On monitoring the atmospheric greenhouse effect from space

Abstract

We propose an analysis technique for monitoring the planetary greenhouse effect from space. 2 quantities are used as a measure of the atmospheric greenhouse effect: (1) Ga, which is the reduction in the clear sky outgoing longwave radiation (OLR) due to the atmosphere; it is the radiative heating of the surface-atmosphere column; (2) Ga*, which is the back radiation from the atmosphere to the surface; it is the radiative heating of the surface by the atmosphere. Ga is obtained from satellite observations of OLR and surface temperature. Here, we develop a technique to obtain Ga* from a combination of OLR (broadband and window channel), surface temperature, column water vapor amount and near-surface air temperature. The difference, Ga - Ga*, yields the net radiative cooling of the atmospheric column. While the technique is applicable for any radiometric observations, it is demonstrated here for the proposed cloud and the earth's radiant energy systems (CERES). CERES will have in addition to the broadband channel (4 to 200 μm), a channel in the atmospheric window (8-12 μm) to provide continuous radiation flux data. The potential benefits of the window channel in estimating the surface downward longwave flux, as also in addressing the water vapor feedback and continuum-related studies are explored in this paper. The success of the method in unravelling the water vapor - radiative interactions is illustrated by 2 case studies using ship and satellite data for the 1985 to 1990 period: (a) the seasonal and latitudinal variation in the northern tropical oceans; (b) seasonal to inter-annual variations averaged over the entire tropical (30°N to 30°S) pacific ocean. In both instances, the column greenhouse effect (Ga) varies significantly on seasonal and inter annual time scales and is larger in the warmer months. Furthermore, variations in Ga* exceed those in Ga, such that the atmospheric cooling increases in the warmer, moist atmosphere. The water vapor continuum plays a significant role in the tropical variations of Ga* and tropospheric radiative cooling.