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Using a Variability Factor to Account for Cloud Microphysical Inhomogeneity in Mesoscale Models

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  • 1 NorthWest Research Associates, Redmond, Washington
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Abstract

Different formulations of the joint probability distribution function (JPDF) based on large-eddy simulation (LES) studies of shallow cumulus and cumulus congestus clouds were evaluated. It was shown that inhomogeneity in both cloud types can be quantified by their respective JPDFs calculated using datasets from the entire simulation time period (“generic” JPDFs). The generic JPDF can be a priori integrated and yield a one-dimensional variability factor (V factor) specific for each cloud type. A quite accurate approximation of V factors by an analytical function in the form of a third-order polynomial was obtained and can be easily implemented in mesoscale models. The effect on precipitation of conversion rates modified by V factors was also evaluated in LES sensitivity studies of shallow cumulus (Cu) and congestus Cu clouds. The surface precipitation increased significantly when V factors were taken into account. The sensitivity experiments revealed that most of the increase resulted from the modified autoconversion process. The effect of accretion rates modified by V factors was much less significant, primarily because of the nearly linear dependence of accretion on its parameters. This fact shows the importance of the most accurate formulation of the autoconversion process.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Yefim Kogan, ykogan@nwra.com

Abstract

Different formulations of the joint probability distribution function (JPDF) based on large-eddy simulation (LES) studies of shallow cumulus and cumulus congestus clouds were evaluated. It was shown that inhomogeneity in both cloud types can be quantified by their respective JPDFs calculated using datasets from the entire simulation time period (“generic” JPDFs). The generic JPDF can be a priori integrated and yield a one-dimensional variability factor (V factor) specific for each cloud type. A quite accurate approximation of V factors by an analytical function in the form of a third-order polynomial was obtained and can be easily implemented in mesoscale models. The effect on precipitation of conversion rates modified by V factors was also evaluated in LES sensitivity studies of shallow cumulus (Cu) and congestus Cu clouds. The surface precipitation increased significantly when V factors were taken into account. The sensitivity experiments revealed that most of the increase resulted from the modified autoconversion process. The effect of accretion rates modified by V factors was much less significant, primarily because of the nearly linear dependence of accretion on its parameters. This fact shows the importance of the most accurate formulation of the autoconversion process.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Yefim Kogan, ykogan@nwra.com
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