Multiple AGCM Hindcasts of 1969-1998

Multiple realizations of the 1979-1998 time period have been simulated by the GISS AGCM to examine its performance, particularly over agricultural regions, and its sensitivity to forcing events and model resolution. A microwave radiative transfer postprocessor produced the AGCM's lower tropospheric, tropospheric and lower stratospheric brightness temperature (Tb) time series for correlations with various Microwave Sounding Unit (MSU) time series available. Seven realizations by the AGCM were forced by observed sea surface temperatures (sst) through 1992 to assess internal model variability. Subsequent runs hindcast January 1979 through April 1998 with an accumulation of forcings: observed ssts, greenhouse gases, stratospheric volcanic aerosols, stratospheric and tropospheric ozone and tropospheric sulfate and black carbon aerosols. Additional runs increased the AGCM's vertical and horizontal resolution. The goal of narrowing gaps between AGCM and MSU temperature time series was complicated by multiple MSU time series, by Tb simulation concerns and by unforced climatic variability in the AGCM and in the real world. Correlations between the AGCM and MSU in the lower stratosphere, mid-troposphere and oceanic lower troposphere are detailed with respect to the applied forcings and the employed model resolution. Of the sensitive agricultural areas considered, Nordeste in northeastern Brazil was simulated best. The two other agricultural regions, in Africa and in the northern mid-latitudes, suffered from higher levels of non-sst variability. Correlations of MSU and AGCM time series notably improved with addition of explicit atmospheric forcings in zonal bands but not in agricultural regional bins each encompassing only six AGCM gridcells.