Natural Variability of Thermodynamic Features Affecting Convective Cloud Growth and Dynamic Seeding: A Comparative Summary of Three High Plain Sites from 1975 to 1977

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  • 1 Office of Atmospheric Resources Research, Bureau of Reclamation, Denver, CO 80225
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Abstract

A statistical summary of the thermodynamic features generally thought to be relevant to convective cloud development and the dynamic seeding hypothesis, as diagnosed by a one-dimensional steady-state model, has been compiled for selected locations on the High Plains. The summary is based primarily on rawinsonde data collected at Miles City, Montana, Goodland, Kansas, and Big Spring, Texas during the summer months from 1975 to 1977, as part of the High Plains Cooperative Program (HIPLEX). Data were stratified according to the occurrence or non-occurrence of convective clouds and, when clouds occurred, according to cloud type as seen in geosynchronous satellite imagery. Geographic comparisons are presented showing significant variations from north to south over the High Plains, the most intense convection occurring in the central and southern plains and the least intense in the north. Large annual variations were noted in mean values of thermodynamic features at each site between the three summers.

Analyses of mesoscale variability of thermodynamic features were made using observations from a mesoscale rawinsonde network. Although large spatial variations in thermodynamic variables on a given day were found, seasonal mean values were similar, suggesting that the variations are related to small-scale dynamics and appear statistically random but that the average values at any one site are representative of a given region.

While no cloud seeding was performed to test the dynamic seeding hypothesis in field experiments, the cloud model indicated that soundings on the High Plains had potential for additional dynamic growth. The dynamic modification potential (DMP) is defined as the post-seeding enhancement of growth that is predicted by the cumulus model to result from heat released by converting supercooled water to ice. This is equivalent to Simpson's (1976) “seedability”, and is similar to model-predicted changes in cloud depth used by other authors. While supercooled water is presumed to exist in sufficient quantity for a long enough time to produce changes in cloud dynamics, no in-cloud measurements of the presence and duration of supercooled water were made in this study. However, the model indicated that DMP was found to exist in about 50% of the soundings analyzed in this study. The maximum frequency of occurrence of DMP was associated with mesoscale convective systems in Montana and Texas; whereas in Kansas, it occurred on days with smaller air mass convective clouds. Large variations in the frequency of modeled DMP occurred as a function of observed cloud types and mesoscale types.

Abstract

A statistical summary of the thermodynamic features generally thought to be relevant to convective cloud development and the dynamic seeding hypothesis, as diagnosed by a one-dimensional steady-state model, has been compiled for selected locations on the High Plains. The summary is based primarily on rawinsonde data collected at Miles City, Montana, Goodland, Kansas, and Big Spring, Texas during the summer months from 1975 to 1977, as part of the High Plains Cooperative Program (HIPLEX). Data were stratified according to the occurrence or non-occurrence of convective clouds and, when clouds occurred, according to cloud type as seen in geosynchronous satellite imagery. Geographic comparisons are presented showing significant variations from north to south over the High Plains, the most intense convection occurring in the central and southern plains and the least intense in the north. Large annual variations were noted in mean values of thermodynamic features at each site between the three summers.

Analyses of mesoscale variability of thermodynamic features were made using observations from a mesoscale rawinsonde network. Although large spatial variations in thermodynamic variables on a given day were found, seasonal mean values were similar, suggesting that the variations are related to small-scale dynamics and appear statistically random but that the average values at any one site are representative of a given region.

While no cloud seeding was performed to test the dynamic seeding hypothesis in field experiments, the cloud model indicated that soundings on the High Plains had potential for additional dynamic growth. The dynamic modification potential (DMP) is defined as the post-seeding enhancement of growth that is predicted by the cumulus model to result from heat released by converting supercooled water to ice. This is equivalent to Simpson's (1976) “seedability”, and is similar to model-predicted changes in cloud depth used by other authors. While supercooled water is presumed to exist in sufficient quantity for a long enough time to produce changes in cloud dynamics, no in-cloud measurements of the presence and duration of supercooled water were made in this study. However, the model indicated that DMP was found to exist in about 50% of the soundings analyzed in this study. The maximum frequency of occurrence of DMP was associated with mesoscale convective systems in Montana and Texas; whereas in Kansas, it occurred on days with smaller air mass convective clouds. Large variations in the frequency of modeled DMP occurred as a function of observed cloud types and mesoscale types.

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