GCM Grid-Scale Evaporation from Mesoscale Modeling

This paper investigates the problem of aggregation of land surface properties in a large area comparable in size with a model grid box of a GCM. The study is based on 3D numerical results obtained for atmospheric situations encountered during HAPEX-MOBILHY 1986-that is, moderate plant water stress at the beginning of summer. Estimating effective surface properties describing the spatial distribution of the vegetation and the soil texture within the large area under study is proposed as a first guess for accounting for spatial variability. Despite the nonlinear dependence of surface fluxes on both vegetation and soil water content, it is found that the effective surface fluxes computed from effective parameters with a ID column model match the areal-averaged fluxes estimated from 3D mesoscale model results with a relative error less than 10%. On the other hand, fluxes computed with prescribed surface properties associated with the dominant land use of the large domain depart significantly from the averaged fluxes. For the cases examined, the effects of nonlinearity are found to be smaller for the vegetation behavior than for the soil water transfers. The parameter aggregation method has been tested successfully for a long lime period within the context of a ID GCM grid cell representing the HAPEX-MOBILHY 1986 instrumented area. Given precipitation and solar radiation fluxes, predictions of soil water content and total evaporation for 25 days compare well with aggregated observations within the large area.