Vertical Structure of Cold Drainage Flow Revealed by Drone Observations

The drainage flow in a valley was observed with meteorological sensors attached on compact drones. We first developed a wind measurement system by mounting an ultrasonic anemometer on a drone and evaluated its performance by comparing the data with those from wind measurements on a meteorological tower and on a pole. With this system, the vertical structure of drainage flow was observed through multiple observation campaigns. By compiling 35 profiles in total, the statistical features of their vertical profiles in temperature, humidity and horizontal winds have been derived. It is found that the flow is characterized by a strong temperature inversion layer with positive humidity anomaly and that the wind profile takes a parabolic shape, with the wind speed taking its maximum around at the center of the layer. This shape seems to agree with the previous findings from measurements at a relatively deep valley while differing from those on a simple plane terrain.