New Snow-Physics to Complement SSiB: Part II: Effects on Soil Moisture Initialization and Simulated Surface Fluxes, Precipitation and Hydrology of GEOS II GCM

The SSiB model, which was forced with GSWP ISLSCP Initiative I surface data, was modified to include more realistic snow physics and snow-melt infiltration. The new snow model in SSiB was again integrated with the GSWP data. The new SSiB simulation produced wetter and warmer soil, with more realistic snow-melt timing and runoff, in regions of significant snow-melt. The simulation was used for initializing land-surface temperature, soil moisture, and snow cover for the GEOS II GCM, which was integrated for JJA to generate an ensemble of runs for both 1987 and 1988. Each ensemble contained six cases starting from an ECMWF analysis for each day starting from 29 May through 3 June of each year. As compared to the old SSiB GCM, the new SSiB GCM with new initial hydrologic conditions significantly improved the prediction of precipitation in mid-to-high-latitude regions of Canada and Russia. Evapotranspiration, soil moisture, and runoff also compared more favorably in the new SSiB GCM simulation than the old SSiB GCM. Also, the 1988-1987 difference in northern India precipitation was more pronounced in the new SSiB GCM. In the U. S., where the old SSiB GCM had failed to simulate the 1988 drought circulation, the new SSiB GCM performed only slightly better. This was also evident in the 1988-1987 differences. In this region, the influence of initial conditions was mostly lost in about one month's time to the evolution of an unrealistic circulation.