Cloud Physics Summary SOURCE - PowerPoint Presentation

1. Cloud Physics SummarySOURCE:www.geos.ed.ac.uk/~dstevens/...DS_L3_Precipitation_development.ppt

2. Precipitation development;Warm and Cold clouds>0°C<0°C

3. Last lectures from me…Cloud droplet formation (micro-scales)Cloud/fog formation processes (macro-scales)This lecture – return to the micro-scales

4. Cloud droplets and Raindrop sizesHow do droplets grow and become raindrops?r = radius in mn = numberconcentrationper litrev = terminalfall speedin cm/s

5. Why doesn’t it always rain when there are clouds?A: Updrafts can keep small cloud droplets suspendedRadius(m)Terminal Velocity(cm s-1)Type of Particle0.10.0001Condensation (Aitken) nuclei101Typical cloud droplet10070Large cloud droplet1000 = 1 mm650Typical raindrop2500 = 2½ mm900Large raindropQNeed stronger updraughts to support larger drops…

6. What do rain drops look like?Qequivalent diameter (mm) of rain dropDrops break up for larger sizes;Max. size ~8-10 mm

7. How do cloud droplets (radius = 10 m) turn into rain drops (1 mm) ?There are 2 main processes:In ‘warm’ clouds with cloud top T > -15 °CIn ‘cold’ clouds with cloud top T <-15 °CInitial growth by condensation, but this is limited by diffusion…They never get a chance to grow into raindrops by condensation alone – this process would take D A Y S . . .Q

8. Raindrop formation by collision and coalescencein warm cloudsIt takes about 106 small cloud droplets (10 mm) to form onelarge raindrop (1000 mm)

9. Stochastic model of collisions and droplet growth‘Statistical’Start with 100 dropsIn 1 timestep, 10% growNext step, repeat…End up with a logarithmic size distribution…Actually, more complicated…

10. Cascade processRaindrops reaching Earth’s surface rarely exceed 5 mm (5000 mm). Collisions or glancing blows between large raindrops break them into smaller drops.Also surface tension is too weak to hold the larger drops together

11. Distribution of raindrop sizes – raindrop spectrathe Marshall-Palmer distribution123456Drop diameter, D (mm)100030002000400050006000123456Drop diameter, D (mm)110010100010000differentrain ratesn(D) = noe-ΛDNo. of drops in each class size per m3 no = 8 x 103 ; Λ= 4.1 Rh-0.21 where Rh is the rainfall rate (mm h-1)

12. Depth of cloud influences type of rainStratus – thin cloud (<500 m) and has a slow upwardmovement (< 0.1 ms-1).Growth by coalescence wouldn’t produce a dropletmore than about 200 mm.If RH below the cloud is high, then the droplets will arrive at the ground as drizzle, defined as diameter of drop < 500 mm (0.5mm).Thicker clouds, formed by convective motion, can have stronger updrafts and can keep larger cloud droplets aloft, permitting them to join (coalesce) with more droplets and grow to greater sizes.

13. 1 Low – Nimbostratus (Ns)

14. 3 Cumulonimbus (Cb)

15. Supplementary feature: virga

16. Cold clouds (temperate latitudes and polewards).Does water always freeze at 0 °C ?It depends … on its volume and the presence of ice nuclei.Ice in your freezer in an ice tray – it’ll freeze at 0 °C.but a 1000 mm (1mm) drop will not freeze until T ≈ -11 °C.For ice to form all the water molecules must align in the proper crystal structure – in a large volume there is a high chance a few of the molecules will line up in the proper manner whereas in a small volume of water the chances are reduced, simply because there are fewer moleculesQA

17. Ice nucleiIce or freezing nuclei aid the freezing processc.f aitken nuclei (<0.2 mm) for condensation nuclei.1 cm3 of pure water in a test tube wouldn’t freeze until T was about -3 to – 5 °C.all-421 in 102-351 in 104-301 in 105-201 in 106-10none0ProportionfrozenT (°C)Proportion ofcloud droplets frozenat different temperatures

18. Ice nuclei- are less common than Aitken nuclei most effective ones have the same crystal shape as ice crystals hence ice can form around and on them easily.- kaolonite (clay) minerals are effective ice nuclei- are most effective at about -10 °C because of the relative sparseness of ice nuclei, ice crystals and supercooled water can coexist at the same time. this last point is crucial in the formation of precipitation in cold clouds as it gives rise to the Bergeron process.

19. vapour pressuretemperature0 °CiceSuper-cooledwaterBergeron process arisessince svpice<svpwaterso ice grows at the expense of supercooled water dropletsIf you look at the area in-between the two SVP curves you’ll see that an air parcel here would be unsaturated with respect to water but supersaturated with respect to ice. That means net evaporation will take place from the water but net condensation to the ice.

20. One of the reasonsyou have to defrostyour freezer regularly…

21. Bergeronprocess

22. Lab ice crystal growing from super-cooled water drops

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24. Why are snowflakes hexagonal? …it’s complicated!Angle ~104°++-Sheets of molecules – viewed from abovehttp://www.uwgb.edu/dutchs/PETROLGY/Ice%20Structure.HTM

25. Shape of H2O molecule and H-bonding gives rise to hexagonal crystalsMelting and re-freezing gives rise to vast variety of snow flakes

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27. Clouds can be a mixture of water droplets and ice

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33. SummaryCloud particle size limited to a few mm by fall velocityDroplets (μm) grow to raindrops (mm) by two main routes:Warm clouds: condensation, collision, coalescence (then break-up)Cold clouds: super-cooled water freezes on ice nuclei – producing larger ice particles – often melt en route to surfacePrecipitation can evaporate en route

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