Chapters 5 and 6 Cloud 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 SOLAR INFRARED

Aerosol Number Distributions for air source: Continental (red curve) Marine (blue curve) and urban polluted (black curve) Aitken nuclei: Particle counted with a very high super saturation, 100% or so, also known as condensation 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 CCN at various supersaturations?

UNR CIMEL SUNPHOTOMETER AEROSOL DATA AEROSOL SPECTRAL OPTICAL DEPTH

UNR CIMEL SUNPHOTOMETER AEROSOL DATA Coarse mode ‘Retrieved’ Aerosol Volume Distributions Fine 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

Summary In Two Images Water droplet coalescence to form raindrops. Ice crystals/snowflakes at top; Mid level mixed clouds, ice consumes water droplets Melt to raindrops near the surface from http://apollo.lsc.vsc.edu/classes/met130/notes/chapter7/cold_clouds.html

Cloud Types

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 Cold Cloud Processes Courtesy ? Warm Cloud Processes Courtesy: Steve Platnick, NASA

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 energy diminshes. NOTE: e > es is needed for 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 the vapor phase for a flat surface compared with a curved surface of radius r and area S.

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

From Köhler, salty water 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 Raoult’s Law From Köhler, salty water

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 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.

Köhler Curves Suppose RH = 100.1% “Saturation ratio” = RH/100 10-19 g 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%

Droplets growing on smaller nuclei behave as before

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.

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 Schematic of pod mounted instrument

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 For laminar flow, at terminal velocity drag 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