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Dependence of surface exchange coefficients on averaging scale and grid size
Author(s) -
Mahrt L.,
Sun Jielun
Publication year - 1995
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49712152803
Subject(s) - turbulence , mesoscale meteorology , scale (ratio) , flow (mathematics) , momentum (technical analysis) , grid , drag coefficient , statistical physics , flux (metallurgy) , mechanics , physics , mathematics , meteorology , drag , materials science , geometry , finance , quantum mechanics , economics , metallurgy
The value of the effective exchange coefficient for area‐averaged fluxes can depend significantly on the averaging scale. This dependence implies that the exchange coefficient in numerical models should depend on grid size. the main goal of this study is the assessment of the importance of such scale‐dependence. When the large‐scale flow is weak, the mesoscale motions on scales smaller than the averaging width (grid size) may generate significant turbulence which is not represented by the area‐averaged wind vector (resolved flow). In this case, the effective exchange coefficient must be larger than that predicted by similarity theory in order to predict the grid‐averaged turbulent flux. This study reviews different approaches for representation of spatially‐averaged surface fluxes over heterogeneous surfaces. the scale‐dependence of the effective exchange coefficient is posed in terms of spatially averaging the bulk aerodynamic relationship. This scale‐dependence is evaluated in terms of observed fluxes from three different field programs. the effective drag coefficient is found to be more scale‐dependent than the effective exchange coefficients for heat and momentum. the effective conductance is found to be less scale‐dependent than the effective exchange coefficient and effective resistance. Except for the case of weak flow over strong surface heterogeneity, the exchange coefficient is nearly independent of grid size.