Impact of Some Simplifying Assumptions in the New ECMWF Surface Scheme

Abstract

Two simplifying assumptions adopted in the current ECMWF surface scheme are explored: a uniform skin temperature for all grid-box fractions with variable latent heat release and a fixed value of an effective heat conductivity defining the soil heat flux density. This paper proposes relatively simple modifications of the ECMWF scheme with a better physical basis, without large input or computer infrastructure requirements. A uniform skin temperature overestimates evaporation from relatively wet surface fractions when the other surface components are dry and warm. This is shown to be the case for an evaporating soil after rain and vegetation evaporation in a sparse Mediterranean vineyard canopy. Allowing different temperatures for each surface fraction significantly reduces the overestimations and introduces only little additional computation. The default effective conductivity value (7 W m⁻²K⁻¹) employed by the current ECMWF scheme is shown to be too low for the sparse vineyard canopy. By raising the conductivity to 17 W m⁻² K⁻¹ for the bare-soil part of the surface, the daytime simulated soil heat flux was improved considerably.

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