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Impact of several hail parameters on simulated supercell storms




Van Den Heever, Susan Claire, author

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Most modeling studies of supercell storms neglect the ice phase. However, supercells are often prolific producers of hail and extend deep enough in the troposphere that active participation by the ice phase in storm dynamics would be expected. Therefore, the sensitivity of simulated supercell characteristics to variations in the settings of several ice parameters is investigated. In the first sensitivity test, decreasing the mean hail diameter affects the hail size distribution by increasing the number of smaller hailstones and decreasing the number of larger hailstones. Melting and evaporation rates are greater in the smaller hail diameter cases, which result in stronger low-level downdrafts and more intense cold pools. The stronger downdrafts enhance the low-level convergence within the updraft, thereby forcing stronger updrafts and greater low-level vertical vorticity through stretching. However, the stronger downdrafts also occlude the low-level updrafts more rapidly, and the storms are shorter-lived. The longevity of the left-moving storm decreases as the mean hail diameter decreases, and is controlled primarily by the low-level buoyant forcing which is negative. The maximum surface precipitation is greater in the larger hail cases, but the precipitation covers a smaller area. The storms in the small hail cases resemble classic supercells, whereas those in the large hail cases possess high-precipitation supercell characteristics. This suggests that the precipitation characteristics of the storm may determine the type of supercell that develops. Increasing the shape parameter of the hail size distribution in the second sensitivity test decreases the low-level downdrafts and associated divergence. However, the distance between the downdraft and updraft also decreases as the shape parameter increases, thereby increasing the low-level convergence within the updraft, and hence the updraft strength and low-level vertical vorticity. Excluding all hail-related processes in the third sensitivity test results in greater rain mixing ratios, however, the rain remains suspended at higher levels. The presence of hail therefore affects the rain distribution within the storm by increasing rain in the lower levels through melting, and reducing rain in the upper levels through droplet freezing. Excluding hail delays the development of the low-level mesocyclone, through the slower evolution of the low-level downdrafts. In the final sensitivity test, the use of two-moment bulk microphysics results in the presence of larger hail below cloud base and smaller hail above the melting level, a wider vertical and horizontal distribution of hail, and reduced rain mixing ratios at the surface. Several supercell characteristics were better developed when using the two-moment scheme. All of the sensitivity tests indicate that if a steady, long-lived updraft and low level mesocyclone are to develop, a balance is needed between the characteristics of the hail size distribution which influence the strength and position of the low-level downdrafts, and the strength of the low-level environmental winds which prevent the gust front from outrunning the storm. The balance is sensitive to the parameters determining the hail size distribution, and thus these parameters need to be determined accurately.


November 2, 2001.
Also issued as author's dissertation (Ph.D.) -- Colorado State University, 2001.

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Storms -- Computer simulation


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