November Colloquium

The nocturnal low-level jet (LLJ) is a common atmospheric boundary layer feature over the U.S. Great Plains. LLJs play important roles in the transport of moisture and air pollutants, and have implications for the regional climate, nocturnal convection, and wind-resource assessments. In this study, data sets from three field campaigns are analyzed to investigate characteristics of the nocturnal boundary layer and their interdependencies with LLJ evolution. The field campaigns considered are the Joint Urban (2003) experiment in Oklahoma City, the Lower Atmospheric Boundary Layer Experiment (LABLE, 2012-2013), conducted in northern Oklahoma, and the Plains Elevated Convection at Night (PECAN; 2015) experiment that targeted central Kansas. The observations are supplemented by numerical simulations. The LLJ strength and turbulent mixing in the nocturnal boundary layer are both found to increase with the geostrophic forcing. During nights with the strongest LLJs, turbulent mixing persists after sunset in the nocturnal boundary layer, and a strong surface temperature inversion does not develop. The strongest increase in LLJ speed relative to the daytime wind speed often occurs when the geostrophic forcing is weak and thermally-induced turbulence in the mixed layer is strong. Numerical tests confirm that weakening of turbulent mixing during the early evening transition is critical for the LLJ formation.