Seminar coordinator: Prof. Jeff Trapp (jtrapp at illinois.edu)
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Samantha Chiu, Graduate Student, DAS, University of Illinois
Room 112 of the Transportation Building
Department of Atmospheric Sciences
This study focuses on the environmental and storm scale dynamics of supercells that produce long-track tornadoes, with modeling emphasis on the central Alabama storms from April 27, 2011 - part of the 2011 Super Outbreak. While most of the 204 tornadoes produced on this day were weaker and short-lived, this outbreak produced 5 tornadoes in Alabama alone whose path-length exceeded 50 documented miles. The results of numerical simulations are inspected for both environmental and storm-scale contributions that make possible the formation and maintenance of such long–track tornadoes.
A two-pronged approach is undertaken, utilizing both idealized and case-study simulations with the Weather Research and Forecasting [WRF] Model. Idealized simulations are designed to isolate the role of the local storm environment, such as instability and shear, on long-track tornadic storm structure. Properties of simulated soundings, including hodograph length and curvature, storm relative helicity and CAPE, are compared to idealized soundings described by Adlerman and Droegemeier 2005 in an effort to identify properties conducive to storms with non-cycling (sustained) mesocyclones.
Case-study simulations are being used to understand the mesoscale forcing and environmental changes along storm tracks. A strong, sustained (duration > 90 minutes) mesocyclone has been successfully modeled at high resolution for the 27 April case (see Figure 1), allowing detailed analysis of the storm evolution and its environment. Model soundings are examined from the inflow of simulated storms in order to diagnose and understand favorable environments and any changes present within them in which the storms may have developed and been sustained. Additional analysis focuses on investigating the importance of surface moisture fluxes in influencing storm morphology, as well as identifying mesoscale boundaries and storm-scale vorticity structures that may have played a role in intensifying and/or sustaining the tornadic cells.
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