An Investigation of Mountain-Valley Circulations Based on Wind Tracer Measurements

This paper presents the results of a field program investigating the mesoscale structure of wind and temperature fields in mountainous terrain. A Doppler lidar system and a number of portable automated weather stations were deployed along a mountain slope. The instruments enhance a network of permanent surface-based meteorological sensors that are located at mountain basin and ridge-top locations. The Doppler lidar was configured for horizontal sweeps at fixed elevation angles and vertical sweeps along a fixed azimuth angle that coincided with a line of surface measurement sensors. Analyses of remotely sensed radial velocities are combined with surface data to provide information for selected cases for a nine-day period of study. These display interactions between ambient winds over the mountain ridge top, boundary layer circulations, and terrain-forced flows. The results show the mountain-valley circulations are characterized by relatively uniform flow, except for near-surface winds, with weak subsidence during the nighttime hours for levels up to 4 km measured by the lidar system. One-minute temperature fluctuations on the order of ±3oC are observed and are associated with weak wind direction variations at the 2-m level. Daytime vertical motions measured by the lidar system display large temporal variations at 20 minute time periods and exhibit coherent structure at 100-m intervals and levels between 400 m and 4 km during the mid-afternoon hours. The afternoon circulations suggest the presence of well-developed convective boundary layer at the base of the mountain slope with thermal structures that exhibit no significant tilt with height.