Transcript Lecture
Lecture 11 Cloud Microphysics Wallace and Hobbs – Ch. 6 Ignore most of the math – concentrate on descriptive conclusions and graphs Cloud Types Outline Cooling Supersaturation Droplet Formation Droplet Growth Precipitation Formation Nucleation Usually refers to the initial formation of a droplet More general definition: AMS Glossary Homogeneous nucleation Droplet spontaneously forms in pure air No particles present Heterogeneous nucleation Droplets form on particles called cloud condensation nuclei (CCN) Homogeneous Nucleation Formation of a curved water surface requires energy maintenance of a small droplet requires large supersaturations RH to Form Droplet of Radius r RH W & H, Fig. 6.2 112% Such large RHs do not occur in nature. 0.01 r (m) Heterogeneous Nucleation Hygroscopic CCN are particularly effective condensation initiators Generally made of soluble salts When droplet forms, solution has a much lower vapor pressure than pure water Condensation begins when RH < 100% Droplet growth requires supersaturations of less than 1% Such supersaturations are achieved in updrafts Köhler Curves Give the equilibrium droplet size for a given RH. “Saturation ratio” = RH/100 Köhler Curves 10-19 g 10-18g 10-17g Numbers indicate mass of dissolved salt (NaCl) Suppose RH = 100.1% 10-19 g 10-18g 10-17g Droplets grow until they reach equilibrium radius 10-19 g 10-18g 10-17g Droplets grow until they reach equilibrium radius 10-19 g 10-18g 10-17g Droplets grow until they reach equilibrium radius 10-19 g 10-18g 10-17g Droplets grow until they reach equilibrium radius 10-19 g 10-18g 10-17g Droplets grow until they reach equilibrium radius Typical cloud droplet radius 10-19 g 10-18g 10-17g Droplets grow until they reach equilibrium radius Droplet Growth If ambient RH < value at peak of curve, droplets stop growing when much smaller than typical cloud drop They are called haze droplets 10-19 g 10-18g 10-17g Suppose RH = 100.3% 10-19 g 10-18g 10-17g Droplets growing on smaller nuclei behave as before 10-19 g 10-18g 10-17g Look at largest nucleus 10-19 g 10-18g 10-17g 10-19 g 10-18g 10-17g 10-19 g 10-18g 10-17g 10-19 g 10-18g 10-17g 10-19 g 10-18g 10-17g 10-19 g 10-18g 10-17g Droplet keeps growing! Droplet “Activation” If ambient RH > peak value, droplet grows indefinitely Once droplet has gotten “over the hump”, it is said to be activated. Slowing of Growth Rate of droplet growth decreases as droplets grow Let r = droplet radius It can be shown that dr 1 dt r Depletion of Water Vapor Also, growth of large number of droplets reduces supersaturation Result: Droplet radius tends to level off at about 10m Fall velocity of such a droplet is < 1 cm-1 droplets tend to be carried upward Droplets must be much larger to actually fall Microphysical Parameters Liquid water content (LWC) grams of liquid water per m3 of cloud Droplet concentration, N Number of droplets per cm3 Mean droplet size, r Usually given in m Not independent – knowledge of any two determines the third Relationship Between Microphysical Parameters 4 3 LWC N L r , 3 where L is the density of liquid water. See W & H, p. 217 for typical values of microphysical parameters Supercooled Water Definition: Liquid water with T < 0C Freezing Homogeneous nucleation occurs at -40C! Heterogeneous nucleation occurs in presence of a freezing nucleus (Typically occurs at temps much higher than -40C) Freezing Point Common experience: Water freezes at 0C This works when mass of water >> cloud droplet Only one nucleation event is required to freeze entire mass Such an event is virtually certain for masses of water normally encountered Cloud droplets very small Probability of a nucleation event at 0C is small Probability increases as temperature falls Ice Crystals When T < 0C, ice crystals can form directly from vapor Homogeneous nucleation requires unrealistically large super-saturations Heterogeneous nucleation occurs on particles called deposition nuclei Ice Nuclei General name for various types of nuclei e.g., freezing nuclei, deposition nuclei Relatively rare 1 particle in 108 suitable! Nucleation Temps Substance Kaolinite Temp. (C) -9 Silver Iodide -4 Bacteria! -3 Source: Table 9.1 in A Short Course in Cloud Physics, 3rd Ed. Rogers, R. and M. Yau. Pergamon Press, 293 pp. Supercooled Water and Ice Let es,w(T) be the saturation vapor pressure over liquid water at temperature T Let es,i(T) be the saturation vapor pressure over ice at temperature T es,i(T) < es,w(T) for T < 0C es,i vs. es,w T (C) 0 -5 -10 -15 -20 -25 -30 -35 -40 es,i(hPa) 6.11 4.02 2.60 1.65 1.03 0.63 0.38 0.22 0.13 es,w(hPa) 6.11 4.21 2.87 1.91 1.25 0.81 0.51 0.31 0.19 (Source: Smithsonian Meteorological Tables, 6th Ed.)