Chapters 5 and 6Cloud and Aerosol Physics

Goals:Understand …• Homogeneous and heterogeneous nucleation of cloud droplets• The central role of aerosol in cloud physics• Precipitation formed in warm and cold clouds• Saturation and super saturation in the atmosphere• Super cooling of cloud droplets• Formation of ice crystals in cirrus clouds and ice fog• Cloud electrification

Figures, etc, from Wallace and Hobbs unless otherwise stated.

Aerosol

Atmosphere as a Photochemical Reactor: OH The Hydroxyl Radical

Section 5.4: Tropospheric Aerosol

SOLARINFRARED

Aerosol Number Distributions for air source: Continental (red curve) Marine (blue curve) and urban polluted (black curve)

Aitken nuclei: Particle countedwith a very high super saturation,100% or so, also known ascondensation nuclei (CN).

Cloud condensation nuclei (CCN) are large CN that are counted with supersaturations of only around a percent or so, as found in the atmosphere.

Particle Surface

Area and Dynamics

(rough idea)

Continental (red curve) Marine (blue curve) and urban polluted (black curve)

What fraction are CCNat various supersaturations?

UNR CIMEL SUNPHOTOMETER AEROSOL DATA

AEROSOL SPECTRAL OPTICAL DEPTH

UNR CIMEL SUNPHOTOMETER AEROSOL DATA

‘Retrieved’ Aerosol Volume Distributions

Fine mode

Coarse mode

Chemistry of Stratospheric Sulfate Layer (strong volcanic eruptions): A form of ‘natural’ geoengineering, aerosol

scatter solar radiation to space

Aerosol removed by tropospheric folds and jet stream dynamics: locations where stratospheric air is forced to mix down to the troposphere.

Stratospheric Aerosol Optical Depth at wavelength 1 micron (40 km to 2 km above tropopause)

Volcanos

Tropospheric Folds

Chapter 6: Cloud And Aerosol Microphysics

CCN size is 25x too large

from http://apollo.lsc.vsc.edu/classes/met130/notes/chapter7/cold_clouds.html

Water droplet coalescenceto form raindrops.

Ice crystals/snowflakes at top;Mid level mixed clouds, ice consumes water dropletsMelt to raindrops near the surface

Summary In Two Images

Cloud Types

Co

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lou

d P

roce

sses

War

m C

lou

d

Pro

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es

Courtesy: Steve Platnick, NASA

Homogeneous Nucleation of Droplets;Kelvin’s Equation

Cloud Condensation Nuclei. Warm Clouds.Growth of Drops by Condensation

Atmospheric Aerosols

Heterogeneous Nucleation of Droplets;Köhler Curves

Growth of Drops by Collisions. Ice Nuclei and Ice Crystal in Clouds

Growth of Ice Particles in Clouds

Chapter 6: First topic: Homogeneous nucleation of cloud droplets (not how most cloud droplets are formed)

Droplets with R < r evaporate:with R > r grow since the energydiminshes. NOTE: e > es is neededfor growth.

Growth begets growth, shrink begets droplet shrink: Unstable equilibrium

Another View …

How Many Water Molecules in a Droplet?

Relative Humidity and Supersaturation at Equilibrium Above a Droplet (with respect to a flat surface of water)

Unrealistically large …super saturations this high are not found

Cloud droplets can’t get past the small size this way:Need another mechanism

Relative Humidity and Supersaturation at Equilibrium Above a Droplet (with respect to a flat surface of water)

Unrealistically large …super saturations this high are not found (broader range of size)

Cloud droplets can’t get past the small size this way:Need another mechanism

Relative probability molecules are in thevapor phase for a flat surface comparedwith a curved surface of radius r and areaS.

Relative Humidity and Supersaturation at Equilibrium Above a Droplet (with respect to a flat surface of water)

Inside water molecules ‘hold’ the surface molecules less strongly for droplets compared with a flat surface.

How to Overcome Problem of High Supersaturation Needed for Homogeneous Nucleation

Nucleation

For typical molecule, r ~ 0.6 nm, would need RH=740% (SS=640%)

In atmosphere maximum SS observed is ~1%

Condensation must be on particles with r ~ 100nm

91 2 1.2 10exp 1

w

svp r

svp r RT r

s.v.p. over curved surface > s.v.p. over flat surface

Radius Name Conc. (cm-3) SS

< 0.1 µm Aitken nuclei 10,000 1%

0.1 - 1 µm large nuclei 100 0.1%

> 1 µm giant nuclei 1 0.01%

Haze and RH Hysteresis

Haze and RH

Hysteresis 150 nm diameter dry aerosol:Start dry at 30% RH:Increase to 80% RH:Deliquescence:Growth for RH>80%:Decrease RH to 38%:Efflorescence:Dry for RH<38%

Raoult’s Law (Wikipedia)

Limitations of Raoult’s Law

Raoult’s Law Example: Fewer water molecules are above a solution droplet

Köhler Theory: Add dissolved ions to reduce the number of water vapor molecules escaping the solution droplet (impure water droplet)

From Lord Kelvin, pure water

From Köhler, salty water

From Raoult’s Law

Köhler Curves for Different Salts: NaCl and (NH4)2SO4: 1 ion unit = 1 million ions

Note the change of scale above 100%

Haze Droplet and Activated Cloud Droplet for Ambient RH=100.4%

Another Look at Köhler Curves

slides from http://www.met.sjsu.edu/~clements/met60_lecture/lecture11_cloud_microphysics.ppt

Heterogeneous NucleationHygroscopic 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.

Köhler Curves

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

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

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

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

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.

Cloud Condensation Nuclei (CCN):Measurement Method, Thermal Diffusion Cloud Chamber

Cloud Condensation Nuclei (CCN):Typical Measurements

Data from Hudson and Yun,JGR, 2002 (DRI)

Warm Clouds (everywhere > 0 C)

Measurements of Cloud Droplets

Schematicof pod mountedinstrument

Warm Cloud Microphysics Example

Warm Cloud Comparison

Cloud Aerosol Indirect Effect:

Clouds with the same liquid

water path, but more CCN and

more cloud droplets, have higher albedo

than clouds with lesser CCN and

fewer cloud droplets.

(Twomey hypothesis)

Cloud Effective Radius From Satellite Retrievals

Generally smaller cloud droplets over land than over ocean.

Ship Tracks (ships emit huge numbers of CCN): Bright lines from numerous

small cloud droplets.

Often present off the California coast

Cloud Liquid Water Content (LWC): Adiabatic LWC

Entrainment of Dry Air

Causes Local Evaporation and Reduced

Cloud LWC

Nonattainment of Adiabatic LWC

Growth of Cloud Droplets in Warm Clouds: Condensation and Collision-Coalescence

Growth of Cloud Droplets in Warm Clouds: Condensational Growth Near Cloud Base

Small and Large Droplet Fall Speed

Terminal velocity of

drops

6mg rv

22

9wgv r

343

6 6wr gmg

vr r

For laminar flow, at terminal velocitydrag force=weight

Faster Falling Droplets Collect Slower Moving Little Ones

• Giant CCN and/or turbulent mixing may produce large collector drops.• Precipitation falls out when collector drop terminal fall speed exceeds the updraft.• Large collector droplets can break into smaller ones during impacts.

Cold Clouds: Ice Crystal Microphysics

Key Idea: Vapor pressure for water is greater than for ice below 0 C.

From http://apollo.lsc.vsc.edu/~wintelsw/MET1010LOL/chapter07/habits.gif

Ice Nuclei are Often Much Less Prevalent than Ice Crystals

Ice Multiplication

Cloud Structure