The Earth's radiation budget and its relation to atmospheric hydrology: 1. Observations of the clear sky greenhouse effect

Abstract

This paper describes an observational study of the clear sky components of the Earth's radiation budget (ERB), the relationship of these components to the sea surface temperature (SST), and the microwave‐derived water vapor amount using observations that are coincident in both space and time. This study uses two sets of ERB data; the Nimbus 7 narrow field‐of‐view, broadband scanning radiometer data from June 1979 to May 1980 and the Earth Radiation Budget Experiment (ERBE) broadband scanning data from March 1985 to February 1986. Clear sky fluxes derived from Nimbus 7 data are compared to the ERBE data. Globally averaged Nimbus 7 clear sky longwave (LW) fluxes are lower than the equivalent ERBE fluxes by about 5 W m−2, whereas clear sky reflected fluxes are higher by about 3 W m−2. These biases are consistent with the possibility of slight cloud contamination of the Nimbus 7 clear sky fluxes, although biases in ERBE fluxes also may partly explain the differences. Comparisons of the global distributions of these clear sky fluxes reveal a cloud contamination in the Nimbus 7 fluxes over the regions of the storm tracks poleward of about 50° latitude. Despite these effects, the comparison of the two data sets generally agree within the uncertainty of the flux measurements and reproduce the gross features of clear sky ERB fluxes reported previously. A strategy for analyzing the emitted clear sky LW flux over the oceans is described using the simple arguments of gray body radiative equilibrium. A greenhouse parameter G, defined in terms of the SST and the emitted flux I, is introduced and a linear relationship between G and the precipitable water w is established from the observations. Using a simple nongray one‐dimensional climate model, we demonstrate how both the observations and the model differ from the simple ideas of gray body radiative equilibrium in significant ways. We demonstrate how the relationships between G and w and G and SST are affected by the assumed vertical profiles of water vapor and temperature. Although it is not possible to observe feedback processes directly, it is argued that the results of the paper are consistent with conventional ideas about the operation of a positive feedback on Earth between the greenhouse effect, SST and w.