92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Wednesday, 25 January 2012
A Numerical Study of the 27th April 2011 Tornado Outbreak in the Southeastern U.S. with the WRF-RTFDDA-LES System
Hall E (New Orleans Convention Center )
Wanli Wu, NCAR, Boulder, CO; and Y. Liu, M. Ge, J. C. Knievel, S. Swedlin, and J. Pace

Devastating tornadoes swept through the southeastern U.S. on the 27th April 2011. There were two waves of tornado outbreaks that day: one occurred in the morning (5-8AM CDT), and another happened in the late afternoon through early evening (3-7PM CDT). Over 300 fatalities and massive destruction were reported. To study and improve forecasts of such deadly storms, the recently formulated WRF Real-Time FDDA and LES System (WRF-RTFDDA-LES; Liu et al. 2011) was utilized to investigate the multiscale event from meso-alpha to LES scales with nested domains from 8.1 kilometers down to 300 meters. The modeling system was able to reasonably capture the evolution of the two waves of tornado outbreaks, and the widespread extreme winds (Wu et al. 2011). This talk will focus on the second outbreak from the late afternoon to early evening. Both the parent mesoscale convective vortex, and the associated tornadic storms were well simulated. On the finest domain, the system simulated key ingredients in tornado genesis: strong convergent flow associated with mesocyclones, persistent rotating updrafts (~40 m/s), and rear flank downdrafts. Simulated tornadoes exhibit a violently rotating column of air and a weak descending tornado eye with surface wind speeds close to Enhanced Fujita Scale 4 (~74 m/s) accompanying with dramatic mean sea level pressure changes. Results and analysis of the simulated tornadic features and mechanisms will be presented.

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