Retrieval of Urban Boundary Layer Structures from Doppler Lidar Data. Part I: Accuracy Assessment

Abstract

Two coherent Doppler lidars from the U.S. Army Research Laboratory (ARL) and Arizona State University (ASU) were deployed in the Joint Urban 2003 atmospheric dispersion field experiment (JU2003) held in Oklahoma City, Oklahoma. The dual-lidar data were used to evaluate the accuracy of a four-dimensional variational data assimilation (4DVAR) method and to identify the coherent flow structures in the urban boundary layer. The objectives of the study are threefold. The first objective is to examine the effect of eddy viscosity models on the quality of retrieved velocity data. The second objective is to determine the fidelity of single-lidar 4DVAR and evaluate the difference between single- and dual-lidar retrievals. The third objective is to inspect flow structures above some geospatial features on the land surface. It is found that the approach of treating eddy viscosity as part of the control variables yields better results than the approach of prescribing viscosity. The ARL single-lidar 4DVAR is able to retrieve radial velocity fields with an accuracy of 98% in the along-beam direction and 80%?90% in the cross-beam direction. For the dual-lidar 4DVAR, the accuracy of retrieved radial velocity in the ARL cross-beam direction improves to 90%?94% of the ASU radial velocity data. By using the dual-lidar-retrieved data as a reference, the single-lidar 4DVAR is able to recover fluctuating velocity fields with 70%?80% accuracy in the along-beam direction and 60%?70% accuracy in the cross-beam direction. Large-scale convective roll structures are found in the vicinity of the downtown airport and parks. Vortical structures are identified near the business district. Strong up- and downdrafts are also found above a cluster of restaurants.