Thursday, 13 February 2003
Observed WSR88D Radar Structrues and MM5 Simulated Kinematic Structures
Tropical Cyclone (TC) Earl (1998) produced several tornadoes in the Tampa area on 2 September and one tornado and one microburst in Georgia and several tornadoes in South Carolina on 3 September 1998. The structures of mesocyclones and the downburst have been analyzed. Vertical cross sections of reflectivity and Doppler velocity were constructed. From these we are able to quantify data on such mesocyclone characteristics as the maximum reflectivity, horizontal and vertical extents, horizontal shears, spectrum width etc. To understand the dynamics of these mesocyclones we employed the MM5 nonhydrostatic model. We used four domains with 81,27,9 and 3 -km scale grid lengths and made short term integrations. Our aim is to understand how the vorticity changes are produced in the mesocyclonic environment.
Preliminary results suggest that a maximum reflectivity of 50 dBZ, persisting over half an hour and horizontal shears exceeding 0.01 per second signal tornado genesis. Some of the mesocyclones themselves were more than 5 km in horizontal and vertical extents. The MM5 results suggest that the domains with 27, 9 and 3-km grid distances represent fairly well the tropical cyclone circulation with mesocyclonic circulation embedded in it. The convergence of streamlines at 925 mb in the 27-km grid depicted well the area where mesocyclones with tornadoes occured. Similarly the 9 and 3-km grid domains represented through the vertical vorticity fields at 925 mb the mesocyclone positions and intensities.