EVALUATION OF THE AMIP SOIL-MOISTURE SIMULATIONS

The Atmospheric Model Intercomparison Project (AMIP) conducted simulat ions by 30 different atmospheric general circulation models forced by observed sea surface temperatures for the 10-year period, 1979-1988. T hese models include a variety of different soil moisture parameterizat ions which influence their simulations of the entire land surface hydr ology, including evaporation, soil moisture, and runoff, and their sim ulations of the energy balance at the surface. Here we compare these p arameterizations, and evaluate their simulations of soil moisture by c omparing them with actual observations of soil moisture, literally gro und truth. We compared model-generated 'data sets' and simulations of soil moisture with observations from 150 stations in the former Soviet Union for 1979-1985 and Illinois for 1981-1988. The spatial patterns, mean annual cycles, and interannual variations were compared to plant -available soil moisture in the upper 1 m of soil. The model-generated 'data sets' are quite different from the observations, and from each other in many regions, even though they use the same bucket model calc ulation method. The AMIP model simulations are also quite different fr om each other, especially in the tropics. Models with 15-cm field capa cities do not capture the observed large high latitude values of soil moisture. In addition, none of the models properly simulate winter soi l moisture variations in high latitudes, keeping soil moisture constan t, while observations show that soil moisture varies in the winter as much as in other seasons. The observed interannual variations of soil moisture were not captured by any of the AMIP models. Several models h ave large soil moisture trends during the first year or two of the AMI P simulations, with potentially large impacts on global hydrological c ycle trends and on other climate elements. This is because the simulat ions were begun without spinning up the soil moisture to the model cli matology. The length of time it took for each to reach equilibrium dep ended on the particular parameterization. Although observed temporal a utocorrelation time scales are a few months, some models had much long er time scales than that. In particular, the three parameterizations b ased on the Simple Biosphere model (SiB) had trends in some regions fo r virtually the entire AMIP simulation period. (C) 1998 Elsevier Scien ce B.V. All rights reserved.