Joshua Gebauer-March 24th

Examining the Cause of Heterogeneous Great Plains Low-Level Jets

During PECAN, north-south lines of convection initiation (CI) occurred on nights without any obvious frontal boundary. These lines of CI were parallel to the LLJ axis and developed within LLJs which had a non-uniform depth and horizontal variations in wind speed. Eastward moisture advection at the top of these LLJs destabilized the CI region and convergence, which was caused by the heterogeneous structure of the LLJ, initiated the convection. Since the LLJ was the primary driver of these CI events, the cause of these heterogeneous LLJs needs to be determined. Our understanding of heterogeneous LLJs, however, is limited, as most theoretical and analytical studies of LLJs are simplified into a 1-D framework. Therefore, the approaches used in previous LLJ studies to examine the forcing and development of LLJs are unfit for examining the LLJs on the nights of these CI events.

By modifying the 1-D framework developed by Shapiro et al. (2016), it will be shown that horizontal buoyancy gradients along the slope are the primary cause of these heterogeneous LLJs. These buoyancy gradients result in vertical and horizontal variations in the geostrophic wind leading to variations in the inertial oscillation amplitude, and therefore, a heterogeneous LLJ develops. The presence of a buoyancy gradient also augments the daytime development of the convective boundary layer (CBL) across the Great Plains slope and creates a non-uniform CBL height. The non-uniform daytime CBL then leads to a non-uniform LLJ height. This non-uniformity in the LLJ height enhances the convergence at the top of the LLJ and provides more support for nocturnal convection.