Transport of Infrared Radiation in Cuboidal Clouds

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  • 1 Department of Meteorology, University of Maryland, college Park 20742
  • 2 Laboratory for Atmospheric Sciences, Goddard Space Flight Center, Greenbelt, MD 20771
  • 3 Space Science and Engineering Censer, University of Wisconsin, Madison 53706
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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.

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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