Shallow Drainage Flows |
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| Authors: | L Mahrt Dean Vickers Reina Nakamura M R Soler Jielun Sun Sean Burns DH Lenschow |
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| Institution: | (1) College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA;(2) Astronomy and Meteorology, University of Barcelona, Barcelona, 08028, Spain;(3) National Center for Atmospheric Research, Boulder, CO 80307, USA |
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| Abstract: | Two-dimensional sonic anemometers and slowresponse thermistors were deployedacross a shallow gully during CASES99. Weak gully flow of a few tenths of m s-1 anda depth of a few metres develops in the earlyevening on most nights with clear skies.Flow down the gully developed sometimes evenwhen the opposing ambient wind exceeded10 m s-1 at the top of the60–m tower. Cold air drainage fromlarger-scale slopes flows over the top ofthe colder gully flow. The gully flowand other drainage flows are generally eliminated in the middle of the night in conjunctionwith flow acceleration abovethe surface inversion layer and downwardmixing of warmer air and highermomentum. As the flow decelerates later inthe night, the gully flow may re-form.The thin drainage flows decouple standard observational levels of3–10 m from the surface.Under such common conditions, eddy correlationflux measurements cannot be used toestimate surface fluxes nor even detect thethin gully and drainage flows. The gentlegully system in this field program is typical ofmuch of the Earth s land surface. |
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| Keywords: | Drainage flows Gravity winds Katabatic winds Nocturnal boundary layer Temperature minimum |
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