Update on the 2-moment stratiform cloud microphysics scheme in CAM

Hugh Morrison and Andrew GettelmanNational Center for Atmospheric Research

CCSM Atmospheric Model Working Group MeetingFeb 12, 2008

Thanks to Steve Ghan (PNNL)

History

• Development began after 2006 AMWG meeting.

• Loosely based on 2-moment microphysics scheme of Morrison et al. (2005), implemented into MM5 and WRF, but with changes to account for long CAM timestep and large grid spacing.

Diagnostic versus prognostic precipitation Fractional cloudiness within grid cell Treatment of sub-grid cloud water variability

Scheme Description

• Prognostic variables: number concentrations and mixing ratios of cloud droplets and cloud ice: Nc, qc, Ni, qi

• Diagnostic variables: number concentrations and mixing ratios of rain and snow

• Droplet activation on aerosol treated by coupling scheme of Abdul-Razzak and Ghan (2000) with a sub-grid vertical velocity.

• Cloud and precipitation particle size distributions assumed to follow gamma functions:

( ) 0D

N D N D e

Key unique features of scheme

• Treatment of sub-grid cloud water distribution

• Allows for consistent treatment of microphysical processes instead of tuning individual processes (e.g., conversion to precipitation)

1''''

( '')( )

cqcc

qP q e

• Extensive testing for numerical stability and truncation errors – substepping of precipitation processes (10 min step).

• Diagnostic treatment of rain and snow mixing ratios and number concentrations

Allows for more robust treatment of precipitation particle size (important for scavenging) in different regimes (drizzle versus heavier rain in deep cloud systems)

Mean rain drop size - SCAM runs for ARM SGP site March 2000

Deep regime Shallow drizzling regime

Rain number predicted

Rain number diagnosed

Global results• 2 x 2.5o FV, 4-month spinup, 5-year analysis

• Aerosol from offline MOZART run (sea salt, sulfur, dust, carbonaceous)

• Stable, global radiative balance to within a few tenths W m-2.

• Relative to existing CAM microphysics scheme: large decrease in liquid water path in mid-latitudes, slight increase in cloud fraction and decrease in longwave and shortwave cloud forcing.

Running CAM with 2-moment Microphysics

• Versions of code after tag cam3_5_08 include an option for the new microphysics

• We recommend bug fixes and additional diagnostics in cam3_5_29

• To run:1. In configure command add ‘-dyn fv -nadv 5’2. In the namelist add: microp_scheme = ‘MG’

New Variables• New Advected prognostic variables:

– NUMLIQ, NUMICE– EFFLIQ and EFFICE are now diagnostic

• Lots of other fun stuff– In cloud variables, averaging arrays, number and size of

precipitation, simulated reflectivity– See:http://www.cgd.ucar.edu/cms/andrew/diag_fields.txt

• More details on wiki:– http://swiki.ucar.edu/cam-dev/microp

Warnings…

• Aerosols now affect climate. Be careful– We have not fully tested this (see Gettelman

talk)

• Please talk to Andrew/Hugh before using code. We just want to know what people are doing, and can help.

Summary• Scheme is available as option after

CAM3_5_08 (papers now accepted in J. Clim.)

• Stable, radiatively balanced with reasonable distribution of cloud forcing

• Good results relative to observations and existing CAM microphysics scheme

Future work• Ice microphysics (Andrew’s talk)

• Coupling with new cloud optics packages (liquid and ice)

• Further coupling/testing with aerosol (Xiaohong’s talk and Andrew’s talk tomorrow)