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Harshvardhan and James A. Weinman

Abstract

A study has been made of infrared radiative transfer through a regular array of cuboidal clouds which considers the interaction of the sides of the clouds with each other and the ground. The theory is developed for black clouds and is extended to scattering clouds using a variable azimuth two-stream (VATS) approximation (Harshvardhan et al., 1981). It is shown that geometrical considerations often dominate over the microphysical aspects of radiative transfer through the clouds. For example, the difference in simulated 10 μm brightness temperature between black isothermal cubic clouds and cubic clouds of optical depth 10, is <2 K for zenith angles <50° for all cloud fractions when viewed parallel to the array.

The results show that serious errors are made in flux and cooling rate computations if broken clouds are modeled as planiform. Radiances computed by the usual practice of area-weighting cloudy- and clear-sky radiances are in error by 2–8 K in brightness temperature for cubic clouds over a wide range of cloud fractions and zenith angles. It is also shown that the lapse rate does not markedly affect the exiting radiances for cuboidal clouds of unit aspect ratio and optical depth 10.

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Harshvardhan, James A. Weinman, and Roger Davies

Abstract

The transport of infrared radiation in a single cuboidal cloud has been modeled using a variable azimuth two-stream (VATS) approximation. Computations have been made at 10 μm for a Deirmendjian (1969) C-1 water cloud of single scattering albedo, ω = 0.638 and asymmetry parameter, g=0.865. Results indicate, that the emittance of the top face of the model cloud is always less than that for a plane parallel cloud of the same optical depth. The hemispheric flux escaping from the cloud top has a gradient from the censor to the edges which are warmer when the cloud is over warmer ground. Cooling rate calculations in the 8–13.6 μm region show that there is cooling out of the sides of the cloud at all levels even when there is heating of the core from the ground below.

The radiances exiting from model cuboidal clouds were computed by path integration over the source function obtained with the two-stream approximation. Results suggest that the brightness temperature measured from finite clouds will overestimate the cloud-top temperature.

Some key results of the model have been compared with Monte Carlo simulations. Overall errors in flux and radiance average a few degrees for most cases.

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