Examining the Nocturnal Stable Boundary Layer and Low-Level Jet during PECAN

PECAN aims to advance the understanding of continental nocturnal warm-season precipitation with a focus on conditions with a nocturnal low-level jet (LLJ) and stable boundary layer (NSBL). A unique aspect of the experimental design was the integration of a wide variety of profiling systems into a fixed and mobile PECAN Integrated Sounding Array (PISA) across northern Oklahoma, central Kansas, and south-central Nebraska. This paper reports the preliminary findings obtained at Fixed-PISA 3 (FP3). Measurements were obtained by the Millersville University Atmospheric Research and Aerostat Facility, which consists of the Vaisala MW41 rawinsonde system, acoustic SODAR with RASS extension, MPL-111 micropulse LIDAR, and 10 m flux tower. Data from FP3, supported by those from other fixed and mobile PISAs, allow for a detailed characterization of the structure and evolution of the boundary layer from late afternoon through early morning, including the development of the LLJ. Three case studies of the Great Plains LLJ will be presented that highlights differences between weak (13 m/s), moderate (23 m/s), and strong (31 m/s) LLJs, and the underlying structure and evolution of the nocturnal boundary layer. Preliminary results suggest that LLJs formed solely by differential heating over sloped terrain are relatively weak and allow for the initiation and growth of a substantial NSBL because momentum transfer to the surface is proportionally weaker. Conversely, relatively strong LLJs are synoptically-forced and occur in the absence of a robust NSBL.