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Ice Microphysics in CAM

Explore improvements in ice microphysics in CAM, focusing on cirrus cloud formation, ice nucleation processes, supersaturation effects, and testing against observations for better model accuracy.

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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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