Daniel T. Dawson II
School of Meteorology
The University of Oklahoma, Norman, OK
CAPS Graduate Research Assistant
03 April 2009, 2:00 PM
National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK
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The accurate parameterization of cloud and precipitation microphysical processes is critically important in simulation and prediction of severe convective storms, including supercell storms and their associated circulations. The 3 May 1999 Oklahoma tornado outbreak was characterized by several discrete tornadic supercells in relatively close spatial and temporal proximity, which in general displayed relatively weak and small cold pools. In this work, a sophisticated multi-moment bulk microphysics parameterization scheme capable of predicting up to three moments of the particle or drop size distribution for several liquid and ice hydrometeor species is evaluated and compared with traditional single-moment schemes through numerical simulations of this event.
First, idealized simulations of this outbreak are conducted at various horizontal grid spacings ranging from 1 km to 250 m, using a sounding extracted from a prior 3-km grid spacing real-data simulation. The impacts of microphysics on cold pool strength and structure and on the overall reflectivity structure of the simulated storms are emphasized. It is shown through microphysics budget and trajectory analyses within the low level downdraft regions that the multi-moment scheme has several important advantages which lead to a weaker and smaller cold pool and better reflectivity structure, particularly in the forward flank region of the simulated supercells. Specifically, the improved treatment of evaporation and melting processes and their effects on the predicted rain and hail DSDs by the multi-moment scheme helps to control the cold bias often found in simulations using typical single-moment schemes. The multi-moment results are more consistent with observed thermodynamic conditions within the cold pools of the discrete supercells of the 3 May 1999 outbreak.
Real-data simulations of this event down to 100 m horizontal resolution are performed, with an emphasis on the prediction of the Moore, Oklahoma F5 tornado. The performance of the multi-moment microphysics scheme is tested and several issues are discussed in relation to the thermodynamic properties of the rear-flank downdrafts of the storms. The multi-moment simulations in general produce a better prediction of the tornado track and intensity than the single-moment simulations; reasons for these differences and their implications for numerical supercell tornado simulation and prediction are discussed.
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