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Two-moment Stratiform Cloud Microphysics & Cloud-aerosol Interactions in CAM

Two-moment Stratiform Cloud Microphysics & Cloud-aerosol Interactions in CAM. H. Morrison, A. Gettelman (NCAR) , S. Ghan (PNNL) Thanks to: P. Field, S. Massie (NCAR), R. Wood (UW-Seattle). Motivation. Aerosol-cloud interactions Better treatment of ice

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Two-moment Stratiform Cloud Microphysics & Cloud-aerosol Interactions in CAM

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  1. Two-moment Stratiform Cloud Microphysics & Cloud-aerosol Interactions in CAM H. Morrison, A. Gettelman (NCAR) , S. Ghan (PNNL) Thanks to: P. Field, S. Massie (NCAR), R. Wood (UW-Seattle) AMWG January 2007

  2. Motivation • Aerosol-cloud interactions • Better treatment of ice • Multi-scale modeling (unify treatments) Method: • Replace bulk microphysics • Current bulk scheme: Rasch & Kristjansson, (1998). • New Scheme based on Morrison et al (2005) AMWG January 2007

  3. History/Progress • Microphysics Task Group (Nov 2005) • Developments reported at AMWG, CCSM • Progress: • Working in versions of CAM (uncoupled) • Coupled to prescribed aerosols • Current status: • Reasonable uncoupled climate simulation • Slight bias improvements over control • Much better moist physics AMWG January 2007

  4. Key features of the new scheme • Two-moment – predicts number concentrations and mixing ratios of cloud water and ice. • Liquid/ice fraction determined by microphysical processes (Bergeron, heterogeneous freezing) instead of simple function of temperature. • Coupled with aerosol by treating droplet nucleation (Abdul-Razzak and Ghan 1998) and ice nucleation (Cooper 1986). • Diagnostic treatment of rain and snow mixing ratio and number concentration. • Self-consistent treatment of sub-grid cloud water distribution for all relevant microphysics processes – straightforward to couple with diagnostic cloud scheme. • Flexibility to allow independent column approach. AMWG January 2007

  5. Aerosol (CCN Number) q = mixing ratio N = number concentration q, N Convective Detrainment Aerosol (IN Number) q, N Cloud Droplets (Prognostic) q, N Cloud Ice (Prognostic) Conversion Processes Evap/Cond Dep/Sub q Water Vapor (Prognostic) Evaporation Sublimation Riming q, N Rain (Diagnostic) q, N Snow (Diagnostic) Sedimentation Sedimentation AMWG January 2007

  6. Mixed Phase Processes Observed Simulated (QJRMS, 2004) AMWG January 2007

  7. Particle Size and Number (JJA) • Zonal mean Lat-height AMWG January 2007

  8. Column Drop Number AVHRR Column Drop Number New Number plot Lohmann et al, 1999 from Han et al 1998 AMWG January 2007

  9. Droplet Effective Radius • New effective radius plot MODIS CAM ‘Average’ CAM ‘Cloud Top’ AMWG January 2007

  10. Droplet Number • New number plot MODIS CAM ‘Average’ CAM ‘Cloud Top’ AMWG January 2007

  11. Performance Relative to Control • TOA balance within 1W/m2 • SW & LW cloud forcing ‘reasonable’ • Little change in precip patterns • 50% reduction in LWP v. Control • Diagnostics available (URL at end) • Taylor metrics: 10% reduction in bias AMWG January 2007

  12. Reduced LWP • Reduction in LWP • Improvement v. control • How? • Smaller Particles • Larger Cloud Fraction • Closer to CERES rad balance (3Wm-2 less) Control New Microphysics AMWG January 2007

  13. Bias Reduction: Taylor Diagram Plot By Rich Neale AMWG January 2007

  14. Tests of Aerosol effects • Test with MODIS observations: • Massie et al (2006): Indian ocean case • Also: Test with sulfur scaled to 30% present • Approx value for pre-industrial Note: MODIFY Abdul Razzak and Ghan with scaling from mass -> nucleated number of Lohmann 99 (Linear to square root like (eponent 0.58) scaling) AMWG January 2007

  15. Scale Sulfate by Latitude • Ratio of Preindustrial/Present Sulfate AMWG January 2007

  16. ‘Indirect’ Effects Liquid Radius PDF 900mb PUT PDFS here • See differences in: • Radiative Forcing • Size and number • Liquid water path • Changes to Radiative forcing: • 1.7 Wm-2 (Direct + Indirect) • Working on discriminating Size Decreases Drop Number PDF 900mb Number Increases Pre-industrial Present AMWG January 2007

  17. Change in Column Number Present Pre-industrial AMWG January 2007

  18. Change in Liquid water Present Day Liquid Water Present- Preindustrial Largest changes in Storm Tracks (where LWP large) Little change in stratocumulus regions Differences larger with Larger difference in sulfate Mean = 148 g/m2 Mean Diff = -12 g/m2 AMWG January 2007

  19. Summary/Conclusions • New scheme performs well • Reasonable drop size distribution • Reasonable number distribution • Aerosols affect clouds • Sizes, Number, Liquid water path & Radiation • ‘Indirect’ effects are still uncertain • Strongly dependent on input aerosols AMWG January 2007

  20. Next steps • Continue analysis • Write up scheme • Single column model/description • Global performance, indrect effects with prescribed aerosol • Coupling to interactive aerosol, scavenging • Move to current dev CAM code • With AMWG approval • Refine aerosol nucleation treatment • Focus on ice phase AMWG January 2007

  21. Please Help! • Diagnostics are available on the web: http://www.cgd.ucar.edu/cms/andrew/diag/#microphys • Runs v. Obs, Control, Aerosol test case • Detailed microphysical comparisons • Let us know the good, bad and ugly! • Also new diagnostics, data sets AMWG January 2007

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