1. 1/26/10MET 61 topic 01 1 MET 61 Topic 1 Precipitation Processes

2. 1/26/10MET 61 topic 01 2 1) How does precip form? A.From warm clouds? –No ice!!! B.From cold clouds? –Ice processes VIP!!!

3. 1/26/10MET 61 topic 01 3 2) Types of precip? –Rain –Drizzle –Snow –Sleet –Hail –Other…

4. 1/26/10MET 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 lower cloud)

5. 1/26/10MET 61 topic 01 5 Warm Clouds Three things we monitor in warm clouds: a)Liquid Water Content (LWC) b)Cloud droplet concentration c)Droplet size distribution Q: Why do these matter? Q: How do these influence precip formation?

6. 1/26/10MET 61 topic 01 6 Goal: grow droplets See Fig. 6.18

7. 1/26/10MET 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

8. 1/26/10MET 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.

9. 1/26/10MET 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

10. 1/26/10MET 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”

11. 1/26/10MET 61 topic 01 11 Growth also requires the “collision and coalescence” process Two processes: a)Collection do droplets collide or not? b)Coalescence having collided, do droplets merge?

12. 1/26/10MET 61 topic 01 12 Collection Imagine a field of droplets of varying radii w1w1 r2r2 r3r3 r1r1 w2w2 w3w3

13. 1/26/10MET 61 topic 01 13 Fall speeds…depend on r and M 100  m radius 25.6 cm/s 1 mm radius403 cm/s = 4.03 m/s 4 mm radius883 cm/s = 8.83 m/s

14. 1/26/10MET 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

15. 1/26/10MET 61 topic 01 15 Coalescence If droplets collide, do they merge? Need: impact force > surface tension

16. 1/26/10MET 61 topic 01 16 We can define a coalescence efficiency, E' And then a collection efficiency E c = E.E' Note … an electric field helps the coalescence process!

17. 1/26/10MET 61 topic 01 17 Now we can do the math again! Compute rate of increase of radius (or mass) of a droplet Result: w l = LWC (as measured) v’s are fall speeds

18. 1/26/10MET 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)

19. 1/26/10MET 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!