Retrieval of convective boundary layer wind field statistics from radar profiler measurements in conjunction with large eddy simulation

Abstract

The daytime convective boundary layer (CBL) is characterized by strong turbulence that is primarily forced by buoyancy transport from the heated underlying surface. The present study focuses on an example of flow structure of the CBL as observed in the U.S. Great Plains on June 8, 2007. The considered CBL flow has been reproduced using a numerical large eddy simulation (LES), sampled with an LES-based virtual boundary layer radar (BLR), and probed with an actual operational radar profiler. The LES-generated CBL flow data are then ingested by the virtual BLR and treated as a proxy for prevailing atmospheric conditions. The mean flow and turbulence parameters retrieved via each technique (actual radar profiler, virtual BLR, and LES) have been cross-analyzed and reasonable agreement was found between the CBL wind parameters obtained from the LES and those measured by the actual radar. Averaged vertical velocity variance estimates from the virtual and actual BLRs were compared with estimates calculated from the LES for different periods of time. There is good agreement in the estimates from all three sources. Also, values of the vertical velocity skewness retrieved by all three techniques have been inter-compared as a function of height for different stages of the CBL evolution, showing fair agreement with each other. All three retrievals contain positively skewed vertical velocity structure throughout the main portion of the CBL. Radar estimates of the turbulence kinetic energy (eddy) dissipation rate (ε) have been obtained based on the Doppler spectral width of the returned signal for the vertical radar beam. The radar estimates were averaged over time in the same fashion as the LES output data. The agreement between estimates was generally good, especially within the mixing layer. Discrepancies observed above the inversion layer may be explained by a weak turbulence signal in particular flow configurations. The virtual BLR produces voltage measurements consistent with the LES data fields. First-, second-, and third-order statistics (mean wind, variance, and skewness) of vertical velocity obtained from BLR output demonstrate its suitability for validating radar-profiler signal processing algorithms.