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A. Raval, A. H. Oort, and V. Ramaswamy

Abstract

The authors have empirically examined the dependence of the outgoing longwave radiation (OLR) on sea surface temperature (T s), precipitable water (W), and height-mean relative humidity (RH¯). The OLR is obtained from 4 yr of data from the Earth Radiation Budget Experiment (ERBE), while T s, W, and RH¯ are obtained from objective analyses of rawinsonde and ship data. It is found that in the midlatitudes, the surface temperature explains over 80% of the variability in the clear-sky OLR (F cs) and almost half of the variability in the total OLR (F tot). It fails badly in the tropics and subtropics, however, where (T s explains only about 20% of the variability in (F cs, and is largely decoupled from F tot. The two-dimensional contour plot of the OLR binned with respect to T s and RH¯ is marked by distinct changes in the gradient that are consistent with inferences from earlier investigations. For low values of T s (<10°C), the OLR depends mainly on T s. For values of T s above 10°C, the OLR depends increasingly on RH¯. Specifically, in the tropics (T s˜25°C), the total and clear-sky OLR depend significantly on both T s and RH¯. The well-known drop in OLR in the tropics with increasing T s correlates directly to an increase in RH¯, and not to changes in T s. The authors suggest that the observed dependence of the OLR on T s and RH¯ be a minimum performance standard for climate models. This approach is illustrated by comparing the observed dependence with the results of a radiative transfer model and an R 15 general circulation model, and by discussing the strengths and limitations of using RH¯ to parameterize the OLR.

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