Mean structure of the nocturnal boundary layer under strong and weak wind conditions: EPRI case study

Abstract

The major objective of this study was to analyze the mean structure and evolution of the nocturnal boundary layer (NBL) under strong and weak wind conditions. Meteorological data collected during the plume-validation experiment conducted by the Electric Power Research Institute (EPRI) over a flat homogeneous terrain at Kincaid, Illinois (39°35′N, 89°25′W), were utilized. A one-dimensional meteorological boundary layer model originally developed by R. A. Pielke, modified with turbulent kinetic energy mixing-length closure, a layer-by-layer emissivity-based radiation scheme, and nonlinear nondimensional temperature and wind profiles in the surface layer, was used. In the four cases that were considered, ranging from strong to weak geostrophic forcing, the model reproduced the observed mean profiles, their evolutions in the NBL, and the inertial oscillations reasonably well. The NBL developed into three layers wherein 1) very close to the surface, radiative cooling dominated over turbulence cooling; 2) a layer above, turbulent cooling was the dominant mechanism; and 3) near the top of the turbulent layer and above, clear-air radiative cooling was the dominating mechanism. However, depending on the geostrophic wind, the structure of these layers varied from one situation to another. The wind maximum, which was at least above 200 m of altitude under windy conditions, was located at an altitude of less than 100 m for the weak-wind case, probably because of weaker diffusion in the boundary layer during transition.