Larson, Vincent E., Carsten Harlass, and Jan Hoft of
University of Wisconsin, Milwaukee
Currently, WRF does not predict
subgrid-scale cloud fraction. However, predicting cloud fraction may aid, for
instance, the representation of microphysics in shallow cumulus clouds, even at
convection-permitting grid spacings.
Unfortunately, parameterizing subgrid
cloud fraction --- due to, e.g., cumulus clouds --- poses difficulties at
convection-permitting grid spacings.
For instance, at finer grid spacings, there is a breakdown in
assumptions for idealized cumulus mass-flux models, such as the assumptions of
steady state and small areal coverage.
We have implemented into WRF a
parameterization of subgrid variability ("CLUBB") that is based on
the assumed probability density function (PDF) method. It predicts the subgrid PDF of heat
content, moisture, and vertical velocity.
Thereby it permits the diagnosis of subgrid cloud fraction. CLUBB is a prognostic parameterization
and does not make any assumption about the areal coverage of cloud or updrafts.
We test WRF-CLUBB by simulating a case of
marine stratocumulus that was observed off the coast of Chile during the VOCALS
field experiment. In this
simulation, CLUBB alone parameterizes all clouds, including stratocumulus,
shallow cumulus, and deep cumulus over the Amazon. Thereby CLUBB is a unified parameterization for clouds. We perform various sensitivity studies
and analyze CLUBB's performance in these simulations.