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

The Great Plains Low-Level Jet (LLJ) has been a topic of study for the past 50 years. There has been considerable scientific debate as to the dynamical mechanisms that initiate, organize, and maintain low-level jet formation. Two main theories have been proposed regarding the low-level jet. First, it has been suggested that the LLJ occurs as a result of an inertial oscillation of the ageostrophic wind about a geostrophic base state as the boundary layer cools and decouples (Blackadar 1957). Second, diurnal variations in the heating and cooling of the sloping terrain allow for a pressure gradient to develop that is necessary for the low-level jet formation (Holton 1967).

Observations as part of the Plains Elevated Convection at Night (PECAN) experiment (1 June-15 July 2015) have allowed an examination of the thermodynamic and dynamic structure of the LLJ. Here we focus on data collected by the University of Wyoming King Air research aircraft (UWKA) to detect the dynamics of the LLJ environment. The UWKA conducted two LLJ mission 4.6-hour flights on 10 June 2015 beginning shortly after 0000 UTC. Aircraft sampling strategy included a series of vertical sawtooth maneuvers between 200-1000 m AGL and isobaric legs along a 125-km geographically-fixed track. Sawtooth maneuvers allow a large volume of the atmosphere to be sampled and thus cross-sectional variations in wind and temperature can be monitored. In addition, differential GPS processing allows the vertical structure of the horizontal pressure gradient to be examined and hence the thermal wind can be diagnosed. The jet was quite shallow with maximum wind speeds near 200 m AGL for the entirety of the mission, but moderate in intensity and increasing in strength as the night progressed. Maximum LLJ winds reached about 20 m s^-1.

A series of numerical simulations of the 10 June 2015 case study LLJ have been made using the Weather Research and Forecasting (WRF) Model. All simulations consisted of a 24-hour simulation from 1200 UTC 9 June to 1200 UTC 10 June 2015. Initialization was completed using either the 0.5- km GFS and 12 km NAM grids. The overall goal of the WRF modeling was to document the dynamics of the jet to allow comparison with the airborne observations.

The purpose of this research is to describe how the temperature field affects the pressure field. Emphasis will be placed on the transition of the jet and the onset of the inertial oscillation. Analyses of the dynamics incorporate D-values to examine the vertical structure of the horizontal pressure gradient.