Parameterization of ice crystal conversion processes in cirrus clouds using double-moment basis functions
Date
1994-04-05
Authors
Harrington, Jerry L., author
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Abstract
With the onset of increased scientific interest in cloud-effects on climate and the need of cirrus forecasts to support military operations and the aviation industry has come the need to develop more credible microphysical parameterizations of the ice transfer processes occurring in cirrus clouds. Herein a parameterization is developed for the transfer between two defined categories of ice; pristine ice (which grows by vapor deposition only and is constrained to have mean diameters less than 125 μm) and snow (resulting from the direct conversion of pristine ice). Each category is assumed to conform to a generalized gamma distribution function, with variations in ice crystal habits allowed. Analytical transfer equations for the flux of number concentration and mass between the pristine ice and snow categories during ice supersaturated and subsaturated atmospheric regimes are derived. A parameterization of ice number concentration loss from each of these distributions during sublimation is also described. These parameterizations are tested in a one-dimensional Lagrangian parcel model for ice supersaturated ascents and ice subsaturated decents. These tests allow analysis of the parameterizations during variations in physical parameters such as the shape of assumed distributions and the ice crystal habit. It is shown that variations in both of these parameters have large impacts on the evolution of the distributions. These results show similarities to other modeling efforts. The ice parameterizations are implemented into the Regional Atmospheric Modeling System (RAMS) developed at CSU and two-dimensional sensitivity tests are conducted using observations from the November 26, 1991 FIRE II cirrus case. Tests of the model using rosette crystals and exponential distribution shapes showed the flexibility of the RAMS model in simulating these systems. The RAMS results compared favorably with data obtained during the FIRE II field project. Tests with larger values of the distribution shape parameter showed possible improvements over the test case in that larger ice masses were found near cloud bases (as was observed). The type of crystal modeled was shown to have a large impact on the microphysical evolution of the simulated cirrus system; cloud depth, ice water content (IWC), number concentrations and updraft profiles were all sensitive to these changes. Tests that examined the profiles of ice nuclei (IN), radiative parameterization, and two moment versus one moment predictions all showed the importance of credible parameterizations and of correct model initialization.
Description
April 5, 1994.
Also issued as author's thesis (M.S.) -- Colorado State University, 1994.
Also issued as author's thesis (M.S.) -- Colorado State University, 1994.
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Subject
Ice clouds
Cloud physics