Brightness temperature maps for global monthly mean general circulation model (GCM) atmospheric and surface fields are calculated with a new microwave radiative transfer postprocessor. This microwave radiative transfer model uses fields from the 4¿ by 5¿ Goddard Institute for Space Studies (GISS) GCM Model II and subsequent developmental versions of the GCM. The absolute brightness temperature maps from the microwave radiative transfer model are shown to be more accurate than maps from temperature weighting functions due to the inclusion of local surface emission and local radiative transfer effects. This permits direct comparison of calculated brightness temperatures to observed brightness temperatures measured by the microwave sounding unit (MSU) channels. Microwave maps are calculated for the January and July Geophysical Fluid Dynamics Laboratory (GFDL) radiosonde upper air and surface climatology and compared to the MSU climatology. This work tests the microwave radiative transfer model and comments on two disparate tropospheric climatologies used for trend analysis and GCM validation. The microwave comparison generally finds good agreement between the MSU channel 2 and the radiosonde climatologies. January brightness temperature differences of a few degrees come from spatial coverage gaps and land biasing in the radiosonde climatology, echoing results in previous comparison studies. The July microwave comparison finds unexpected warmth brightness temperatures from the radiosonde climatology over land masses. This July warmth may be related to diurnal sampling differences and to recent study by other researchers into radiosonde time series' sensitivity to instrumentation and to solar heating of the radiosonde.

Comparison of calculated GCM microwave maps to observed monthly mean MSU maps aids development of the GISS GCM. MSU channel 2R and channel 2 are used to evaluate the impacts of GCM developments near the ocean surface and the midtroposphere, respectively. These microwave channels show large sensitivity to the planetary boundary layer scheme. MSU channel 4 confirms improvements in the lower stratosphere with addition of a parameterized gravity wave drag beneath the GCM's rigid model top. Overall, the MSU climatology with its global coverage, several broad vertical samplings, and horizontal resolution comparable to GCM grid cells complements more traditional observations used to validate general circulation models. ¿ American Geophysical Union 1995