A case study of the nocturnal boundary layer over a complex terrain

A case study of the structure of the nocturnal boundary layer (NBL) over complex terrain is presented. Observations were made during the third night of Project STABLE (Weber and Kurzeja, 1991), whose main goal was to study turbulence and diffusion over the complex terrain of the Savannah River Site (SRS) near Augusta, Georgia.The passage of a mesoscale phenomenon, defined as a turbulent meso-flow (TMF) with an explanation of the nomenclature used, and a composite structure of the lowest few hundred meters over complex terrain are presented. The spatial extent of the TMF was at least 30–50 km, but the forcing is not well understood. The TMF occurred without the presence of a synoptic-scale cold front, under clear conditions, and with no discernible discontinuity in a microbarograph pressure trace. The structure of the NBL over the complex terrain at SRS differed from the expected homogeneous terrain NBL. The vertical structure exhibited dual low level wind maxima, dual inversions, and a persistent elevated turbulent layer.The persistent elevated turbulent layer, with a spatial extent of at least 30 km, was observed for the entire night. The persistent adiabatic layer may have resulted from turbulence induced by shear instability.