Modeling

How Do we Address Aerosol-Cloud Interactions? The Scale Problem Process Models ~ 10s km Mesoscale Models Cloud resolving Models Regional Models 10s km – 1000s km Large Eddy Simulations; microphysical models; Aerosol  cloud interactions Aerosol transport and its effect on clouds Forcing on regional and global scale Predictive GCM Regional/Global scale 0 10 km PM2.5

The Scope of the Aerosol  Cloud Problem Involves complexity in both aerosol and clouds Range of spatial scales –Aerosol particles 10s – 1000s nanometres –Cloud drops/ice particles:  m – cm –Cloud scales: ~ 10 2 m – 10 3 km Range of temporal scales –Activation process: seconds –Time to generate precipitation ~ 30 min –Cloud systems: days

Myriad Coupled Processes

Tools for modeling aerosol-cloud interactions Eulerian (fixed) grid Lagrangian (moving) grid Drop size Bin size distribution Number per bin Fixed size grid Discrete point distribution Drop size Number Irregular size grid

Eulerian Models 3-D modeling at the Cloud Scale (Large Eddy Simulations) Solve Navier-Stokes equations Large Eddy Simulations –Grid size ~ 50 – 100m –Time step ~ 2 s –Domain ~ 10 km x 10 km Bin Microphysics –Moment conserving (two moments in each bin; Tel Aviv University) OR Bulk microphysics (mass only or mass + number) Radiation Land Surface Model Aqueous Chemistry etc…….

Lagrangian Models Parcel Model Predetermined trajectory –adiabatic or other Details of aerosol activation and growth Moving grid microphysics Aqueous Chemistry –Cloud processing –Inorganic (sulfate) –Organic

r – radius w - updraft T – temperature P – pressure  – mass accommodation S - Supersaturation Droplet Growth in an Updraft Kelvin term (sfc tension effects) Solute term (composition effects) Growth by condensation

r – radius w - updraft T – temperature P – pressure  – mass accommodation S - Supersaturation Droplet Growth in an Updraft Kelvin term (sfc tension effects) Solute term (composition effects) Growth by condensation Source of supersaturation (updraft) No condensation S Ht

r – radius w - updraft T – temperature P – pressure  – mass accommodation S - Supersaturation LWC – liquid water content Droplet Growth in an Updraft Kelvin term (sfc tension effects) Solute term (composition effects) Solve coupled equations including - thermodynamic equations - mass conservation Growth by condensation Source of supersaturation (updraft) Sink of supersaturation (condensation) Supersaturation equation: Updraft + condensation S Ht

10 m Cloud base Level of S max Unactivated particles Activated drops Solute term dominates Kelvin term dominates Saturation ratio, S Expansion of condensation term

Differences in N d much larger at low w Effect of surface tension is relatively since particles become dilute Molecular weight M s (if very high) has most influence on N d Competing effects of M s and surface tension Effect of composition on N d  = f(c) M s = 500 g mol -1 (NH 4 ) 2 SO 4 M s = 500 g mol -1,  = f(c) w = m s -1 w = 3 m s -1 Drop number concentration Time, s NdNd Sfc tension = f(carbon conc)

Sensitivity of N d to size, composition and updraft 70,000 runs of adiabatic parcel model N a > 1000 cm -3 N a < 1000 cm -3 Aerosol Size distr. parameters updraft Soluble mass fraction  w gg rgrg NaNa PollutedCleanAllXiXi S i = d ln N d / d ln X i Most sensitive to aerosol number, N a Least sensitive to composition,  Size is important ( r g,  g ) Updraft important in polluted conditions

CCN and N d closure seem to require  = ~ 0.05 (e.g., van Reken, Conant, Nenes)  = mass accommodation When might composition matter? External mixtures of aerosol, some hygroscopic, some hydrophobic Film forming compounds that affect mass accommodation

Composition is Important for “Direct Effect” Low RHHigh RH Atmospheric particles swell as they take up water As particles grow they scatter more sunlight Growth factor Relative humidity 30%85% 1 10 Below cloud remote sensing Controlled RH sampling 99%0% Aerosol type determines amount of growth Aerosol type (and growth) varies by location Oklahoma J. Ogren and colleagues

Modeling Exercise

Lagrangian Parcel Model Captures details of aerosol growth, activation, and condensation Moving grid microphysics Adiabatic (no mixing with environment) Most accurate growth calculations Discrete point distribution Aerosol/Drop size Number Irregular size grid Progressively larger water content Continuum: particle-haze-drop Rising air parcel updraft w

r i – radius of size class i r v – vapor mixing ratio LWC – liquid water content w - updraft T – temperature P – pressure Q 1,2 – f(T,P)  – mass accommodation Equations Kelvin term (sfc tension effects) Solute term (composition effects) Solve coupled equations 1. Growth by condensation Source (updraft)Sink (condensation) 2. Supersaturation Equation 3. Mass conservation 4. Other ;

10 m Cloud base Height of S max Unactivated particles Activated drops Solute term dominates Kelvin term dominates Saturation ratio, S Multiple size classes ~ separation between haze and drops

Drop size Number xx Drop conc, cm -3 Effective radius,  m Dispersion LWC, gm -3 HeightHeight Cloud Base: Temp Pressure Updraft: 1.0 ms-1 Aerosol: M 1.0x Lifting: m Model Output More Soluble (5 types of nuclei) Nuclei diameters supersaturation size classes “doublet” Identifies original size 10 m RH% or Height above cloud base: 100 m