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Thesis Defense

Department of Atmospheric Science

Thurs., Nov. 10, 3:00 pm, EN6085

June 9-10, 2015: A Case Study of the Great Plains Low-Level Jet During PECAN (Plains Elevated Convection at Night)

Sharon M. Sullivan

University or Wyoming

Abstract

Observations as part of the Plains Elevated Convection at Night (PECAN) campaign have allowed for an examination of the thermodynamic and dynamic structure of the low-level jet (LLJ) using ground-based and airborne measurements in central Kansas. A shallow jet with wind speeds near 20 m s-1 formed during the nighttime hours on 10 June 2015. The University of Wyoming King Air research aircraft conducted two research flights beginning at sunset and ending near dawn, capturing the full development of the LLJ.  Each flight included a series of vertical sawtooth maneuvers and isobaric legs along a 125-km fixed track at 38.7°N between 98.89°W and 100.3°W. This case study featured classic signatures of the LLJ, including but not limited to the inertial oscillation of the ageostrophic wind consistent with the Blackadar (1957) mechanism. Forcing of the LLJ was described using cross sections of D-values (an alternate measure of pressure) that allowed the vertical structure of the horizontal pressure gradient and hence a diagnosis of the thermal wind to be performed.

A series of numerical simulations of the 10 June 2015 case study were made using the Weather Research and Forecasting (WRF) model to compare with observations. Output grids indicated that a fine temperature gradient of 6°C per 500 km was present between the surface and 850 hPa, implying a northerly thermal wind of 5 m s-1 must be present between 950 hPa and 850 hPa alone. Warmer temperatures existed to the west from the surface up to 600 hPa. Additionally, the 600 hPa geostrophic winds were from the north. As a result, only weak southerly geostrophic winds were able to develop at the surface. The terrain-induced thermal wind was sufficiently large to overcome the adverse pressure gradient in the free atmosphere, but could only produce weak southerly geostrophic winds at the surface of about 11.4 m s-1


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