Dual-doppler lidar measurements of flow over a suburban area

This study presents an analysis dual-Doppler lidar data acquired during the JU2003 field program. Dual-Doppler observations were processed to reveal the structure and evolution of the horizontal velocity field in the surface layer upstream of the Oklahoma City CBD. The retrieved velocity fields clearly show a tendency for turbulence structures to be elongated in the direction of the mean flow. This tendency for linear organization became more apparent as the stratification changed from unstable to weakly stable. The transverse separation between low speed regions was estimated to be 1.4 km in the shear driven convective boundary layer, and about 500 m in the weakly stable case. Future work will include estimation of the streamwise extent of these structures. This information can be used to determine the aspect ratio and how that ratio changes with stability. This study also investigated the response of the mean upstream flow to the CBD. In the unstable case the dual-Doppler retrievals show a decrease in wind speed from the southern most point in the dual-Doppler over lap area toward the CBD. The slowing is most abrupt within 1.0 km upstream of the CBD. Within this range the wind speed decreases by roughly 10%. In the weakly stable case the slowing is more intense and more concentrated immediately upstream of the CBD. Within 1 km immediately upstream of the CBD the wind speed decreased by approximately 25 %. The retrievals also indicate a channeling of the flow around the CBD in the weakly stable case. This effect is not observed in the unstable case. The wind retrievals indicate a significant counterclockwise rotation in the wind direction over a broad area extending upwind of the CBD. It is suggested that this rotation is caused by a reduction in the Coriolis acceleration over the CBD and surrounding suburban area as a result of increased drag. A preliminary analysis of independent measurements appears to confirm these observations. However, a more comprehensive investigation should be performed to determine if this effect is observed on other days. Future work will incorporate observations from a network of sodars deployed at varying distances from the CBD (Berg et al. 2004).