Abstract
This study examines the dependence of the surface drag coefficienton 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–Obukhovstability 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 effectsand 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.
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Andreas, E. andClaffey, K.: 1995, ‘Air-Ice Drag Coefficients in the Western Weddell Sea. I. Values Deduced from Profile Measurements’ J. Geophys. Res. 100, 4821-4831.
Beljaars, A. C.: 1995, ‘The Parametrization of Surface Fluxes in Large Scale Models under Free Convection’ Quart. J. Roy. Meteorol. Soc. 121, 255-270.
Blanken, P. D.,Black, T. A.,Neumann, H. H.,den Hartog, G.,Yang, P. C.,Nesic, Z.,Staebler, R.,Chen, W., andNovak, M. D.: 1998, ‘Turbulent Flux Measurements above and below the Overstory of a Boreal Aspen Forest’ Boundary-Layer Meteorol. 89, 109-140.
Bosveld, F.: 1997, ‘Derivation of Fluxes from Profiles over a Moderately Homogeneous Forest’ Boundary-Layer Meteorol. 84, 289-327.
Brunet, Y.,Finnigan, J. J., andRaupach, M. R.: 1994, ‘A Wind Tunnel Study of Air Flow in Waving Wheat: Single-Point Velocity Statistics’ Boundary-Layer Meteorol. 70, 95-132.
Businger, J. A.: 1973, ‘A Note on Free Convection’ Boundary-Layer Meteorol. 4, 322-326.
Deacon, E.: 1957, ‘Wind Profiles and the Shearing Stress-An Anomaly Resolved’ Quart. J. Roy. Meteorol. Soc. 537-540.
Donelan, M.: 1990, ‘Air-Sea Interaction’ in B. Le Mehaute andD. M. Hanes (eds.), Ocean Engineering Science, John Wiley and Sons, pp. 239-291.
Edson, J.,McGillis, W., andFrew, N.: 1998, ‘Atmospheric Forcing and Energy Exchange during the 1997 COPE Gas Exchange Experimen t’ in Thirteenth Symposium on Boundary Layers and Turbulence, Dallas, TX, Amer. Meteorol. Soc. pp. 351-354.
Fairall, C. W.,Bradley, E. F.,Rogers, D. P.,Edson, J. B., andYoung, G. S.: 1996, ‘Bulk Parameterization of Air-Sea Fluxes for Tropical Ocean-Global Atmosphere Coupled-Ocean Atmosphere Response Experiment’ J. Geophys. Res. 101, 3747-3764.
Grachev, A.,Fairall, C., andLarsen, S.: 1998, ‘On the Determination of the Neutral Drag Coefficient in the Convective Boundary Layer’ Boundary-Layer Meteorol. 86, 257-278.
Godfrey, J. S. andBeljaars, A. C. M.: 1991, ‘On the Turbulent Fluxes of Buoyancy, Heat and Moisture at the Air-Sea Interface at Low Wind Speeds’ J. Geophys. Res. 96, 22,043-22,048.
Högström, U.: 1990, ‘Analysis of Turbulence Structure in the Surface Layer with a Modified Similarity Formulation for Near Neutral Conditions’ J. Atmos. Sci. 47, 1949-1972.
Howell, J. andMahrt, L.: 1997, ‘Multiresolution Flux Decomposition’ Boundary-Layer Meteorol. 83, 117-137.
Howell, J. andSun, J.: 1999, ‘Surface Layer Fluxes in Stable Conditions’ Boundary-Layer Meteorol.. 90, 495-520.
Inoue, J.: 1989, ‘Surface Drag over the Snow Surface of the Antarctic Plateau. 1. Factors Controlling Surface Drag over the Katabatic Wind Region’ J. Geophys. Res. 94, 2207-2217.
Jensen, N. O. andHummelshøj, P.: 1995, ‘Derivation of Canopy Resistance forWater Vapour Fluxes over a Spruce Forest, Using a New Technique for the Viscous Sublayer Resistance’ Agric. For. Meteorol. 73, 339-352.
Johnson, R.,Ramey, G., andO'Hagan, D.: 1982, ‘Wind Induced Forces on Trees’ J. Fluids Eng. 104, 25-30.
Katul, G. and Brani Vidakovic, 1996: ‘The Partitioning of Attached and Detached Eddy Motion in the Atmospheric Surface Layer Using LorentzWavelet Filtering’ Boundary-Layer Meteorol. 77, 153-172.
