EFFECTIVE RESISTANCE TO SENSIBLE-HEAT AND LATENT-HEAT FLUX IN HETEROGENEOUS TERRAIN

Numerical-weather-prediction models resolve mean variables at the scale of their grid length only. Effective surface parameters must, therefore, be used to relate the mean properties of humidity, temperature and wind speed to the mean surface fluxes. The effective parameters can be calculated using a model with a finer grid. In this paper effective aerodynamic and surface resistances to sensible- and latent-heat flux for a surface which varies over a horizontal scale of one kilometre are calculated from a two-dimensional model with a grid size of 30 m, and compared with simpler averaging techniques. The averaging techniques considered include taking the mean of the aerodynamic and surface resistances in parallel or in series, the mean of the solutions given by these two, and a heuristic model that treats the different surface covers independently but assumes that they are exposed to the same mean meteorological variables at a blending height. The heuristic model consistently estimates the effective resistances to within 25% but, in comparison to the other methods, requires more information. For heterogeneous surfaces, the effective parameters for momentum and sensible- and latent-heat transfer do not bear the same relationship to each other as they do for homogeneous surfaces. Effective parameters for each type of flux, therefore, have to be calculated separately. In situations where negative sensible-heat fluxes occur over rough ground, the effective resistance to sensible-heat flux can be very large, becoming undefined in the extreme case where the area-average sensible-heat flux is zero.