Editorial Type:
Article
Article Type:
Research Article

The Angular Distribution of UV-B Sky Radiance under Cloudy Conditions: A Comparison of Measurements and Radiative Transfer Calculations Using a Fractal Cloud Model

Christopher Kuchinke School of Geography and Environmental Studies, University of Tasmania, Hobart, Tasmania, Australia

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Kurt Fienberg School of Geography and Environmental Studies, University of Tasmania, Hobart, Tasmania, Australia

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Manuel Nunez School of Geography and Environmental Studies, University of Tasmania, Hobart, Tasmania, Australia

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Print Publication:
01 May 2004
DOI:
https://doi.org/10.1175/2095.1
Page(s):
751–761
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Abstract

In recent years, global warming concerns have focused attention on cloud radiative forcing and its accurate encapsulation in radiative transfer measurement and modeling programs. At present, this process is constrained by the dynamic movement and inhomogeneity of cloud structure. This study attempts to quantify the UV sky radiance distribution induced by a partial and overcast stratiform cloud field while addressing some of the inherent spatial and temporal errors resulting from cloud. For this purpose, high-quality azimuthally averaged 2-min measurements of erythemal UV-B sky radiance distribution were undertaken by a variable sky-view platform at Hobart, Australia (42.90°S, 147.33°E). Measurements were subsequently compared with Monte Carlo radiative transfer simulations using both a multifractal and plane-parallel homogenous (PPH) cloud field. Data were also compared with several empirical parameterizations. Results at solar zenith angles of 30° and 50° show that for overcast conditions, the multifractal model is superior to the PPH model. For broken cloud conditions, the radiance measurements are biased toward higher instances of direct-beam interruption by cloud. This tends to smooth the near-sun sky radiance field whereas the multifractal model under the same conditions continues to exhibit the circumsolar effect, indicating that its performance may be still valid for radiation modeling. An empirical parameterization of the same multifractal model produced similar sky radiance profiles, warranting its use in radiative transfer models.

* Current affiliation: Department of Physics, University of Miami, Coral Gables, Florida

Corresponding author address: Dr. Christopher Kuchinke, Department of Physics, University of Miami, P.O. Box 248046, Coral Gables, FL 33124-0530. kuchinke@physics.miami.edu

Abstract

In recent years, global warming concerns have focused attention on cloud radiative forcing and its accurate encapsulation in radiative transfer measurement and modeling programs. At present, this process is constrained by the dynamic movement and inhomogeneity of cloud structure. This study attempts to quantify the UV sky radiance distribution induced by a partial and overcast stratiform cloud field while addressing some of the inherent spatial and temporal errors resulting from cloud. For this purpose, high-quality azimuthally averaged 2-min measurements of erythemal UV-B sky radiance distribution were undertaken by a variable sky-view platform at Hobart, Australia (42.90°S, 147.33°E). Measurements were subsequently compared with Monte Carlo radiative transfer simulations using both a multifractal and plane-parallel homogenous (PPH) cloud field. Data were also compared with several empirical parameterizations. Results at solar zenith angles of 30° and 50° show that for overcast conditions, the multifractal model is superior to the PPH model. For broken cloud conditions, the radiance measurements are biased toward higher instances of direct-beam interruption by cloud. This tends to smooth the near-sun sky radiance field whereas the multifractal model under the same conditions continues to exhibit the circumsolar effect, indicating that its performance may be still valid for radiation modeling. An empirical parameterization of the same multifractal model produced similar sky radiance profiles, warranting its use in radiative transfer models.

* Current affiliation: Department of Physics, University of Miami, Coral Gables, Florida

Corresponding author address: Dr. Christopher Kuchinke, Department of Physics, University of Miami, P.O. Box 248046, Coral Gables, FL 33124-0530. kuchinke@physics.miami.edu

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Journal of Applied Meteorology and Climatology

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