Variability of radiative properties of clouds from aircraft measurements

Abstract

Radiative properties of stratiform clouds in different geographical conditions have been discussed using FGGE data. Characteristic features have been described, in particular, of the spectral and total radiative characteristics of clouds over urban and rural areas, Arctic ice, and water bodies depending on the optical thickness of clouds and sun elevation. The dependence of emissivity of clouds in high, middle and tropical latitudes on their thickness has been analyzed. Variability of radiative properties of stratus clouds in different conditions requires further accumulation of observational data and a search for techniques for their parameterization to consider the interaction between clouds and radiation in numerical modeling of the general atmospheric circulation and climate.The results of studies have shown that the total (full spectrum) albedo of clouds is somewhat less than the albedo in the visible, but within an accuracy of about 10 percent they can be considered identical, which is essential for radiative energetics of the atmosphere. Low-level clouds are characterized by shortwave radiation absorption not only in the molecular absorption bands but also in the visible spectrum. The absorption-induced cloud warming is smaller, as a rule, than cooling due to radiative heat exchange, with the exception of clouds over large industrial areas, in the regions of forest, and clouds interacting with optically active aerosols in the case of strong dust transport from deserts.Cloud absorptivity over water, rural areas and ice does not exceed 0.05 . . . 0.15, but for optically thick clouds with r>30 it may reach 0.20 . . . 0.30. The albedo of a cloudy atmosphere, A, over water bodies descreases with increased altitude, with a lapse rate of about 0.003 km-1, and above the sea about 0.01 km-1. At a sun elevation he=15 - 18° the gradient ^A/^he changes its sign which is related to the effect of clouds macro-inhomogeneity at low sun elevations.The results of comparing calculated data with observations of the evolution of advective fog and clouds over ice as well as complex sub-satellite experiments indicate the necessity of consideration of radiative heat flux divergence in a cloudy atmosphere. A decrease has been found of clouds emissivity in high latitudes as compared to that in middle and tropical latitudes. Data are given on the dependence of clouds emissivity on their thickness and level.