Advanced Research WRF (ARW) Technical Note - MMM - University ...

and it is the water vapor and total condensate that are advected in the model. Local storage arrays
retain first-guess information that extract contributions of cloud water, rain, cloud ice, and
precipitation ice of variable density in the form of snow, graupel, or sleet. The density of precipitation
ice is estimated from a local array that stores information on the total growth of ice by
vapor deposition and accretion of liquid water. Sedimentation is treated by partitioning the timeaveraged
flux of precipitation into a grid box between local storage in the box and fall out through
the bottom of the box. This approach, together with modifications in the treatment of rapid
microphysical processes, permits large time steps to be used with stable results. The mean size
of precipitation ice is assumed to be a function of temperature following the observational results
of Ryan (1996). Mixed-phase processes are now considered at temperatures warmer than -30 ◦ C
(previously -10 ◦ C), whereas ice saturation is assumed for cloudy conditions at colder temperatures.
Further description of the scheme can be found in Sec. 3.1 of the November 2001 Technical
Procedures Bulletin (TPB) at http://www.emc.ncep.noaa.gov/mmb/mmbpll/eta12tpb/ and on
the COMET page at http://meted.ucar.edu/nwp/pcu2/etapcp1.htm.
8.1.7 Thompson et al. scheme
The Thompson et al. (2004) microphysical parameterization scheme includes improvements to
the earlier bulk scheme of Reisner et al. (1998) and has been extensively tested and compared
with both idealized case studies and documented real case studies of mid-latitude wintertime
observations. The scheme includes six classes of moisture species plus number concentration
for ice as prognostic variables. The scheme was designed to improve the prediction of freezing
drizzle events for aircraft safety warnings. Generally microphysical parameterizations have had
problems of overpredicting the amount of snow and graupel fields and under predicting the ice
in outflow regions and often not accurately predicting freezing drizzle. Key improvements are
the following:
• Primary ice nucleation as in Cooper (1986), replaces the Fletcher (1962) curve.
• Auto-conversion as in Walko et al. (1995), replaces the Kessler (1969) scheme.
• A generalized gamma distribution for graupel replaces the exponential distribution.
• The associated intercept parameter depends on mixing ratio instead of remaining constant.
• Riming growth of snow must exceed depositional growth of snow by a factor of 3 before
rimmed snow transfers into the graupel category.
• The intercept parameter of the snow size distribution depends on temperature.
• The intercept parameter for the rain size distribution depends on rain mixing ratio, thereby
simulating the fall velocity of drizzle drops as well as raindrops.
8.2 Cumulus parameterization
These schemes are responsible for the sub-grid-scale effects of convective and/or shallow clouds.
The schemes are intended to represent vertical fluxes due to unresolved updrafts and downdrafts
and compensating motion outside the clouds. They operate only on individual columns where
the scheme is triggered and provide vertical heating and moistening profiles. Some schemes
additionally provide cloud and precipitation field tendencies in the column, and future schemes
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