Transcript Document
Changes to the ECMWF cloud scheme
Recent package of changes for 25r3:
COMPLETE REWRITE OF NUMERICS FOR CLOUD
SCHEME – ALL CLOUD PROCESSES TREATED ON
SAME TIME LEVEL
Showed that method of solution of equations was
equally important as parameterization methodology
ICE FALL SPEEDS CHANGED
Large ice proportion diagnosed as before
Large ice fall speed as HD90 (function of mass)
Small ice fall speed 0.15 m/s
Considerable changes to cloud top entrainment process
OVERALL EFFECT: More ice, less liquid
Cloudnet Workshop April 2003
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Near Future Upgrades
New PBL scheme under development
This has been combined with a diagnostic statistical cloud
scheme
Symmetrical beta distribution used to diagnose cloud cover
Beta distribution moments fixed from cloud mass and
vapour mixing ratio
Cloud cover is no longer a prognostic variable under this
new scheme
Cloudnet Workshop April 2003
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Statistical Schemes for Cloud
Horizontal GCM grid points
Parameterization
qv
ql+qi
Cloudy Region
G(qt)
qsat
Total Water qt=
vapour + liquid + ice
1. The PDF form G(qt)
2. The moments of the distribution
3. The mean saturation mixing ratio qsat
Cloudnet Workshop April 2003
Cloud cover
can be derived
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Far Future developments
A prognostic variance equation will be added to give the
statistical scheme more flexibility
Separate ice microphysics will be included:
Homogeneous nucleation of ice phase
Autoconversion etc etc…
Supersaturation wrt ice will be allowed
Treatment of ice fall out still to be determined (timestep
problems)
Consistent use of subgrid information in microphysics and
radiation
Cloudnet Workshop April 2003
A.Tompkins
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