Estimation of earth OLR using a radiation model calculation and satellite radiance observations

The sensitivity of outgoing longwave radiation (OLR) estimation techniques, based on either Advanced Very High Resolution Radiometer (AVHRR) or High Resolution Infrared Sounder (HIRS) data, has been simulated using radiation transfer model calculations for different atmospheric conditions. The simulations have focused on the effects of near-surface temperature discontinuity over desert surfaces and on variations in the strength of temperature and moisture inversion over subtropical high pressure regions. Sensitivity tests show that the AVHRR technique overestimates the reference model OLR when skin temperature exceeds the surface air temperature, with the overestimate increasing with skin–air temperature differences. The magnitude of the overestimates appears similar to what was found in comparisons of AVHRR derived and Earth Radiation Budget Experiment (ERBE), OLR fluxes over desert surfaces. Unlike AVHRR, the HIRS technique shows no tendency to over-or-under estimate model calculations for such conditions. In subtropical high-pressure regions, the AVHRR OLR fluxes tend to be smaller than those observed by ERBE. As temperature inversion becomes stronger, the OLR difference between model calculations and AVHRR estimations increases, but the strength of the inversion does not have much effect on the HIRS technique. As the H 2O content in the inversion layer increases, the model and HIRS derived fluxes decrease while the AVHRR derived fluxes do not follow the variations of the radiation balance between the surface and near surface levels. Results indicate that large flux differences between ERBE and AVHRR over desert and subtropical oceanic regions are due to shortcomings in the AVHRR technique while the HIRS technique is successfully sensitive to the variations in air temperature and water vapor.