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The absence of the very shallow, near surface negative KF
theta change when using the altered KF scheme is due to the
severe reduction in precipitation produced by the KF
scheme. The downdraft model uses a fraction of the
precipitation to generate an evaporatively cooled downdraft
that detrains in the lowest model layer. In the altered
version of the KF scheme, precipitation is nearly absent and
the water vapor is partitioned into cloud material so that the
downdraft model produces very little cooling.
Skamarock, W. C., J. B. Klemp, and J. Dudhia, Prototypes
for the WRF (Weather Research and Forecasting)
model, Preprints, Ninth Conf. on Mesoscale Processes,
Fort Lauderdale, FL, Amer. Meteor. Soc., J11—J15,
2001.
4. Summary
An altered version of the KF scheme in which water vapor
flux is converted into cloud material rather than precipitation
is examined. The results from simulations of a two-month
period in which large propagating MCSs were frequent
showed dramatic differences in diabatic heating profiles.
At the workshop, additional results will be discussed that
show just as dramatic differences in precipitation
characteristics. In particular, the phase of the diurnal cycle
of precipitation is significantly delayed and more accurate in
when using the altered KF scheme, and with this modified
scheme the eastward, nocturnal propagation of precipitation
is more pronounced. The sensitivity to microphysics
parameterization and grid point spacing will be discussed as
well.
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