Estimation of surface shortwave radiation components under all sky conditions: Modeling and sensitivity analysis

• Published in: Remote sensing of environment Vol. 123; pp. 457 - 469
• Main Authors: Chen, Ling, Yan, Guangjian, Wang, Tianxing, Ren, Huazhong, Calbó, Josep, Zhao, Jing, McKenzie, Richard
• Format: Journal Article
• Language: English
• Published: New York, NY Elsevier Inc 01-08-2012; Elsevier
• Subjects: Animal, plant and microbial ecology; Applied geophysics; Biological and medical sciences; Earth sciences; Earth, ocean, space; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; General aspects. Techniques; Global sensitivity analysis; Hemispherical effective cloud fraction; Internal geophysics; MODTRAN-CF; Teledetection and vegetation maps; Global sensitivity analysis; MODTRAN-CF; Hemispherical effective cloud fraction; Modification; components; solar energy; Prototype; sensitivity analysis; radiative transfer; accuracy; Parallel; clouds; Modeling; System; Field; physical models; radiance; short period waves; Earth atmosphere; Atmospheric condition; Modulator; Approximation; Estimation; Independence; radiation; geometry; Pixel; dimensions
• ISSN: 0034-4257; 1879-0704
• DOI: 10.1016/j.rse.2012.04.006

Clouds are the most important modulator of the amount of solar energy absorbed by the earth–atmosphere system. Traditional one-dimensional (1D) plane-parallel atmospheric radiative transfer models which use the independent pixel approximation (IPA) can only consider two extreme conditions, i.e., either cloud-free or overcast cases. In this paper, two cloud fraction related factors (hemispherical effective cloud fraction and regional cloud fraction) are calculated and incorporated into MODTRAN 4 (one of the most popular radiative transfer packages) to simulate the surface shortwave radiation components and the top-of-atmosphere (TOA) radiance for all possible solar-cloud-viewing geometries. The accuracy of this modified solar radiative transfer model (named as MODTRAN-CF) is consistent with its prototype (MODTRAN 4) which has been widely used and validated in radiative transfer modeling. Some field measurements are used to validate the superiority of MODTRAN-CF. For further understanding and simplifying of this physical model, a global sensitivity analysis (GSA) method is employed to analyze the effect of model parameters on each surface shortwave radiation component. Five parameters including solar zenith angle, surface albedo, hemispherical effective cloud fraction, ground altitude and atmospheric visibility show non-negligible impacts on almost all surface shortwave fluxes, which indicates that these five parameters should be carefully considered in the future modeling of the surface shortwave radiation fluxes. Two cloud optical thickness related parameters (cloud extinction coefficient and cloud thickness) exhibit obvious importance only under cloudy illumination condition especially with optically thin clouds. These findings on the improved model will enhance our knowledge on how to accurately model the surface shortwave radiation fluxes under all sky conditions.
► Proposed a modified atmospheric solar radiative transfer model named as MODTRAN-CF ► Involved two cloud fraction related factors and all solar-cloud-viewing geometries ► Simulation results can reflect the solar radiation enhancement by clouds. ► A global sensitivity analysis method has been implemented. ► The superiority of MODTRAN-CF has been validated by field measurements.