Friday, 24 June 2016: 11:15 AM
Bryce (Sheraton Salt Lake City Hotel)
As part of the Plains Elevated Convection at Night (PECAN) campaign, detailed airborne measurements were conducted of the summertime Great Plains low-level jet (LLJ) in central Kansas during the nighttime hours of 03 June 2015 using the University of Wyoming King Air research aircraft. Two flights were conducted that covered the evolution of the LLJ from its development near sunset to the diminution phase near dawn. Each flight included a series of vertical sawtooth and isobaric legs along a fixed track at 38.7°N between longitudes 98.9°W and 100°W. Goals of the research flights were to document changes in the vertical wind profile and the forcing of the jet. Aircraft observations show rapid changes in the horizontal pressure field during the early evening in response to stabilization of the lower atmosphere. Forcing of the LLJ was detected using cross-sections of D-values that allow measurement of the vertical profile of the horizontal pressure gradient force. Combined with observations of the actual wind, ageostrophic components normal to the flight track can be detected. Comparisons are made between the 03 June case and that of a two-year climatological average LLJ based on results from the operational 12-km horizontal resolution North American Mesoscale Forecast System. Observations show that the 03 June 2015 LLJ displayed the classic features of the LLJ including an inertial oscillation of the ageostrophic wind. Aircraft observations indicated that frictional decoupling occurred just prior to sunset. In contrast to climatological average case, the low-level horizontal pressure gradient actually increased during early morning hours for the 03 June case. Thus, the marked oscillation in the geostrophic wind that arises from the daytime heating and nighttime cooling of the sloping terrain found in the climatological average was seen to a lesser degree for the 03 June 2015 case.
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