An analysis of the dependence of clear‐sky top‐of‐atmosphere outgoing longwave radiation on atmospheric temperature and water vapor

We have analyzed observations of clear‐sky top‐of‐atmosphere outgoing longwave radiation (OLR) measured by the Clouds and the Earth's Radiant Energy System (CERES). These measurements were obtained during March 2005 at night and over the ocean and cover latitudes from 70°N to 70°S. First, we compare the OLR measurements to OLR calculated from two radiative transfer models. The models use as input simultaneous and collocated measurements of atmospheric temperature and atmospheric water vapor made by the Atmospheric Infrared Sounder (AIRS). We find excellent agreement between the models' predictions of OLR and observations, well within the uncertainty of the measurements. We also analyze the sensitivity of OLR to changing surface temperature T s , atmospheric temperature T a , and atmospheric water vapor q. We find that OLR is most sensitive to unit changes in T a when that change occurs in the lower troposphere. For q, the altitude distribution of sensitivity varies between the midlatitudes, subtropics, and the convective region. We also partition the observed variations in OLR into contributions from changing T s , T a , and q. In the midlatitudes, changes in T s and T a contribute approximately equally, and are partially offset by changes in q. In the subtropics, changes in T a dominate, with a smaller contribution from changes in T s and a relatively small offsetting contribution from q. In the tropical convective region, a rapid increase in q in the midtroposphere leads to a dramatic reduction in OLR with increasing T s , which has been termed the “super greenhouse effect”.