THE IMPACT OF UPPER TROPOSPHERIC HUMIDITY ON CLEAR-SKY OUTGOING LONGWAVE RADIATION AS ESTIMATED FROM SATELLITE OBSERVATIONS

摘要

The effects of upper tropospheric humidity (UTH) variations on the clear-sky outgoing longwave radiation (OLR) at the top of the atmosphere are significant when simulated by radiative transfer model calculations. In order to access and validate the impact of UTH on clear-sky OLR, we inter-compared three sets of satellite-estimatedOLR data with meteorologicalparametersin the domain of the tropical oceans. Our results show monthly clear-sky OLR anomalies due to the El Nino / La Nina events that amount to 20 W m~(-2) over large geographical locations. They are geographically associated with the anomalies in the meteorological fields in most events. ERBE agrees with HIRS that the clear-sky OLR has the strongest dependence on UTH and negligible dependence on SST. However, the AVHRR shows the strongest dependence on the SST and that can be explained by the nature of the single-channel window-only algorithm. The sensitivities of clear-sky OLR to the UTH vary with atmospheric states and are quite different. In terms of the 300 hPa specific humidity, they are -11, -19, -3 (W m~(-2)) per (g/kg) for ERBE, HIRS, and AVHRR, respectively. Primarily, they are due to the difference in the HIRS and AVHRR OLR algorithms, the deficiency of ERBE clear-sky determining technique, and differences between the atmospheric states.