1. Lecture 15-16: Precipitation Processes (Ch 7) scores just in, average score 10.9 (top score 19, eleven scores 15 or over) < 50% does not mean “fail” – quiz was too long scores will be posted after class cloud droplet size spectrum collision-coalescence process cold clouds over Edmonton today Bergeron process forms and distribution of precip NSF(Nat.Sci.Fnd.) Cloud Research NCAR Hercules, Electra Covered 13 Oct

2. Liquid water at the base of a cloud initially forms onto condensation nuclei (“CCN”), of which there are a very large number. Within a few tens of meters of the LCL all the available condensation nuclei have attracted moisture… they quickly attain diameters of about a micrometer (p178; repeated p191) These are in competition for water vapour… “cloud water is spread across numerous small droplets rather than being concentrated in fewer large drops” (p190) Water vapour + lifting parcel → CLOUD CLOUD → PRECIPITATION? How do some cloud droplets grow large enough to fall as precip? How do some cloud droplets grow large enough to fall as precip?

3. Fig. 7-2 Cloud droplet “size spectrum” “Condensation can lead to rapid droplet growth, but only up to a radius of order 20 micrometer”

4. An aside on cloud physics research: measuring cloud droplet size spectrum Shadow images of cloud particles in a size range between 25 and 800 µm are focused on a diode array with 32 elements which is read out with a maximum repetition frequency of up to 5 MHz… calibration accomplished by a rotating glass disk with opaque spots on it simulating particles. A similar device handles precipitation particles (diam 0.2 - 6.4 mm)

5. An “autonomous Unmanned Aerial Vehicle” (UAV) Technische Universitat Braunschweig An aside on meteorol. research

6. Growth in Warm Clouds: collision-coalescence process Fig. 7-4 larger drops with greater terminal velocity assimilate smaller collision efficiency low for droplets of size nearly equalling that of the “collector” droplet, and for droplets very much smaller than collector

7. 12Z Mon 16 Oct, 2006 Stony Plain Sounding low cloud sub-zero

8. slack flow over Ab T-T d < 2 o C

9. Cumulonimbus clouds = ice (top, fuzzy cloud margins), liquid (bottom, sharp margins) and mix of ice and liquid (middle) Fig. 7-6

10. Growth in Cool and Cold Clouds: Bergeron process - - depends on co-existence of mixture of ice and liquid water Fig. 7-5 Equilib. vapour pressure over ice is less than over supercooled water at same temp: Equilib. vapour pressure over ice is less than over supercooled water at same temp: (H 2 O molecules bound in a crystal lattice require more energy to “escape”) Fig. 7-7a T[ o C]e s (T), mb (Ice) e s (T), mb (Water) 562568 -2517528

11. Fig. 7-7 (See tutorial on CD)

12. Riming (accretion) supercooled droplets Ice crystal falls through which freeze on (“graupel” is heavily rimed ice crystal - nucleus of hailstone Ice crystal hits another Aggregation and Aggregation is easier if there is a film of water on the ice: bigger flakes from warmer clouds “What happens to these crystals as they fall determines the type of precipitation that occurs”

13. Fig. 7-10a Distribution & Forms of Precip Snow: ice crystals in clouds can have wide variety of shapes (dendrites, plates, columns) infinite variety of snowflack forms (even multiple forms within one flake) because each regime of T,T d will favour a different structure Lake- effect

14. Rain: first drops tend to be large – because due to their larger size they fall fastest. Their partial evaporation humidifies the air column so that the smaller drops, initially greatly diminished by evaporation, survive to the ground due to droplet breakup, raindrop diameter seldom exceeds 5 mm

15. Graupel & Hail: graupel – heavily rimed ice crystals, diameter up to about 5 mm, may fall to ground or if remain suspended, form nuclei of hailstones Successive passages above/below freezing level result in layered hailstone; not much air due to the melting stages

16. Cold, dry air (cP) Heat & Vapour Slope-lift and frictional- convergence LAKE-EFFECT SNOWS ice-free lake maximal T water - T air long overwater fetch