Effects of uncertainties in universal functions, roughness length, and displacement height on the calculation of surface layer fluxes

A sensitivity analysis of the turbulent momentum and sensible heat fluxes a s functions of wind speed and temperature gradients in the surface layer wa s made on the basis of Monin-Obukhov similarity theory. The sensitivity of parameterized fluxes to the choice of a number of universal functions found in the Literature was examined. The relative standard deviations of sensib le heat fluxes calculated using these universal functions are in the range of 10 to 20%. Differences between the minimum and maximum Values of the hea t fluxes generally are 30 to 50% of the mean values. Smaller differences ar e found in the calculation of momentum fluxes. These results demonstrate th e difficulty in defining an optimal universal function that would be applic able to all conditions. The uncertainties originating from the choice of wi nd and temperature roughness lengths and displacement height were also anal yzed. These uncertainties are very important when a complex surface is char acterized by single values of roughness length and displacement height. Amo ng the surface parameters, the largest error is caused by the uncertainty i n estimating the wind and temperature roughness lengths. In the parameteriz ation of the surface layer, the most frequently used model (that of BUSINGE R et al., 1971) gives roughly the same result for the turbulent diffusion c oefficient for sensible heat and the sensible heat flux as the mean of all the universal functions considered. However, the turbulent diffusion coeffi cient for the momentum and the momentum flux calculated using the BUSINGER et al. (1971) function gives substantially lower values than the average of the universal functions considered.