Surface layer processes and nocturnal low-level jet development--an observational study during PECAN

Abstract

Low-level jets (LLJ) occur in many regions around the world and exhibit a diverse range of impacts across a variety of climate and weather-related applications, including U.S. Department of Defense assets and operations. A team from the Naval Postgraduate School participated in the 2015 Plains Elevated Convection at Night (PECAN) research project and collected high-resolution stable boundary layer data as it evolved through the night. The objective of this study was to use this dataset to identify the impact of LLJ presence on surface layer properties, such as thermal stability, dynamic stability, surface fluxes, and turbulence. Additionally, this study investigated pre-LLJ daytime surface layer conditions that might promote LLJ development and intensification. The subsequent analysis found that both nocturnal thermal stability and dynamic stability, while in the presence of a LLJ, were only marginally stable, a result consistent with previous literature that related LLJ development primarily to boundary layer properties above the surface layer. This study also found that nocturnal surface fluxes of momentum, sensible heat, and latent heat were significantly enhanced during LLJ events, owing mostly to larger-than-normal generation of shear-driven turbulence. Interestingly, this research also found that the presence of the nocturnal LLJ was highly correlated with values of thermal and dynamic stability that were close to neutral. This result appears to be inconsistent with previous literature in that LLJ presence is suggested to occur under clear, cloud-free conditions; this finding warrants further analysis. Finally, after examining the relationship between daytime turbulence and subsequent LLJ presence, jet development appeared to be well-correlated to higher levels of turbulence during the preceding daytime.