The Atmospheric Model Intercomparison Project (AMIP) conducted simulations by 30 different atmospheric general circulation models forced by observed sea surface temperatures for the 10-year period, 1979-1988. These models include a variety of different soil moisture parameterizations which influence their simulations of the entire land surface hydrology, including evaporation, soil moisture, and runoff, and their simulations of the energy balance at the surface. Here we compare these parameterizations, and evaluate their simulations of soil moisture by comparing them with actual observations of soil moisture, literally ground truth. We compared model-generated "data sets" and simulations of soil moisture with more than 200 observations taken in the former Soviet Union for 1979-1985, the People's Republic of China for 1981-1991, and Illinois for 1981-present. The spatial patterns, mean annual cycles, and interannual variations were compared to plant-available soil moisture in the upper 1 m of soil.
The "data sets" of Mintz and Serafini and Schemm et al. and the AMIP model output are quite different from the observations. The "data sets" are quite different from each other in many regions, even though they use the same calculation method. The model simulations are also quite different from each other, especially in the tropics. Models with 15-cm field capacities do not capture the large high latitude values of soil moisture. In addition, none of the models properly simulate winter soil moisture variations in high latitudes. They all keep soil moisture constant when the temperature is below freezing, 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 have large soil moisture trends during the first year or two of the AMIP simulations, with potentially large impacts on global hydrological cycle trends and on other climate elements. This is because the simulations were begun without spinning up the soil moisture to the model climatology. The length of time it took for each to reach equilibrium depended on the particular parameterization. Although observed temporal autocorrelation time scales are a few months, some models had much longer time scales than that. In particular, the 3 parameterizations based on SiB had severe trends in some regions for virtually the entire AMIP simulation period.