Cloud Microphysics: Understanding Precipitation Processes

1. Cloud Microphysics Original Materials by Liz Page NWS/COMET (minor modifications/additions by SMR)

2. Introduction • Meteorology and hydrology are linked by the processes that produce precipitation • A greater understanding of cloud microphysics will help determine which clouds will be most efficient in producing precipitation

4. Vapor Pressure • Dalton’s Law of Partial Pressure • Saturation vapor pressure (es) • Saturation is a dynamic process

5. Dalton’s Law of Partial Pressure • Total pressure = partial pressure of dry air + partial pressure of water vapor • e  vapor pressure (actual) • es saturation vapor pressure [f (T) only] • S  saturation ratio = e/es • RH relative humidity = S*100%

7. Condensation and Cloud Formation • Cloud Condensation Nuclei • dust • salt particles from sea spray • natural aerosols • human created pollution • Hygroscopic nuclei • ‘attract’ water • allow saturation at RH < 100% • different nuclei have varying ‘degrees’ of condensation efficiency

8. Process of Cloud Formation • Air rises and cools to saturation • Most effective (hygroscopic) nuclei are activated • Saturation vapor pressure decreases as parcel continues to rise and cool • The parcel becomes supersaturated • More CCN activate at the higher humidity • Recall that not all CCN are created equally!

10. Cloud Droplet Growth by Condensation (Diffusion) • Driven by the difference in saturation vapor pressures • between droplet and environment • between droplets • Vapor is transported from higher to lower saturation vapor pressure • Recall that es is a function of temperature only

15. Collision and Coalescence • Two-step process • Will the droplets collide? • If so, will they coalesce?

16. Collision and Coalescence • Collisions begin at radius of 18 microns • Collision efficiency increases as the size of the colliding drop increases • why? • larger drops mean more collisions • faster terminal velocities

18. Collision and Coalescence • Not all collisions result in coalescence • Coalescence is affected by: • turbulence • surface contaminants • electric fields and charges • Broad droplet spectra (varying sizes) favor more collisions

19. Marine vs. Continental Environments 1 • Droplet concentrations • marine ~ 100 cm-3 • continental ~ 300 cm-3 • Does this make sense? • It should (more ‘crud’ [CCN] over land), however: • Where would clouds/precip more likely form? • Marine! • Why?

20. Marine vs. Continental Environments 2 • Droplet concentrations are not the whole story • Size DOES matter! • CCN: • more numerous over land than over water, but… • larger size range over water (many tiny CCN over land) • more CCN competing for available moisture  unable to grow via condensation (haze instead) • marine environment (w/fewer CCN of larger size range) better able to create precipitation

21. Marine vs. Continental Environments 3 • Not only larger CCN in marine environment, but larger droplets as well • Larger (size range of) droplets means greater collision efficiency (see previous slide) • Smaller (continental) droplets more prone to evaporation  cumulus clouds with ‘sharper’ edges • Oceanic cumulus cloud can produce precipitation more efficiently than a continental cumulus cloud: • shallower cloud • weaker updrafts • almost counterintuitive, no?

26. Droplet Breakup and Multiplication • Falling drops sweep out a cone-shaped volume • Drops are unstable just after coalescence • Droplet breakup broadens the spectra and limits the maximum size • most raindrops are 5 mm in diameter • larger droplets prone to breakup (unstable)

28. Precipitation Formation through Ice Processes 1 • Bergeron process • Dependent upon different saturation vapor pressures • es (ice) < es (water) • supercooled water and ice can (and do) coexist in same cloud • supersaturated wrt ice, but saturated wrt water • ice crystals will grow at expense of droplets

29. Precipitation Formation through Ice Processes 2 • Ice forms on Ice Nuclei (IN) • silicates • clays • combustion products • industrial products • Similar in principle to CCN • not as numerous as CCN • must be similar in nature as ice crystal

30. Nucleation of Ice • IN activate as a function of temperature (~ -10°C) • Heterogeneous (contact) nucleation • IN necessary • more common • Homogeneous (spontaneous) nucleation • no IN needed • occurs ~ -40°C • less common • Warm-top clouds (> -10oC) rarely have ice

32. Ice Crystal Growth • Ice crystals grow by: • vapor deposition • growth at expense of water vapor (direct deposit?) • dominant crystal growth mechanism • ‘cold’ process • accretion of cloud droplets • freezing of supercooled water onto surface of IN/crystal • growth at expense of liquid droplets • ‘cold’ process • graupel forms via accretion • aggregation • snowflakes stick upon collision (‘wet’ snow) • ‘warm’ process

35. Growth by Deposition

37. Ice Particle Multiplication • Three processes • Fracture (collisions of fragile crystals) • Splintering during riming • rapid freezing of supercooled water onto crystal • ejects splinters upon freezing • possibly most important/efficient process of the three • Fragmentation of large drops during freezing • ‘isolated’ drop freezes from outside in (forms shell) • water expands on freezing • shell cracks, forming splinters

38. Parting Thoughts 1 • Not all clouds are ‘cold’ or ‘warm’ • contain water in all three phases • relative ‘lack’ of IN allows coexistence of ice crystals and supercooled water in the same cloud • top of cloud dominated by cold-cloud (Bergeron) processes • bottom of cloud governed by warm-cloud (collision/coalescence) processes

39. Parting Thoughts 2 • Neither process solely responsible for precipitation development (BOTH contribute) • Bergeron process dominant in mid-latitudes and polar regions • most mid-latitude precipitation starts frozen • collision/coalescence dominant in tropics • C/C also important in increasing raindrop size