87th AMS Annual Meeting

Thursday, 18 January 2007: 11:30 AM
Dual-Doppler lidar measurements of flow over a suburban area
207B (Henry B. Gonzalez Convention Center)
Rob K. Newsom, PNNL, Richland, WA; and R. Calhoun, D. Ligon, and K. J. Allwine
Poster PDF (2.6 MB)
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 in the summer of 2003. This study focuses specifically on a ten hour 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 (CBD). 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 evening transition periods.

The retrieved velocity fields clearly show a tendency for turbulence structures to be elongated in the direction of the mean flow throughout the entire ten hour study period. As the stratification changed from unstable to weakly stable the turbulence structures became increasingly more linearly organized, and the spanwise separation between low speed “streaks” decreased. Estimates are given of the streamwise and spanwise dimensions of these linearly organized turbulent structures as a function of stability.

This study also investigates the response of the velocity fields to the CBD and surrounding suburban area. For neutral and weakly stable stratification the retrievals show the effects of blocking due to the high concentration of tall buildings within the CBD. The retrievals also show a reduction in the wind speed on the order of 10%, and a counterclockwise turning of the winds over a broad area extending several hundred meters upwind of the CBD. The observed deviation in the wind direction is consistent with a cyclonic rotation caused by increased drag over the CBD and surrounding suburban area.

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