Shallow drainage flows

Two-dimensional sonic anemometers and slow response thermistors were deployed across a shallow gully during CASES99. Weak gully flow of a few tenths of 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 colder gully flow. The gully flow and other drainage flows are generally eliminated in the middle of the night in conjunction with flow acceleration above the surface inversion layer and downward mixing of warmer air and higher momentum. 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 from the surface. Under such common conditions, eddy correlation flux measurements cannot be used to estimate surface fluxes nor even detect the thin gully and drainage flows. The gentle gully system in this field program is typical of much of the Earth's land surface.