Comparison of Model Infrared and Solar Fluxes

Radiative fluxes obtained from a numerical model were compared with those obtained from aircraft-based radiometers. Separate comparisons were made for thermal infrared (wavelength X > 2.9g) and solar (2.9g > X > 0.6 g) radiation. The model ignores atmospheric scattering, and assumes that H20, C02, 03 and 02 are the only absorbers. Clouds are assumed to be black bodies in the thermal region, while in the solar region they are assumed to be non-emitting and partially reflecting. The radiometer data were taken from four flights of the high-altitude ER-2 aircraft during the STEP experiment in Darwin, Australia in January 1987. Both radiometers had hemispheric fields of view. The infrared instrument recorded both upward and downward flux, from which the net upwelling flux was calculated; only the upwelling component of the net flux was obtained from the solar instrument. The model results correspond to the measured thermal infrared and solar fluxes in a cloudy atmosphere to within instrument error. A solar value computed for a cloudless atmosphere was low compared to the single set of measurements taken in clear skies; this could be due to either unreported low cloud, or a low assumed albedo.