DESCRIPTION AND VALIDATION OF THE ATMOSPHERE-LAND-SURFACE INTERACTIONSCHEME (ALSIS) WITH HAPEX AND CABAUW DATA

A new land surface parameterization scheme (ALSIS), with emphasis on s oil moisture prediction, is described and validated with observations from HAPEX-MOBILHY and Cabauw. An important feature of the scheme is t he inclusion of vertical heterogeneity of soil hydraulic parameters is modelling unsaturated flow. The simulated soil moisture for HAPEX sit e using a Vertically homogeneous soil has a positive bias in the upper soil layers and a negative bias in the deep soil layers. Taking into account the soil vertical heterogeneity greatly eliminates this discre pancy and results in an excellent agreement between annual cycles of m odelled and observed soil moisture profiles. The mean annual soil mois ture in the top 1.6 m of soil increased from 394 mm for homogeneous ca se to 433 mm for the heterogeneous case, consistent with 435 mm observ ed. The improvement in soil moisture simulation resulted in an improve d skill in predicting the mean and the diurnal cycles of surface fluxe s for the intensive observational period (28 May-3 July). The simulate d monthly averages of surface temperature and fluxes follow observatio ns over the year, except for January when the model overestimates the latent heat flux due to its failure in simulating high rates of dew fa ll. The deviation of modelled monthly mean surface fluxes from observa tions are well within the estimated observational errors. The simulate d mean daily surface temperature, and surface fluxes are generally con sistent with observations, except for some times in the winter period. The modelled diurnal cycles of temperature and fluxes are in agreemen t with those observed. However, the model overestimates the night-time latent heat flux, especially during January. (C) 1998 Elsevier Scienc e B.V. All rights reserved.