Retrieval of Thermal and Microphysical Variables in Observed Convective Storms. Part II: Sensitivity of Cloud Processes to Variation of the Microphysical Parameterization

Conrad L. Ziegler National Severe Storms Laboratory/NOAA, Norman, Oklahoma

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Abstract

The hydrometeor content and thermal fields in a thunderstorm are estimated from a three-dimensional kinematic cloud model employing Doppler wind fields and parameterized microphysical processes. The sensitivity of the cloud model calculations to variation of the microphysical parameterization is determined by comparing results of model runs with modified parameterizations to the results of a standard or control model run with complete warm/cold cloud microphysics. Changes of certain calculated or specified model parameters and alternate exclusion or inclusion of the ice phase modulate extreme values of precipitation content. Differences between the model solutions, which result from altering the balance between predominant precipitation processes, are traced through analysis of model output to some major change of the precipitation accretion mechanism. The largest differences in maximum retrieved graupel/hail content and radar reflectivity associate with the parameterizations which fix the graupel/hail distribution intercept parameter or accomplish the riming of supercooled cloud droplets by graupel/hail of a fixed high density. A reduction of the collection efficiency of cloud droplets by snow crystals, while significantly weakening the Bergeron precipitation process, has negligible impact on either the content or riming growth of graupel/hail. The sensitivity of precipitation content to variation of model parameters such as CCN concentration and dispersion of the cloud droplet distribution is relatively weak because the balance among predominant precipitation processes is not significantly altered. Exclusion of the ice phase results in a cooling of up to 2°C in the main updraft region. The major impact on cloud dynamical forcing by presence of the ice phase is a reduction of negative buoyancy in the upper half of the main updraft region.

Abstract

The hydrometeor content and thermal fields in a thunderstorm are estimated from a three-dimensional kinematic cloud model employing Doppler wind fields and parameterized microphysical processes. The sensitivity of the cloud model calculations to variation of the microphysical parameterization is determined by comparing results of model runs with modified parameterizations to the results of a standard or control model run with complete warm/cold cloud microphysics. Changes of certain calculated or specified model parameters and alternate exclusion or inclusion of the ice phase modulate extreme values of precipitation content. Differences between the model solutions, which result from altering the balance between predominant precipitation processes, are traced through analysis of model output to some major change of the precipitation accretion mechanism. The largest differences in maximum retrieved graupel/hail content and radar reflectivity associate with the parameterizations which fix the graupel/hail distribution intercept parameter or accomplish the riming of supercooled cloud droplets by graupel/hail of a fixed high density. A reduction of the collection efficiency of cloud droplets by snow crystals, while significantly weakening the Bergeron precipitation process, has negligible impact on either the content or riming growth of graupel/hail. The sensitivity of precipitation content to variation of model parameters such as CCN concentration and dispersion of the cloud droplet distribution is relatively weak because the balance among predominant precipitation processes is not significantly altered. Exclusion of the ice phase results in a cooling of up to 2°C in the main updraft region. The major impact on cloud dynamical forcing by presence of the ice phase is a reduction of negative buoyancy in the upper half of the main updraft region.

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