Michelle Pitcel, Graduate Student
Department of Atmospheric Sciences
University of Illinois
Date: Wednesday, December 9, 2009
3:00 pm: Conversation and Cookies in Room 108 Atmospheric Sciences Building
3:30 pm: Seminar in Room 114 Transportation Building
Mesoscale Gravity Waves (MGWs) are large pressure perturbations that tend to occur behind Mesoscale Convective Systems (MCSs) in summer and over warm frontal surfaces behind elevated convection in winter. MGWs are associated with damaging winds, moderate to heavy precipitation, and occasional heat bursts at the surface. The forcing mechanism for MGWs in this study is hypothesized to be evaporative cooling occurring behind the convective line in the area of the stratiform region. This evaporatively-cooled air generates a downdraft that then depresses the surface-based stable layer and causes pressure decreases, strong wind speeds and MGW genesis. Using the Weather Research and Forecast Model (WRF) version 3.0, evaporative cooling is simulated using an imposed cold thermal. Sensitivity studies are conducted to examine the response of MGW structure to different thermal and shear profiles where the strength and depth of the inversion are varied, as well as the amount of wind shear. MGWs are characterized in terms of wind speed perturbations (U’), temperature perturbations (T’), pressure perturbations (P’), potential temperature perturbations (Θ’), and the correlation coefficient (r) between U’ and P’. “Regime Diagrams” that relate these variables are used to measure MGW activity. These diagrams portray the response of the MGW to the above variables in order to better understand the formation, causes, and intensity of MGWs.
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