Shallow drainage flows

Two-dimensional sonic anemometers and slow response thermistors were deploy ed across a shallow gully during CASES99. Weak gully flow of a few tenths o f m s(-1) and a depth of a few metres develops in the early evening on most nights with clear skies. Flow down the gully developed sometimes even when the opposing ambient wind exceeded 10 m s(-1) at the top of the 60-m tower . Cold air drainage from larger-scale slopes flows over the top of the cold er gully flow. The gully flow and other drainage flows are generally elimin ated in the middle of the night in conjunction with flow acceleration above the surface inversion layer and downward mixing of warmer air and higher m omentum. As the flow decelerates later in the night, the gully flow may re- form. The thin drainage flows decouple standard observational levels of 3-10 m fr om the surface. Under such common conditions, eddy correlation flux measure ments cannot be used to estimate surface fluxes nor even detect the thin gu lly and drainage flows. The gentle gully system in this field program is ty pical of much of the Earth's land surface.