Observations of Low-Level Jets during PECAN

Observations of the Great Plains low level jet on the nights of 10, 20, and 22 June 2015 during the Plains Elevated Convection At Night (PECAN) field experiment provide an opportunity for detailed investigation of the differences between weak (13 m/s), medium (23 m/s), and strong (33 m/s) jets, and the underlying structure and evolution of the nocturnal boundary layer (NBL). Measurements were obtained primarily by the Millersville University Atmospheric Research and Aerostat Facility (MARAF), an integrated observing system that was deployed in Ellis, KS (FP3), one of six sites making up the Fixed PECAN Integrated Sounding Array (Fixed-PISA; FP). MARAF is an integrated observing system that includes a rawinsonde system, tethered balloon system, flux tower, acoustic sodar with RASS extension, and lidar. During LLJ IOPs, radiosondes were launched every 30 minutes starting at 0000 UTC or earlier to capture the formation, evolution, and dissipation of the LLJ. While observations support the dominant characteristics of conceptual models (e.g. Blackadar and Holton) that reasonably describe the body forces and velocity field, significant variability across the synoptic and mesoscale domains ensure that no two LLJs are alike. Three case studies are presented here to illuminate the influence that local and regional gradients can have on the structure and behavior of the boundary layer winds.

The 10 June LLJ episode occurred under weak synoptic forcing with winds being driven by a pressure gradient only slightly enhanced by a trough to the west. This case exhibited many classic features associated with a pressure gradient force generated by differential heating over sloped terrain. Decoupling of the LLJ accompanied the growth of the inversion and led to a rapid reduction the frictional influence and an acceleration of the wind field. A non-uniform collapse of the BL stimulated undulations in the shallow nocturnal stable layer inducing an isallobaric velocity component that modified the characteristic inertial oscillation of the total ageostrophic velocity field, helping to maintain a weaker LLJ.

The 22 June LLJ was distinctly different. Occurring under stronger synoptic forcing with a stationary/warm front across northern Kansas, a dryline to the west, and a developing MCS in northern Nebraska, strong gradients and convergence persisted across Kansas during this IOP. The boundary layer remained deep and well-mixed, and the LLJ was elevated and strong (33 m/s) for the duration. The inertial turning was not as complex as the 10 June case since any isallobaric effects would essentially be overwhelmed by the stronger synoptic and mesoscale forcings. Richardson numbers belie sustained boundary layer mixing under conditions of weak dynamic stability over the entire episode. Observations of TKE and kinematic momentum and heat fluxes are evidence of this strong and sustained mixing.

The 20 June LLJ had characteristics of both and the results were a LLJ of moderate intensity. Isallobaric effects were evident in the early evening. The non-uniform collapse of the BL led to a rapid acceleration of the velocity field that is not described by conceptual models. In addition, considerable structure in the wind field developed later as the nocturnal stable layer deepened and the Richardson number exhibited a marked increase toward dynamic stability. Three case studies are investigated elucidate the key features of the LLJ under different forcing conditions. Over the 1.5 month duration of PECAN, 26 IOPs were conducted that addressed four key scientific objectives: MCS, bores, convective initiation, and LLJs. MARAF data are critical to each of the four scientific missions. Seven IOPS were dedicated specifically to LLJs or LLJs in conjunction with convective initiation. Analyses are ongoing on the remaining LLJ IOPs, and preliminary studies suggest a broad range of complex BL flows and features that characterize this recurring feature of the mesoscale environment of the summertime Great Plains.