weight (gravity) must overcome drag (air resistance). The droplet mass must grow.

10. Terminal Velocity • Gravity increases faster than drag with radius. WHY? • Weight = r 4/3 p r3 depends on volume • Drag = 4 p r2 kv2 depends on area Vterminal = r1/2weight = drag • The terminal velocity increases as drop radius increases. • Both gravity and drag increase with radius, one because of mass the other because of surface area. Weight is faster. • This is even ignoring updrafts. So size does matter!

11. Drop Size • For a drop to fall, weight (r3) must overcome drag (r2). • Therefore, it has to grow to a large enough size. • Cloud drops are 1 million times smaller in total volume than rain drops.

12. How Fast A typical raindrop (2000 mm, 6.5 m/s) falls 600x faster than a typical cloud droplet (20 mm, 0.01 m/s). Using an average cloud height of 6500 meters and D = v t, the time required for a drop to fall to the ground, train = 6500 m/6.5 m/s = 1000 s ~ 16 minutes tdroplet = 6500 m/0.01m/s = 160 hours Occur in CALM air, ignoring wind or updrafts.

13. N Size Collision/Coalescence With a random distribution of sizes, droplets interact because of their differences in terminal velocities.

14. Clouds Most clouds do not produce precipitation as condensation and evaporation continue to work at similar rates on small droplets within the clouds. Condensation nuclei and condensation working alone would take days to form large enough drops to fall to the ground.

15. Warm Clouds Warm clouds: Clouds that do not have ice crystal formation. Collision/Coalescence is the ONLY mechanism for droplet growth. Largest rain drops hit the ground first.

16. Stratus Stratus: cloud droplets can be small Very little vertical motions (updrafts), so small droplets with smallterminal velocities may reach the ground. There is less time for collision and coalescence in the thin stratus layer, so small drops are numerous. Drizzle and/or misty rain (雨霧) is common.

17. Cumulus Cumulus: cloud droplets can be very large. Very large vertical motions (updrafts), so only large enough heavy drops can make it to the ground. There is more time for collision and coalescence in the thick cumulus cloud with large vertical development, so large drops are numerous. Towering cumulus can produce heavy rain and even hail.

18. Ice Nuclei In order for non-homogeneous freezing to take place at higher temperatures, ice nuclei that are structurally similar to ice must be present in order to initiate freezing. Silver and lead iodide, cupric sulfide (硫酸銅), kaolinite (高嶺土) are examples. Natural sources include clay soils, plant material, bacteria and of course ice. Without these ice nuclei, saturated air will be supercooled (often to ~ -40oC) before freezing would actually take place.

19. Cloud Seeding a form of weather modification (often refer to increase rainfall.) Providing ice nuclei with the right geometry to facilitate condensation, growth and precipitation. (CO2 drop; i.e., dry ice)

20. Cool Clouds Colder clouds have ice nuclei which allow faster growth rates for cloud droplets.

21. Water Versus Ice es = eo exp{L/Rv (1/To-1/T)} Saturation Vapor Pressure

22. Saturation Vapor Pressure For equilibrium between condensation and evaporation, more water vapor is needed over water than over ice. (Curvature and Temperature are the difference)

23. Maximum SVP Difference Supercooled water vs. Ice occurs when T ~ - 12 oC

24. Ice Crystals Some water vapor sublimates onto ice. This removes water vapor from the air and then causes more evaporation of water in order to establish equilibrium. BUT adding more water vapor to the air, increases ice crystal growth.

25. Bergeron Process Evaporation is larger near droplet, condensation is larger around ice nuclei. Water diffuses from drop to ice. Removing water vapor increases evaporation around droplet. Ice crystals grow at the expense of water droplets.

26. Ice Crystal Growth • Twomechanisms facilitate the growth of ice crystals. • Accretion: the growth of ice crystals as supercooledwaterfreezes onto them. • Aggregation: the coalescense of ice crystals through adhesion (falling ice crystals collide and stick to other ice crystals).

27. Accretion Most precipitation begins with falling ice crystals as they have much higher growth rates than water droplets acting under condensation alone. The physical conditions those ice crystals encounter as they fall ultimately determines the type of precipitation that reaches the ground.

28. Aggregation Growth of dendrite ice crystal otherwise known as snow through aggregation and adhesion. Dendrite (snow) is favored because its growth rate is the highestat the maximum difference in S.V.P. between water and ice near -12oC.

29. Snow Flakes • Dendrite Ice Crystals

30. Fracturing Collision and fracturing provide more ice nuclei. Ratios of ice nuclei to droplets 1:100,000 precipitation 1:1 nothing 100,000:1 nothing Even ice needs to get heavy to fall.

31. Ice Crystal Habits Ice crystal growth is dependent upon pressure and temperature. Growth rates are highly dependent upon the S.V.P. difference between water and ice.

32. Precipitations • LIQUID • Virga幡狀雲(wispy rain) • Mist • Drizzle • Rain • ICE (Graupel:雪丸) • Freezing Drizzle • Freezing (Icy) Rain (Glaze) • Sleet (雨夾雪) • Snow • Hail virga sea mist

33. Rain or Snow? Melting Level is a function of season, higher up in summer.

34. Rain Profile • A deep warming layer always results in rain.

35. Rain Rain Intensities (inch/hour) Light 0.01- 0.10 Moderate 0.11- 0.30 Heavy > 0.30

36. Rain Gauges • All official rain gauge instruments have 8-inch diameter openings. SimpleStandardAdvanced

37. Precipitation Frequency

38. Virga • Precipitation that evaporates before reaching the ground

39. Snow Profile • Temperatures well below freezing allow snow to reach the ground.

40. Snowfall Intensity • Snowfall intensity is not measured by accumulation as in the case of rainfall, usually. • Intensity is measured by visibility (or lack thereof). Heavy < 0.25 mile (~ 6 inch/day) Moderate 0.25 - 0.50 mile Light > 0.50 mile

41. Snowfall Frequency

42. Sleet Profile • A very deep freezing layer causes supercooled water droplets to freeze into tiny ice pellets before reaching the ground.

43. Sleet Formation • The formation of sleet along a warm front.

44. Glaze Profile • A shallow freezing layer allows supercooled water droplets to freeze upon contact creating an ice coating.

45. Glaze Very beautiful, very heavy, very dangerous, very damaging.

46. Glaze

47. Glaze

48. Glaze

49. Graupel • The tiny ice pellets that collect from ice crystals, can form very large raindrops (upon melting), very clumpy snow, and is the source for hail.

50. Hail Storm Frequency