Linearly Organized Turbulence Structures Observed Over a Suburban Area by Dual-Doppler Lidar

Dual-Doppler lidar observations are used to investigate the structure and evolution of surface-layer flow over a suburban area. The observations were made during the Joint Urban 2003 (JU2003) field experiment in Oklahoma City, U.S.A. in the summer of 2003. This study focuses specifically on a 10-h sequence of scan data beginning shortly after noon local time on 7 July 2003. During this period two coherent Doppler lidars performed overlapping low elevation angle sector scans upwind and south of Oklahoma City’s central business district. Radial velocity data from the two lidars are processed to reveal the structure and evolution of the horizontal velocity field in the surface layer throughout the afternoon and during the evening transition period. The retrieved velocity fields clearly show a tendency for turbulence structures to be elongated in the direction of the mean flow throughout the entire 10-h study period. In order to quantify the observed anisotropy and its dependence on stability, integral length scales are estimated directly from the spatially resolved velocity retrievals. As the flow became more stably stratified the characteristic cross-stream dimension of the linear structures decreased. The streamwise component was consistently more anisotropic than the cross-stream component, and both velocity components exhibited maximum anisotropy under neutral conditions. The ratio of the streamwise to cross-stream length scale was estimated to be about eight for the streamwise component, and four for the cross-stream component under neutral conditions.

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