Kerzenmacher, T. andGardiner, B.: 1998, ‘A Mathematical Model to Describe the Dynamic Response of a Spruce Tree to the Wind’ Trees 12, 385-394.
Lenschow, D. H.,Mann, J., andKristensen, L.: 1994, ‘How Long Is Long Enough When Measuring Fluxes and Other Turbulence Statistics?’ J. Atmos. Oceanic Tech. 11, 661-673.
Mahrt, L.: 1998, ‘Flux Sampling Strategy for Aircraft and Tower Observations’ J. Atmos. Oceanic. Tech. 15, 416-429.
Mahrt, L. andSun, J.: 1995, ‘Multiple Velocity Scales in the Bulk Aerodynamic Relationship for Spatially Averaged Fluxes, Mon. Wea. Rev. 123, 3032-3041.
Mahrt, L,Lee, X.,Black, A.,Neuman, H., and R. Staebler: 2000, ‘Nocturnal Mixing in a Forest Subcanopy’ Agric. For. Meteorol. 101, 67-78.
Mahrt, L.,Sun, J.,Blumen, W.,Delany, A.,McClean, G., andOncley, S.: 1998, ‘Nocturnal Boundary-Layer Regimes’ Boundary-Layer Meteorol. 88, 255-278.
Mahrt, L.,Vickers, D.,Howell, J.,Edson, J.,Hare, J.,Højstrup, J., andWilczak, J.: 1996, ‘Sea Surface Drag Coefficients in RASEX’ J. Geo. Res., Oceans 101, 14,327-14,335.
Mann, J. andLenschow, D. H.: 1994, ‘Errors in Airborne Flux Measurements’ J. Geophys. Res. 99, 14,519-14,526.
Mayhead, G.: 1973, ‘Some Drag Coefficients for British Forest Trees Derived from Wind Tunnel Studies’ Agric. Meteorol. 12, 123-130.
Mourad, P. D. andBrown, R. A.: 1990, ‘Multiscale Large Eddy States inWeakly Stratified Planetary Boundary Layers’ J. Atmos. Sci. 47, 414-438.
Oncley, S. P.,Friehe, C. A.,LaRue, J. C.,Businger, J. A.,Itsweire, E. C., andChang, S. S.: 1996, ‘Surface-Layer Fluxes, Profiles, and Turbulence Measurements over Uniform Terrain under Near-Neutral Conditions’ J. Atmos. Sci. 53, 1029-1044.
Paulson, C. A.: 1970, ‘The Mathematical Representation ofWind Speed and Temperature Profiles in the Unstable Atmospheric Surface Layer’ J. Appl. Meteorol. 9, 857-861.
Pilegaard, K.Hummelshøj, P.,Jensen, N. O., andChen, Z.: 2001, ‘Two Years of Continuous CO2 Eddy-Flux Measurements over a Danish Beech Forest’ Agric. For. Meteorol. 107, 29-41.
Raupach, M. R. andThom, A. S.: 1981, ‘Turbulence in and above Plant Canopies’ Ann. Rev. Fluid Mech. 13, 97-129.
Schuepp, P.: 1993, ‘Leaf Boundary Layers’ New Phytol. 125, 477-507.
Smedman, A.-S.: 1988, ‘Observations of a Multi-Level Turbulence Structure in a Very Stable Atmospheric Boundary Layer’ Boundary-Layer Meteorol. 44, 231-253.
Sun, J.: 1999, ‘Diurnal Variation of Thermal Roughness Height over a Grassland’ Boundary-Layer Meteorol. 92, 407-427.
Sun, J.,Howell, J.,Esbensen, S. K.,Mahrt, L.,Greb, C. M.,Grossman, R., andLeMone, M. A.: 1995,’ Scale Dependence of Air-Sea Fluxes over the Western Equatorial Pacific’ J. Atmos. Sci. 53, 2997-3012.
Vickers, D. andMahrt, L.: 1997, ‘Quality Control and Flux Sampling Problems for Tower and Aircraft Data’ J. Atmos. Oceanic Tech. 14, 512-526.
Wyngaard, J.: 1981, ‘The Effects of Probe-Induced Flow Distortion on Atmospheric Turbulence Measurements’ J. Appl. Meteorol. 20, 784-794.
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Mahrt, L., Vickers, D., Sun, J. et al. Determination Of The Surface Drag Coefficient. Boundary-Layer Meteorology 99, 249–276 (2001). https://doi.org/10.1023/A:1018915228170
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DOI: https://doi.org/10.1023/A:1018915228170