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

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

5. Bergeron Process MG Micro Rotstayn Gettelman - Ice AMWG 2008
Green: Existing (Morrison), Black: Rotstayn 2000.
Rotstayn scheme reduces peak liquid at small sizes and shifts to larger numbers
Also broadens ice number PDF.
Increases magnitude of cloud forcing by 3W/m2, with change to TOA of -0.5W/m2
(SW increases slightly more than longwave). IWP changes less than 5%
Not clear which is better.
Gettelman - Ice
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
Gettelman - Ice
AMWG 2008

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

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

12. Status Done initial coding: Works with MG scheme & ice nucleation
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
Gettelman - Ice
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!
Gettelman - Ice
AMWG 2008