Examining Common Features of the Low-Level Jet during PECAN

During the Plains Elevated Convection at Night (PECAN) field campaign, four intensive operation periods (IOPs) were conducted to capture the structure and evolution of the low-level jet (LLJ) with an unprecedented array of profiling instrumentation. Out of the four IOPs, three of the nights (June 3, June 20, June 22) had a veering with height LLJ with wind speeds exceeding 25 m s^-1. On the other night (June 10), the LLJ developed more slowly, was shallower, and had a more uniform wind direction. Despite these differences, the strength of all four LLJs was enhanced by a synoptic-scale, slope-following buoyancy gradient, not to be confused with the typical horizontal temperature gradient associated with the sloped terrain. To identify if these PECAN LLJs developed in a unique environment, a climatology of buoyancy gradients over the region was created using 42 years of radiosonde data. This climatology reveals that these synoptic-scale buoyancy gradients are a persistent feature over the region, and therefore need to be accounted for in LLJ theories. Possible explanations for the cause of this persistent buoyancy gradient include land surface effects and also a climatological west-to-east soil moisture gradient across the Great Plains. Another interesting finding was that the most significant temperature advection in the PECAN LLJs was not associated with the southerly wind maxima. Instead, the westerly winds at the top of the LLJ caused eastward potential temperature advection since the LLJ major axis was aligned parallel to the potential temperature gradient. The veering with height structure which caused the advection may be a product of the inertial oscillation, as analytical experiments of inertial oscillation of Ekman boundary layer produce a similar structure. While no convection occurred on these nights, other studies have found that eastward advection at the top of the LLJ can create conditions favorable for convection, and therefore awareness of these feature is paramount for improving nocturnal convection forecasts.