The Earth's radiation budget and its relation to atmospheric hydrology: 2. Observations of cloud effects
Authors: Stephens Graeme L. , Greenwald Thomas J. 
Published: February 2008

This paper describes an observational study of the relationship between the cloudy sky components of the Earth's radiation budget (ERB) and space/time coincident observations of the sea surface temperature, microwave‐derived cloud liquid water and cloud cover. The study uses two ERB data sets; Nimbus 7 narrow field‐of‐view, broadband scanning radiometer data from June 1979 to May 1980 and the Earth Radiation Budget Experiment broadband scanning data from March 1985 to February 1986. Cloud fluxes are derived from the ERB fluxes and estimates of the clear sky fluxes are described in a related paper. A new method that extends the cloud forcing analysis of ERB data is also introduced to estimate the cloud albedo. The zonally and seasonally averaged cloud flux components of the ERB are within 6 W m−2 for the two data sets. The general gross features of the global distributions of these fluxes also reproduce those reported in recent studies with the largest differences in mid‐to‐high latitude regions characterized by persistent cloud cover where the estimation of Nimbus 7 clear sky fluxes is suspect. A quantitative assessment of the impact of clouds on the greenhouse effect is given in terms of the greenhouse parameter introduced in a related study. This impact is significant, especially for deep convective clouds that form over the warmest waters of the oceans. It is also shown how the greenhouse effect of clouds increases as the liquid water path (LWP) of clouds increases in a manner analogous to that observed for water vapor. This increase is in direct contrast to many recent model studies of cloud feedback that ignore this influence. Cloud albedo data are grouped in categories corresponding to ranges of solar zenith angle. Albedos and longwave fluxes for the latitudinal ranges of these categories suggest that brighter, colder clouds exist over tropical land masses in comparison to tropical oceanic regions and vice versa for middle and high latitudes. While microphysical effects cannot be ruled out as an explanation, the general reciprocal change of albedo and longwave flux support the assertion that these differences originate from gross macrophysical differences of clouds. The albedo of clouds and the relationships between the cloud albedo and LWP are also shown to be significantly different for midlatitude oceanic clouds compared to clouds over tropical oceans. The cloud albedo differences are substantial and cannot be explained simply in terms of cloud amount effects. Based on comparison with theory, it is unlikely that realistic differences in the microphysics of clouds are large enough to explain the observations. An explanation for these differences in terms of gross macroscopic effects is proposed. The major conclusion of this study is that the largest, and hence most important, observed influence of cloud on the ERB is more consistent with macrophysical properties of clouds as opposed to microphysical properties, which have received much more attention in recent literature.