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 U. S. southern Great Plains. During the early morning hours of 2 June 2015 convection was initiated over eastern Kansas along the edge of a LLJ in a region without any obvious frontal boundary. Although the numerical models used by the forecasters during PECAN consistently predicted CI in this region, the forecasters were not confident in the model 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 of the convection, but was correct in the longitudinal placement of much of the CI). An investigation of RAP forecast fields thus provided a convenient starting point for studying what appear to be subtle processes involved in the event. The RAP forecast indicated that slight isentropic lift and differential moisture advection caused by a veering with height LLJ destabilized the CI region. In addition, the convergence on the eastern flank of the LLJ increased as the LLJ veered with time which aided in initiating the convection.

The Weather Research and Forecasting (WRF) model was also run for this CI case. It provided better spatial and temporal resolution of the LLJ and CI than the RAP. Back trajectories were computed to determine how air parcels were lifted across the LLJ axis. In addition, due to the over-amplified CI forecast by the RAP, a boundary layer scheme sensitivity study was conducted to identify which scheme produced the most accurate LLJ and CI for this night