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

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. During the early morning hours of 2 June 2015, convection was initiated in a north-south line in eastern Kansas along the edge of a LLJ in a region without any obvious frontal boundary. Although the numerical models used by the forecasters in the PECAN operations center consistently predicted CI in this region, the forecasters were not confident in the predictions and only issued low and moderate probabilities of CI. Because of the forecasting challenge posed by this event and the apparent association of the LLJ with the CI, we have selected this case for a detailed analysis.

The Rapid Refresh (RAP) model output and PECAN observations were used to examine the atmospheric conditions and evolution of the LLJ in the PECAN domain leading to this event. The RAP model was selected because it provided a clear signal of CI occurrence in eastern Kansas (the RAP actually over-predicted the spatial extent and intensity of the convection, but was correct in the placement of much of the CI). In addition, the LLJ predictions by RAP were in reasonably good agreement with wind profiles derived from radar data by applying a velocity azimuth display technique and also with wind data collected with radiosondes and wind profilers at fixed PECAN sites. An investigation of RAP forecast fields thus provided a convenient starting point for studying what appear to be subtle processes involved in the event.

During the afternoon of 1 June 2015, an east-west temperature and moisture gradient was present across central Kansas. The moisture over western Kansas had been advected there by the previous night's LLJ and by persistent daytime southerly flow. The LLJs that developed during 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. During the night of 2 June, baroclinicity and inertial oscillation produced a strongly veering LLJ wind profile that caused eastward differential moisture advection. Due to slight isentropic ascent over the sloping terrain, this differential moisture advection saturated the region near the capping inversion at the CI location. Slight convergence at the eastern edge of the LLJ caused by the inertial oscillation provided the lifting mechanism for the convection. Data from the PECAN observations revealed that the RAP model significantly overestimated the moisture in the boundary layer in western Kansas, which was likely the cause of the amplified CI signal in the model. Future work will include further analysis of these preliminary findings and identifying the prevalence of this nocturnal CI mechanism.