P2.9 The Unique Microphysical Signature of Severe Storms Clouds that Produce Tornado and Large Hail

Wednesday, 12 July 2006
Grand Terrace (Monona Terrace Community and Convention Center)
Amit Lerner, The Hebrew University of Jerusalem, Jerusalem, Israel; and D. Rosenfeld

During the last decade, a new method was developed to retrieve information about the microphysical evolution of convective clouds from meteorological satellites with an emphasis on understanding the roll of aerosols and air pollution on precipitation forming processes. In this method, the growth of cloud drops with height is determined using the retrieved cloud top temperature of neighboring clouds in different developmental stages. We used this method on NOAA polar-orbiter satellite images over regions of the United States to analyze the microphysical development of 96 storms clouds formed during the years 1998-2002 and categorized as clouds producing tornadoes, clouds producing large hail (diameter ≥ 0.75"), and clouds exhibiting no severe storm phenomena. We found that clouds that produce tornadoes can be distinguished statistically from ones that do not. In tornado producers, the drop size at cloud base is smaller, there is no warm rainout zone (i.e., zone where drop effective radius exceeds 14 ┬Ám precipitation threshold in temperatures warmer than 0oC), there is no coalescence zone at warm temperatures, and there is a linear growth of effective radius up to the level of homogenous freezing with a smaller drops size at that level. Clouds exhibiting no severe phenomena have larger drops at cloud base and an effective mixed phase zone in which there is a rapid growth of drops to the maximum detectable size limit of the instrument of 35 μm. In contrast, in large hail and tornado producing clouds this growth is much slower with height, and in tornado producers freezing occurs at temperatures as low as the homogeneous freezing temperature of -38oC. These unique signatures of tornado and large hail microphysical development were observed in the AVHRR snapshots more than an hour before the occurrence of the tornado and large hail, suggesting that the retrieved cloud microstructure has the potential for a vast improvement in forecasting severe storm phenomena.
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