The sensitivity of winter evaporation to the formulation of aerodynamic resistance in the ECMWF model

In atmospheric models, the roughness length for momentum, heat and moisture are often taken equal, and tuned to the momentum budget problem. In this paper, it is shown that the roughness lengths have considerable impact on the evaporation in winter. One-column simulations of the land-surface scheme are driven with a long time series of observations for Cabauw in The Netherlands. It is shown that with the operational roughness lengths for this location (as in use at ECMWF in May 1993), evaporation in January, February and March is overestimated by more than a factor 2. More realistic parameters, as documented for this site, virtually eliminate the error. This study shows the importance of the surface roughness lengths in determining evaporation from wet surfaces. It also illustrates the strength of long observational time series in identifying model deficiencies.

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