Boundary Layer, Urban Meteorology and Land-Surface Processes

The Role of the Nocturnal Low-Level Jet in Convection Initiation on 1-2 June 2015

Joshua Gebauer

School of Meteorology

22 April 2016, 2:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

An important objective of the Plains Elevated Convection at Night (PECAN) project is to elucidate the role(s) of the nocturnal low-level jet (LLJ) in convection initiation (CI) over the southern Great Plains. Two similar cases of CI occurred on consecutive days during the early morning hours of 1 and 2 June 2015. In both cases convection was initiated in a north-south direction along the edge of a LLJ in a region without any obvious frontal boundary. Numerical models used by the PECAN forecasters consistently predicted CI on both occasions, but the mechanism for initiation was not that clear, so only low and moderate probabilities of CI were issued. Because of the forecasting challenge posed by the two events and the apparent association of the LLJ with the CI events, these cases were selected for a detailed analysis.
Rapid Refresh (RAP) model output and PECAN observations were used to examine the atmospheric conditions and evolution of the LLJ leading to this event. The RAP model was selected because it provided a clear signal for CI in eastern Kansas (the model actually over-predicted the spatial extent and intensity of the convection but was correct in the longitudinal placement of the CI). In addition, the LLJ predictions by the RAP were in reasonably good agreement with wind data collected with radiosondes and wind profilers at fixed observation sites. An investigation of RAP model forecast fields thus provided a convenient starting point for studying what appear to be subtle processes involved in the event.

During the time period of the CI events, an east-west temperature and moisture gradient was present across central Kansas. The LLJs that developed on the nights of 1 and 2 June were likely enhanced by baroclinicity caused by the sloping terrain as well as by a synoptic scale temperature gradient. This baroclinicity combined with an inertial oscillation resulted in veering of the LLJ which caused height-differential eastward moisture advection. This differential moisture advection along with a weak ascent caused by the convergence within horizontally heterogeneous LLJ flows led to saturation of the elevated layers of the CI regions, making the regions unstable and initiating convection. Future work will include further analysis of these preliminary findings and identifying the prevalence of the proposed mechanism for these two nocturnal CI cases.

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