ESTIMATION OF SURFACE HEAT AND MOMENTUM FLUXES USING THE FLUX-VARIANCE METHOD ABOVE UNIFORM AND NONUNIFORM TERRAIN

Eddy-correlation measurements above an uneven-aged forest, a uniform-i rrigated bare soil field, and within a grass-covered forest clearing w ere used to investigate the usefulness of the flux-variance method abo ve uniform and non-uniform terrain. For this purpose, the Monin and Ob ukhov (1954) variance similarity functions were compared with direct m easurements. Such comparisons were in close agreement for momentum and heat but not for water vapor. Deviations between measured and predict ed similarity functions for water vapor were attributed to three facto rs: 1) the active role of temperature in surface-layer turbulence, 2) dissimilarity between sources and sinks of heat and water vapor at the ground surface, and 3) the non-uniformity in water vapor sources and sinks. It was demonstrated that the latter non-uniformity contributed to horizontal gradients that do not scale with the vertical flux. Thes e three factors resulted in a turbulence regime that appeared more eff icient in transporting heat than water vapor for the dynamic convectiv e sublayer but not for the dynamic sublayer. The agreement between edd y-correlation measured and flux-variance predicted sensible heat flux was better than that for latent heat flux at all three sites. The flux -variance method systematically overestimated the latent heat flux whe n compared to eddy-correlation measurements. It was demonstrated that the non-uniformity in water vapor sources reduced the surface flux whe n compared to an ''equivalent'' uniform terrain subjected to identical shear stress, sensible heat flux, and atmospheric water vapor varianc e. Finally, the correlation between the temperature and water vapor fl uctuations was related to the relative efficiency of surface-layer tur bulence in removing heat and water vapor. These relations were used to assess critical assumptions in the derivation of the flux-variance fo rmulation.