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MET 61

MET 61. Topic 1 Precipitation Processes. 1) How does precip form? From warm clouds? No ice!!! From cold clouds? Ice processes VIP!!!. 2) Types of precip? Rain Drizzle Snow Sleet Hail Other…. Refresher … Warm versus Cold Clouds A warm cloud has T > 0 C throughout

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MET 61

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  1. MET 61 Topic 1 Precipitation Processes MET 61 topic 01

  2. 1) How does precip form? • From warm clouds? • No ice!!! • From cold clouds? • Ice processes VIP!!! MET 61 topic 01

  3. 2) Types of precip? • Rain • Drizzle • Snow • Sleet • Hail • Other… MET 61 topic 01

  4. Refresher … Warm versus Cold Clouds A warm cloud has T > 0C throughout A cold cloud has T < 0C in part or all • e.g., cirrus (Ci) is a cold cloud • e.g., status (St) is a warm cloud • e.g., cumulonimbus (Cb) is a cold cloud (T < 0C in upper cloud, T > 0C in lower cloud) MET 61 topic 01

  5. Warm Clouds Three things we monitor in warm clouds: • Liquid Water Content (LWC) • Cloud droplet concentration • Droplet size distribution Q: Why do these matter? Q: How do these influence precip formation? MET 61 topic 01

  6. Goal: grow droplets See Fig. 6.18 MET 61 topic 01

  7. Can droplets grow to raindrop size by condensation alone? We can do some math! • Take a droplet of radius r and mass M • The droplet is surrounded by water vapor molecules with water vapor density v(r)→ v() • The vapor diffuses towards the droplet with diffusion coefficient (rate) D MET 61 topic 01

  8. Note…need a gradient of v to get diffusion! Then we can show: where e = SVP and l is liquid water density. Tells us how fast the droplet grows! Fig. 6.15 = blue line. MET 61 topic 01

  9. Fig. 6.15… • Growth is rapid at first  • Then growth slows  Fig. 6.17… • Calculated results after 5 minutes • Observations 244 m above cloud base MET 61 topic 01

  10. Looks good, but… • Droplets are still small (10 m) • Growth is slowing • All droplets tend to the same size (monodispersed – read p. 223 column 1) • Further calculations show that “condensation alone in warm clouds is much too slow to produce raindrops with radii of several millimeters” MET 61 topic 01

  11. Growth also requires the “collision and coalescence” process Two processes: • Collection • do droplets collide or not? • Coalescence • having collided, do droplets merge? MET 61 topic 01

  12. Collection Imagine a field of droplets of varying radii r1 r3 w3 r2 w2 w1 MET 61 topic 01

  13. Fall speeds…depend on r and M MET 61 topic 01

  14. Fig. 6.19 → not all droplets can be “captured” • Due to flow around larger drop following streamlines • Like flow around a boulder • We can define a collision efficiency, E • (Eq. 6.25) Fig. 6.20 → plots of E MET 61 topic 01

  15. Coalescence If droplets collide, do they merge? Need: impact force > surface tension MET 61 topic 01

  16. We can define a coalescence efficiency, E' • And then a collection efficiency • Ec = E.E' Note … an electric field helps the coalescence process! MET 61 topic 01

  17. Now we can do the math again! Compute rate of increase of radius (or mass) of a droplet Result: wl = LWC (as measured) v’s are fall speeds MET 61 topic 01

  18. Fig. 6.15 = red line. • More rapid growth than with condensation alone  • Growth rate continues to increase  Above needs to be modified for a real cloud which has non-zero uplift (w  0 … p.228) Result: • growth to raindrops takes about 1 hour  • Deeper cloud + strong updraft → rain sooner (per expectation)  MET 61 topic 01

  19. For more realism…p.229 (quiz/homework) • Include possibility of giant CCN • Include effects of turbulence • Include radiative effects • Include “stochastic” effects • Include effects of variable LWC within clouds Finally… • Computer modeling! MET 61 topic 01

  20. Cold Clouds Ice crystals exist and contribute to the formation of precipitation as follows… Let’s assume only deposition is working vapor → solid And compute rate of crystal growth… MET 61 topic 01

  21. Eq. 6.36… • Spherical ice particle (easy math!) • D = diffusion rate again • C = capacity (electrical analogy) • C/o = 4r Eqn.  max. growth rate at about -14C MET 61 topic 01

  22. Table 6.1 → ice crystal behavior p.244 … growth by deposition alone is too slow to produce large raindrops Example: • Hexagonal plate at - 5C • Grows to 7 g mass = 0.5 mm radius in ½ hour • Then grows more slowly MET 61 topic 01

