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

8.1.2 Purdue Lin scheme
Six classes of hydrometeors are included: water vapor, cloud water, rain, cloud ice, snow, and
graupel. All parameterization production terms are based on Lin et al. (1983) and Rutledge
and Hobbs (1984) with some modifications, including saturation adjustment following Tao et al.
(1989) and ice sedimentation. This is a relatively sophisticated microphysics scheme in WRF,
and it is more suitable for use in research studies. The scheme is taken from the Purdue cloud
model, and the details can be found in Chen and Sun (2002).
8.1.3 WRF Single-Moment 3-class (WSM3) scheme
This scheme follows Hong et al. (2004) including ice sedimentation and other new ice-phase
parameterizations revised from the older NCEP3 scheme (Hong et al., 1998) that was in WRF
Version 1. A major difference from other schemes is that a diagnostic relation is used for
ice number concentration that is based on ice mass content rather than temperature. Three
categories of hydrometers are included: vapor, cloud water/ice, and rain/snow. As with Dudhia
(1989), this is a so-called simple-ice scheme wherein the cloud ice and cloud water are counted
as the same category. They are distinguished by temperature: namely, cloud ice can only exist
when the temperature is less than or equal to the freezing point; otherwise, cloud water can
exist. The same condition is applied to rain and snow. Though the ice phase is included, it is
considered efficient enough for using in operational models.
8.1.4 WSM5 scheme
This scheme is similar to the WSM3 simple ice scheme. However, vapor, rain, snow, cloud ice,
and cloud water are held in five different arrays. Thus, it allows supercooled water to exist, and
a gradual melting of snow as it falls below the melting layer. Details can be found in Hong et
al. (2004). It replaces WRF Version 1’s NCEP5 scheme (Hong et al., 1998).
8.1.5 WSM6 scheme
The six-class scheme extends the WSM5 scheme to include graupel and its associated processes.
Many of these processes are parameterized similarly to Lin et al. (1983), but there are differences
for the accretion calculation and in some other parameters. The freezing/melting processes are
computed during the fall-term sub-steps to increase accuracy in the vertical heating profile of
these processes. The order of the processes is also optimized to decrease the sensitivity of the
scheme to the time step of the model. As with WSM3 and WSM5, saturation adjustment follows
Dudhia (1989) and Hong et al. (1998) in separately treating ice and water saturation processes,
rather than a combined saturation such as the Purdue Lin (above) and Goddard (Tao et al.,
1989) schemes.
8.1.6 Eta Grid-scale Cloud and Precipitation (2001) scheme
This is also known as EGCP01 or the Eta Ferrier scheme. The scheme predicts changes in
water vapor and condensate in the forms of cloud water, rain, cloud ice, and precipitation ice
(snow/graupel/sleet). The individual hydrometeor fields are combined into total condensate,
53