Determination of the surface drag coefficient

L. Mahrt, D. Vickers, J.L. Sun, N.O. Jensen, Hans Ejsing Jørgensen, E. Pardyjak, H. Fernando

    Research output: Contribution to journalJournal articleResearchpeer-review

    Abstract

    This study examines the dependence of the surface drag coefficient on stability, wind speed, mesoscale modulation of the turbulent flux and method of calculation of the drag coefficient. Data sets over grassland, sparse grass, heather and two forest sites are analyzed. For significantly unstable conditions, the drag coefficient does not depend systematically on z/L but decreases with wind speed for fixed intervals of z/L, where L is the Obukhov length. Even though the drag coefficient for weak wind conditions is sensitive to the exact method of calculation and choice of averaging time, the decrease of the drag coefficient with wind speed occurs for all of the calculation methods. A classification of flux calculation methods is constructed, which unifies the most common previous approaches.

    The roughness length corresponding to the usual Monin-Obukhov stability functions decreases with increasing wind speed. This dependence on wind speed cannot be eliminated by adjusting the stability functions. If physical, the decrease of the roughness length with increasing wind speed might be due to the decreasing role of viscous effects and streamlining of the vegetation, although these effects cannot be isolated from existing atmospheric data.

    For weak winds, both the mean flow and the stress vector often meander significantly in response to mesoscale motions. The relationship between meandering of the stress and wind vectors is examined. For weak winds, the drag coefficient can be sensitive to the method of calculation, partly due to meandering of the stress vector.
    Original languageEnglish
    JournalBoundary-Layer Meteorology
    Volume99
    Issue number2
    Pages (from-to)249-276
    ISSN0006-8314
    DOIs
    Publication statusPublished - 2001

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