  23. What can we add to speed things up? • Aggregation • Where crystals stick together • Riming • Where crystals collide with supercooled liquid water droplets → coating MET 61 topic 01

  24. Aggregation (p.242) Aggregation depends on crystal shape Tend to bounce off each other Tend to stick together MET 61 topic 01

  25. Riming (p.241) As supercooled liquid drops collide with an ice crystal, they freeze  coating → larger crystal / frozen particle Graupel http://en.wikipedia.org/wiki/Graupel Many different types – see photos in book MET 61 topic 01

  26. Hail !!! best-known example of effects of riming Multiple layers of ice form during repeated trips up / down within a Cb cloud Note: only a Cb cloud can → hail Animation & quiz  http://apollo.lsc.vsc.edu/classes/met130/notes/chapter7/52_Hail/A_52.html MET 61 topic 01

  27. Precipitation types – Rain Water drops over 0.5 mm in diameter Smaller = drizzle (e.g., locally from St clouds) Max observed raindrop size up to  1 cm Raindrops fall in “jellyfish” shape See p.231 MET 61 topic 01

  28. Rainfall stats: Station model (Ackerman & Knox) MET 61 topic 01

  29. Rainfall stuff: Global rainfall distribution p. 20 Fig. 1.25 MET 61 topic 01

  30. Rainfall stuff: Where does it rain most / least? • Tutunendo, Colombia, with an average rainfall of 463.4 inches per year. • Cherrapunji, India holds the record for highest rainfall in one year, which was 904.9 inches between August 1860 and July 1861. • On average, it rains about 450 inches a year in Cherrapunji, most of which falls during the monsoon season of their winter months. • Cherrapunji also holds the record for the highest rainfall within one month…366 inches in July 1861. • The place that has the most rainy days per year is Mount Wai-'ale'ale on the island of Kauai, Hawaii. It has up to 350 rainy days annually. MET 61 topic 01

  31. The longest rainless period in the world was14 years, from October 1903 to January 1918, at Arica, Chile. http://www.world66.com/southamerica/chile/arica/lib/climate(ha ha) http://www.huffingtonpost.com/2009/07/21/tiny-drizzle-wreaks-havoc_n_242057.html (ha ha ha) • In the United States the longest dry spell was 767 days at Bagdad, California – from October 3, 1912, to November 8, 1914. MET 61 topic 01

  32. Precipitable water… The total amount of water vapor in a column which could precipitate out. r = average mixing ratio in column (or layer) pbottomand ptopare pressures @ top & bottom of column (e.g., 200-1000 hPa) MET 61 topic 01

  33. Global map (Earth) http://weather.unisys.com/upper_air/ua_con_prec.html http://www.suominet.ucar.edu/ Coolest… http://www.osdpd.noaa.gov/bTPW/Overview.html Mars…measured in microns!! http://humbabe.arc.nasa.gov/MarsToday/MarsWater.html MET 61 topic 01

  34. Precipitation types - Snow When ice crystals falling do not melt below cloud → snow MET 61 topic 01

  35. Rain – snow equivalence … Roughly, 10 snow = 1 rain But… dry powder snow… 20 : 1 heavy wet snow… 5 : 1 Identical snowflakes … who? Find out! MET 61 topic 01

  36. Snowfall records … NOAA / NCDC data.. http://www.ncdc.noaa.gov/ussc/USSCAppController?action=extremes&state=110 MET 61 topic 01

  37. Sleet and freezing rain • Closely related (I think) • Suppose snow falls from a cloud MET 61 topic 01

  38. Sleet and freezing rain • Closely related (I think) • Suppose snow falls from a cloud MET 61 topic 01

  39. Sleet and freezing rain • Closely related (I think) • Suppose snow falls from a cloud MET 61 topic 01

  40. Sleet and freezing rain Sleet http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/prcp/slt.rxml Freezing rain http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/prcp/zr/frz.rxml Thus – the depth of the low-level sub-freezing layer is what matters {WW2010 site http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/prcp/home.rxml } MET 61 topic 01

  41. Graupel http://en.wikipedia.org/wiki/Graupel i.e., “none of the above”!!! Weatherwise article… MET 61 topic 01

  42. “On the afternoon of November 28, 2008, a dark, elongated, featureless cloud passed overhead. • Ice somewhat larger than pea-size fell with a temperature of 40°F. • Some snow crystals seemed to adhere to the ice. • Several National Weather Service spotters reported hail. • Was this really hail? I didn't hear thunder. Could it have been sleet?” MET 61 topic 01

  43. In pairs, decide what happened… Turn in a write up! MET 61 topic 01

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