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Modeling

Modeling. Forcing on regional and global scale. Predictive GCM Regional/Global scale. Mesoscale Models Cloud resolving Models Regional Models 10s km – 1000s km. Aerosol transport and its effect on clouds. PM2.5. Large Eddy Simulations; microphysical models;

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Modeling

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  1. Modeling

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

  3. 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: mm – cm • Cloud scales: ~ 102 m– 103 km • Range of temporal scales • Activation process: seconds • Time to generate precipitation ~ 30 min • Cloud systems: days

  4. Myriad Coupled Processes

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

  6. 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…….

  7. 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

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

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

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

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

  12. Sfc tension =f(carbon conc) (NH4)2SO4 Ms = 500 g mol-1, s = f(c) s= f(c) Ms = 500 g mol-1 Drop number concentration w = 0.10 m s-1 w = 3 m s-1 Time, s Time, s Effect of composition on Nd Nd • Differences in Nd much larger at low w • Effect of surface tension is relatively since particles become dilute • Molecular weight Ms (if very high) has most influence on Nd • Competing effects of Ms and surface tension

  13. Na < 1000 cm-3 Na > 1000 cm-3 Xi All Clean Polluted Na 0.88 0.92 0.73 Aerosol Size distr. parameters rg 0.32 0.28 0.39 sg -0.39 -0.31 -0.53 updraft w 0.29 0.18 0.47 e 0.11 0.09 0.13 Soluble mass fraction 70,000 runs of adiabatic parcel model Sensitivity of Ndto size, composition and updraft • Most sensitive to aerosol number, Na • Least sensitive to composition, e • Size is important (rg, sg) • Updraft important in polluted conditions Si = d ln Nd / d ln Xi

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

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

  16. Modeling Exercise

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

  18. Equations 1. Growth by condensation Kelvin term (sfc tension effects) Solute term (composition effects) ri – radius of size class i rv – vapor mixing ratio LWC– liquid water content w - updraft T – temperature P – pressure Q1,2 – f(T,P) a – mass accommodation 2. Supersaturation Equation Source (updraft) Sink (condensation) 3. Mass conservation 4. Other ; Solve coupled equations

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

  20. Model Output H e i g h t xx xx xx xx Drop conc, cm-3 Effective radius, mm Dispersion LWC, gm-3 Number More Soluble (5 types of nuclei) Drop size RH% or Height above cloud base: 100 m Cloud Base: Temp Pressure Updraft: 1.0 ms-1 Aerosol: M 1.0x Lifting: 125.0 m “doublet” Identifies original size 10 m supersaturation 0.1 10. 1. Nuclei diameters 55 size classes 55 size classes

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