OBSERVED DEPENDENCE OF OUTGOING LONGWAVE RADIATION ON SEA-SURFACE TEMPERATURE AND MOISTURE

The authors have empirically examined the dependence of the outgoing l ongwave radiation (OLR) on sea surface temperature (T(s)), precipitabl e water (W), and height-mean relative humidity (RHBAR). The OLR is obt ained from 4 yr of data from the Earth Radiation Budget Experiment (ER BE), while T(s), W, and RHBAR are from objective analyses of rawinsond e and ship data. It is found that in the midlatitudes, the surface tem perature 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) explain s 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 RHBAR is marked by distinct changes in the gradien t that are consistent with inferences from earlier investigations. For low values of T(s)(< 10-degrees-C), the OLR depends mainly on T(s). F or values of T(s) above 10-degrees-C, the OLR depends increasingly on RHBAR. Specifically, in the tropics (T(s) approximately 25-degrees-C), the total and clear-sky OLR depend significantly on both T(s) and RHB AR. The well-known drop in OLR in the tropics with increasing T(s) cor relates directly to an increase in RHBAR, and not to changes in T(s). The authors suggest that the observed dependence of the OLR on T(s) an d RHBAR be a minimum performance standard for climate models. This app roach is illustrated by comparing the observed dependence with the res ults of a radiative transfer model and an R15 general circulation mode l, and by discussing the strengths and limitations of using RHBAR to p arameterize the OLR.