Local Structure of the Convective Boundary Layer from a Volume-Imaging Radar

Abstract

The local structure and evolution of the convective boundary layer (CBL) are studied through measurements obtained with a volume-imaging radar, the turbulent eddy profiler (TEP). TEP has the unique ability to image the temporal and spatial evolution of both the velocity field and the local refractive index structure-function parameter, C?2n. Volumetric images consisting of several thousand pixels are typically formed in as little as 1 s. Spatial resolutions are approximately 30 m by 30 m by 30 m. CBL data obtained during an August 1996 deployment at Rocks Springs, Pennsylvania, are presented. Measurements of the vertical C?2n profile are shown, exhibiting the well-known bright band near the capping inversion at zi, as well as intermittent plumes of high C?2n. Horizontal profiles show coherent 100-m-scale C?2n and vertical velocity (w) structures that correspond to converging horizontal velocity vectors. To quantify the scales of structures, the vertical and streamwise horizontal correlation distances are calculated within the TEP field of view. To study the statistics and scales of larger structures, effective volumes larger than the TEP field of view are constructed through Taylor?s hypothesis. Statistics of C?2n and w time series are compared to an appropriately scaled large eddy simulation (LES). While w time series comparisons agree very well, the LES C?2n predictions agree only with some of the measured data. Finally, the scales of C?2n structures in the TEP time series measurements are calculated and compared to the scales in the LES spatial domain. Good agreement is found only near the capping inversion layer, the area of largest structures. This study highlights the unique capabilities of the TEP instrument, and shows what are believed to be the first statistical comparisons of measured C?2n data with LES derived results.