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Latest Microphysics Developments & Ice Microphysics and Indirect effects for CAM4

Explore the latest developments in ice microphysics and indirect effects for CAM4 model, including new ice nucleation methodology, aerosol indirect effects, and sensitivity experiments. Key findings indicate reasonable effects compared to observations, with future steps focusing on refining radiation interface and final aerosol code.

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Latest Microphysics Developments & Ice Microphysics and Indirect effects for CAM4

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  1. Latest Microphysics Developments & Ice Microphysics and Indirect effects for CAM4 A. Gettelman, X. Liu, H. Morrison, S. Ghan AMWG 2008

  2. Outline • Motivation • CAM Microphysics Status • New Ice Nucleation Description • Basic Results • Status and future plans

  3. MG Microphysics Status • Code Running in Coupled Model • Produces ‘reasonable’ climate • Microphysics look good • ENSO good in coupled system • Too much sea ice in coupled run • See R. Neale talk • Indirect effects diagnosed • New Optics being built for RRTM (Conley talk)

  4. MG Coupled Run: ENSO • ENSO

  5. MG Coupled Run: Cloud Forcing Cloud Forcing v. CERES Obs Longwave Shortwave

  6. MG Coupled Run: NH Sea Ice Sea Ice too thick/extensive • Cloud Forcing v. Obs • ENSO • Sea Ice concentration

  7. Aerosol Indirect Effects (AIE) 1750 Aerosols – 2000 Aerosols

  8. AIE Key Findings • Model produces reasonable effects compared to observations • Not a strong global constraint • Aerosol Indirect Effects are ~1-2 W/m2 with direct effects of 0.4-0.7W/m2 • Depends on Pre-industrial emissions • AIE numbers are only weakly dependent (10%) on whether aerosol mass is prescribed or prognostic • Oxidant levels also seem to matter for AIE • Changes AIE by 20-30%

  9. AIE Next Steps • Waiting on: • RRTM radiation interface: more flexible • New Radiation and New Cloud Optics • Final Aerosol Code (BAM/Modal, scavenging) • Near final Configuration • PBL, Macrophysics • Options (BAM): • Add aerosol species (Biogenic), • Change size distributions • Minimum Droplet Number • Modify Microphysics (Park) Gettelman - Ice 9

  10. Ice Processes: Motivation • Cirrus and ice nucleation uncertain • Supersaturation over ice common • CAM does not permit it • Cirrus radiative forcing important • Cirrus affects stratospheric H2O • Better description allows process level testing with observations

  11. Ice Microphysics in MG Micro • New microphysics has limited ice nucelation • Ni=f(T) following Cooper (1986) • Ni=const below -35C • Goal • Explore ice processes & sensitivity • Bergeron • Hallet Mossop ice multiplication • Parameterize ice nucleation • Allow supersaturation

  12. Ice Nucleation • Add ice nucleation treatment of Liu & Penner 2005, following Liu et al 2007 • Homogenous and Heterogenous immersion nucleation • Relax Zhang et al closure for ice to allow supersaturation (C-E w.r.t. liquid) • Based on parameterizing nucleation results from a detailed parcel model • Pro: detailed model • Con: lots of fixed numbers

  13. Liu et al 2007: in CAM3.0 MLS RHi LIU CAM AMWG 2008

  14. Allowing Supersaturation • Goal: adjust closure for supersaturation • Principle: • Separate ice and liquid cloud fractions • Bulk condensation only w.r.t water (liquid). • Ice formation requires • Existing ice (vapor deposition onto ice) • Liu et al 2007 parameterized ice nucleation • Process rates & sedimentation govern ice • Ice cloud fraction closed using IWC • Empirical fit to mid-lat cirrus observations (Wang & Sassen 2002, JAS)

  15. Key Results: Current Version • Reasonable TOA distributions • Reduction of High Latitude Cloud • Especially mid-high clouds • Increased UT humidity • SW & LW Cloud Forcing 3W/m2 less • 30% lower IWP

  16. Cloud Fraction New Ice Base Case

  17. Downward JJA SW New Ice - Base Case (avg +8W/m2) Gettelman - Ice 17

  18. Ice Cloud Indirect Effects • Sensitivity Experiments • Increase ice nuclei by factor of 10, 100 (global) • Results: • x10: IWP + 25%, tropical CF ±8-10W/m2 • x100: IWP +100%, tropical CF ±16-20W/m2 • X100: regional decreases in precipitation rate • Need to explore sensitivity to aerosols • Vertical distribution of aerosols AMWG 2008

  19. Change in Ice Number High IN (x10) Std IN

  20. Change in Precip • Base • High IN-Base • (x100)

  21. Status and Plans • Finalize and Harmonize code • Rewriting mixed phase code now • Perhaps separate process rates by individual cloud fractions • Write up description • Target for end of summer • Port to latest version of trunk • Harmonize macrophysics and closure with Park/Rasch work • Propose for CAM4

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