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

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 (42.90ЊS, 147.33ЊE). 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 30Њ and 50Њ 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.

Spectral attenuation of global and diffuse UV irradiance and actinic flux by clouds

“…Sobolev, ; Mayer and Kylling, ) and empirical (e.g. Grant and Heisler, ; Kuchinke et al , ; Román et al , ) studies have proved that sky radiance under overcast conditions is non‐isotropic and reaches its maximum in the zenith direction. We performed model calculations (not shown here) for cloud‐free and overcast conditions to better interpret the observations.…”

Technical note: The libRadtran software package for radiative transfer calculations - description and examples of use

“…Sobolev, 1975) and experimentally (e.g. Grant and Heisler, 1997;Kuchinke et al, 2004). Equation (7) clearly shows that the sky radiance under overcast conditions is non-isotropic and reaches its maximum in the zenith direction.…”

Short-term variability of overcast brightness

“…Changing a MODTRAN atmosphere's fixed ⌬z over time or h 0 is easy, but adding the radiative-transfer consequences of overcasts' horizontal inhomogeneity will require new modeling tracks, 4 -8 perhaps including the use of fractal clouds. 18…”