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Offline validation of the ERA40 surface scheme.

A new surface scheme is introduced in the upcoming ecmwf global reanalysis covering a 40-year period 1957-1997 (known as era40). The new scheme is characterized by a tiled treatment of subgrid fractions of evaporating surfaces, the use of a global vegetation database plus an associated set of model parameters, and changes into the parameterization of snow processes, the coupling of the skin layer to the soil, and the vertical soil moisture transport. Parallel to the evaluation of the global impact in the ecmwf 3D model, this tiled scheme was tested in oine mode with data from 7 long term surface eld campaigns. The datasets cover a wide range of climate regions and surface land use and vegetation types, ranging from boreal (Boreas), tropical (Arme) and temperate (Garderen) forest, via savannah (Sebex) and prairie (Fife) to crop (Hapex-Mobilhy) and grassland (Cabauw). The oine evaluation was primarily used for validation purposes, but gave also rise to adjusting parameter settings and/or parameterization approaches. After the combined calibration/validation exercise we can conclude that (a) the description of snow processes and surface uxes in the Boreal areas has improved signicantly, (b) the tiling approach does not give rise to major changes in the model correspondence to data in case of grass-and cropland, and (c) model performance becomes increasingly dependent on a proper choice of vegetation dependent model parameters, which gives rise to an increased number of degrees of freedom.

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Van den Hurk, B.J.J.M., P. Viterbo, A.C.M. Beljaars and A. K. Betts, 2000: Offline validation of the ERA40 surface scheme. ECMWF Tech Memo, 295, 43 pp., Eur. Cent. For Medium-Range Weather Forecasts, Shinfield Park, Reading RG2 9AX, England,UK. http://www.ecmwf.int/publications/library/ecpublications/_pdf/tm/001-300/tm295.pdf