Validation of Clear-Sky Fluxes for Tropical Oceans from the Earth Radiation Budget Experiment

The existence and magnitude of a systematic bias in the clear-sky longwave fluxes from the Earth Radiation Budget Experiment (ERBE) is investigated. The bias is apparently introduced because the ERBE method for scene identification does not account for large zonal gradients in longwave absorption by water vapor. The ERBE fluxes are compared to fluxes calculated with a radiative transfer model from ship radiosonde measurements. The comparison is based upon an analysis of 5 yr of coincident satellite and radiosonde observations for equatorial ocean regions. The differences between the ERBE and model fluxes are examined as functions of sea surface temperature (SST) and relative humidity. The authors use height-mean relative humidity R̄H̄ as an index of atmospheric moisture. The average offset between the model and ERBE fluxes ranges between +2 and +6 W m−2 for SSTs above 295 K, and the gradients with respect to SST are nearly identical. However, the difference between the model and ERBE depends significantly on the tropospheric relative humidity. ERBE fluxes exceed model fluxes for R̄Hmacr; above 70%, and the maximum offset of +9 to +12 W m−2 is consistent with previous estimates. There are also indications that the clear-sky fluxes for R̄Hmacr; below 25% may be underestimated by about 10–15 W m−2. Since extreme values of height-mean humidity are relatively infrequent, the net bias introduced in the ERBE monthly mean clear-sky fluxes is generally less than the systematic error in estimates of the instantaneous fluxes. These findings support earlier work on the coupling between, SST and the atmospheric greenhouse effect, in particular the existence of a super greenhouse effect for oceans warmer than 300 K. Recent reports of much larger systematic differences are not supported by this analysis. The results indicate that comparison of GCM and ERBE clear-sky longwave fluxes will depend explicitly on atmospheric humidity.