Lecture 11
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Transcript of Lecture 11
Lecture 11Lecture 11
Cloud MicrophysicsCloud Microphysics
Wallace and Hobbs – Ch. 6
Ignore most of the math – concentrate on descriptive conclusions and graphs
NucleationNucleation
Usually refers to the initial formation of a Usually refers to the initial formation of a dropletdroplet More general definition: More general definition: AMS GlossaryAMS GlossaryHomogeneous nucleationHomogeneous nucleation Droplet spontaneously forms in pure airDroplet spontaneously forms in pure air No particles presentNo particles present
Heterogeneous nucleationHeterogeneous nucleation Droplets form on particles called cloud Droplets form on particles called cloud
condensation nuclei (CCN)condensation nuclei (CCN)
Homogeneous NucleationHomogeneous Nucleation
Formation of a curved water surface Formation of a curved water surface requires energyrequires energy maintenance of a small droplet requires large maintenance of a small droplet requires large
supersaturations supersaturations
RH to Form Droplet of Radius rRH to Form Droplet of Radius r
r (m)
RH
0.01
112%
W & H, Fig. 6.2
Such large RHs do not occur in nature.
Heterogeneous NucleationHeterogeneous NucleationHygroscopicHygroscopic CCN are particularly effective CCN are particularly effective condensation initiatorscondensation initiators Generally made of soluble saltsGenerally made of soluble salts
When droplet forms, solution has a much When droplet forms, solution has a much lower vapor pressure than pure waterlower vapor pressure than pure water Condensation begins when RH < 100%Condensation begins when RH < 100%
Droplet growth requires supersaturations of Droplet growth requires supersaturations of less than 1%less than 1% Such supersaturations are achieved in updraftsSuch supersaturations are achieved in updrafts
KKöhler Curvesöhler Curves
Give the equilibrium droplet size for a Give the equilibrium droplet size for a given RH.given RH.
Köhler Köhler CurvesCurves
Suppose RH = 100.1%
10-19 g
10-18g
10-17g
Numbers indicate mass of dissolved salt (NaCl)
“Saturation ratio” = RH/100
10-19 g
10-18g
10-17g
Droplets grow until they reach equilibrium radius
Typical cloud droplet radius
Droplet GrowthDroplet Growth
If ambient RH < value at peak of curve, If ambient RH < value at peak of curve, droplets stop growing when much smaller droplets stop growing when much smaller than typical cloud dropthan typical cloud drop
They are called They are called hazehaze droplets droplets
Droplet “Activation”Droplet “Activation”
If ambient RH > peak value, droplet grows If ambient RH > peak value, droplet grows indefinitelyindefinitely
Once droplet has gotten “over the hump”, Once droplet has gotten “over the hump”, it is said to be it is said to be activated.activated.
Slowing of GrowthSlowing of Growth
Rate of droplet growth decreases as Rate of droplet growth decreases as droplets growdroplets grow
Let r = droplet radiusLet r = droplet radius
It can be shown thatIt can be shown that
rdt
dr 1
Depletion of Water VaporDepletion of Water Vapor
Also, growth of large number of droplets Also, growth of large number of droplets reduces supersaturationreduces supersaturation
Result: Droplet radius tends to level off at Result: Droplet radius tends to level off at about 10about 10mm
Fall velocity of such a droplet is < 1 cmFall velocity of such a droplet is < 1 cm-1-1
droplets tend to be carried droplets tend to be carried upwardupward
Droplets must be much larger to actually Droplets must be much larger to actually fallfall
Microphysical ParametersMicrophysical Parameters
Liquid water content (LWC)Liquid water content (LWC) grams of liquid water per mgrams of liquid water per m33 of cloud of cloud
Droplet concentration, NDroplet concentration, N Number of droplets per cmNumber of droplets per cm33
Mean droplet size, Mean droplet size, Usually given in Usually given in mm
Not independent – knowledge of any two Not independent – knowledge of any two determines the thirddetermines the third
r
Relationship Between Relationship Between Microphysical ParametersMicrophysical Parameters
,3
4 3rNLWC L
where L is the density of liquid water.
See W & H, p. 217 for typical values of microphysical parametersSee W & H, p. 217 for typical values of microphysical parameters
Supercooled WaterSupercooled Water
Definition: Liquid water with T < 0Definition: Liquid water with T < 0CCFreezing Freezing Homogeneous nucleation occurs at -40Homogeneous nucleation occurs at -40C!C! Heterogeneous nucleation occurs in presence Heterogeneous nucleation occurs in presence
of a of a freezing nucleusfreezing nucleus(Typically occurs at temps much higher than -40(Typically occurs at temps much higher than -40C)C)
Freezing PointFreezing Point
Common experience: Water freezes at 0Common experience: Water freezes at 0CCThis works when mass of water >> cloud dropletThis works when mass of water >> cloud droplet Only one nucleation event is required to freeze entire Only one nucleation event is required to freeze entire
massmass Such an event is virtually certain for masses of water Such an event is virtually certain for masses of water
normally encounterednormally encountered
Cloud droplets very smallCloud droplets very small Probability of a nucleation event at 0Probability of a nucleation event at 0C is smallC is small Probability increases as temperature fallsProbability increases as temperature falls
Ice CrystalsIce Crystals
When T < 0When T < 0C, ice crystals can form C, ice crystals can form directly from vapordirectly from vapor
Homogeneous nucleation requires Homogeneous nucleation requires unrealistically large super-saturationsunrealistically large super-saturations
Heterogeneous nucleation occurs on Heterogeneous nucleation occurs on particles called particles called deposition nucleideposition nuclei
Ice NucleiIce Nuclei
General name for various types of nucleiGeneral name for various types of nuclei e.g., freezing nuclei, deposition nucleie.g., freezing nuclei, deposition nuclei
Relatively rareRelatively rare 1 particle in 101 particle in 1088 suitable! suitable!
Nucleation TempsNucleation Temps
-3-3Bacteria!Bacteria!
-4-4Silver IodideSilver Iodide
-9-9KaoliniteKaolinite
Temp. (Temp. (C)C)SubstanceSubstance
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 IceSupercooled Water and Ice
Let eLet es,ws,w(T) be the saturation vapor pressure (T) be the saturation vapor pressure
over liquid water at temperature Tover liquid water at temperature T
Let eLet es,is,i(T) be the saturation vapor pressure (T) be the saturation vapor pressure
over ice at temperature Tover ice at temperature T
ees,is,i(T) < e(T) < es,ws,w(T) for T < 0(T) for T < 0CC
T (T (C)C) ees,is,i(hPa)(hPa) ees,ws,w(hPa)(hPa)
00 6.116.11 6.116.11
-5-5 4.024.02 4.214.21
-10-10 2.602.60 2.872.87
-15-15 1.651.65 1.911.91
-20-20 1.031.03 1.251.25
-25-25 0.630.63 0.810.81
-30-30 0.380.38 0.510.51
-35-35 0.220.22 0.310.31
-40-40 0.130.13 0.190.19
ees,is,i vs. e vs. es,w s,w
(Source: (Source: Smithsonian Meteorological Tables, 6th EdSmithsonian Meteorological Tables, 6th Ed.).)