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Ice Microphysics in CAM

Ice Microphysics in CAM. A. Gettelman, X. Liu, H. Morrison. Outline. Motivation Tests of ice processes Exploration of explicit ice nucleation Allowing ice supersaturation Status and future plans. Motivation. Cirrus and ice nucleation uncertain Supersaturation over ice common

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Ice Microphysics in CAM

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  1. Ice Microphysics in CAM A. Gettelman, X. Liu, H. Morrison AMWG 2008

  2. Outline • Motivation • Tests of ice processes • Exploration of explicit ice nucleation • Allowing ice supersaturation • Status and future plans AMWG 2008

  3. 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 AMWG 2008

  4. 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 AMWG 2008

  5. Bergeron Process MG Micro Rotstayn AMWG 2008

  6. 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 AMWG 2008

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

  8. Initial Testing w/ MG micro • Code runs in new microphysics • Produces lots of supersaturation • Not enough ice generated • Need to work out: • Depletion of vapor in presence of ice (vapor deposition appears insufficient) • Adjustments to reflect sub-grid variability AMWG 2008

  9. SCAM & Global Tests • Doubled High Cloud fraction • Increase in RH, lots of supersaturation • 70% Decrease in Cld Ice • 30% Decrease in net CF (LW & SW) Main issues: • Need better constraints on ice processes • Account for sub-grid variability AMWG 2008

  10. 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 • Explicit (parameterized) ice nucleation • Process rates & sedimentation govern ice • Ice cloud fraction determined using IWC closure AMWG 2008

  11. New ‘ICECLDF’ Cloud Fractions. SCAM, ARM Site IOP Ice Liquid Total AMWG 2008

  12. Status • Done initial coding: • separate ice and liquid clouds • structure for closure in place • Works with MG scheme & ice nucleation • Testing IWC closure now • Use empirical relation for IWC =f(T) • Expand to better IWC = f(T, Rei, Ni, etc) • Needs a good conceptual re-think • Goal: still try for CAM4 • Don’t see major impact on run-time • Better physical basis, easier to compare to obs AMWG 2008

  13. Summary • Ice microphysics changes are small, but not negligible • Modifies relative sizes/numbers of drops and ice in mixed phase • Partitioning of mixed phase affects CF • Ice nucleation treatments show promise • Separate ice cloud variable possible: • better physical representation, • better constrained by observations • More work to be done: stay tuned! AMWG 2008

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