Detection of structures in the horizontal wind field over complex terrain using coplanar Doppler lidar scans

Coplanar scans from three Doppler lidars are used to retrieve the horizontal wind field in a horizontal plane of about 5 km x 5 km in size above the city of Stuttgart in south-western Germany. Stuttgart is located in moderate mountainous terrain that is characterized by a basin-shaped valley (Stuttgart basin) which opens into the larger Neckar Valley. Using the retrieved horizontal wind field, which is available on 22 days with a temporal resolution of 1 min and a horizontal resolution of 100 m, we investigate the mesoscale structure of the horizontal flow in the valleys with respect to time of the day, stratification and wind above the mean ridge height, and determine how fast the cells in the convective boundary layer move downstream, i.e. we estimate the convection velocity. The measurements reveal a large spatial and temporal variability of the flow. During stable conditions, the flow below the mean ridge height is decoupled from the flow aloft and downvalley wind dominates in the valleys. At the opening of the Stuttgart basin into the Neckar Valley outflow dominates during nighttime, whereas inflow into the basin prevails in the early morning. During thermally unstable conditions the flow in the valleys is mainly coupled to the flow aloft with a preference for upvalley wind direction. Convective cells moving downstream are detected in the horizontal wind field and a method to estimate the convection velocity from the horizontal wind field measurements is presented. The mean convection velocity is found to be higher by 24 % than the mean horizontal wind speed at the same height and about similar to the wind speed 100 m further up.