25th Conference on Severe Local Storms


ZDR columns as a predictive tool for hail growth and storm evolution

Joseph C. Picca, CIMMS/Univ. of Oklahoma, Norman, OK; and A. V. Ryzhkov

Within the updraft of a convective cell, liquid water in the form of relatively large raindrops can be carried aloft to an environment characterized by sub-freezing temperatures. Due to the oblate shape of these drops, they contribute positively to differential reflectivity ZDR. If upward vertical velocities are strong enough, the number and size of the lofted drops results in a columnar region of enhanced ZDR (sometimes as great as 5 dB at S band) which has been termed the ZDR column. Therefore, the size and intensity of the column are closely related to the strength of the updraft itself; in turn, ZDR columns likely provide a simple, yet powerful tool to assess and predict hail growth, which is highly dependent upon updraft strength, as well as overall storm intensity. To estimate this predictive capability, quantitative analyses relating the volume of ZDR columns above the environmental freezing level and reflectivity ZH below this level were performed for several cases. The results show that indeed an approximate lag of 20 minutes exists between an increase (decrease) in ZDR column strength and an increase (decrease) in overall ZH intensity, and presumably hail size, within the precipitation core near the surface. Furthermore, a negative correlation is observed at an approximate lag of 5-10 minutes, likely resulting from the ability of an updraft to loft hydrometeors. Lastly, as previous research has demonstrated that updraft weakening tends to precede tornadogenesis, the evolution of ZDR column intensity within tornadic supercells is investigated to assess a possible connection between the two.

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Session 11, Forecasting Techniques and Warning Decision Making: Advances in the Use of Radar, Satellite, and Lightning Data
Wednesday, 13 October 2010, 10:30 AM-12:00 PM, Grand Mesa Ballroom F

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