Cloud modification factor parametrization for solar UV based on the GOES satellite: Validation using ground-based measurements in São Paulo city, Brazil

Abstract

A novel approach for parametrization of the cloud effect on ultraviolet (UV) sun wavelengths is proposed. Diagnostic and observational studies show that solar reflectance at 0.65 μm (R0.65μm) could be a proxy for cloud optical depth, and it can be applied to parametrize the cloud modification factor (CMF), which is often applied to estimate the UV index. For all sky conditions, a linear relation between broadband CMF (0.2–0.4 μm) and R0.65μm is derived, given by CMF(theory) = 1.09–1.19·R0.65μm using a radiative transfer model, which is close to CMF(observed) = 1.15 − 1.55 Rch1 based on ground-based radiation measurements and geostationary satellite channel 1 reflectance data (Rch1). The observationally derived CMF versus Rch1 fitting was applied for four different cloud classes from the satellite scene classification products: cirrus, cumulus, stratus and deep convective clouds. CMF varies with Rch1 from 1 for optically thin clouds to 0.05 for stratus and convective clouds, while CMF >0.7 denotes cirrus clouds. These values differ significantly from the constant CMF values suggested in the literature, particularly for convective clouds. The parameterization found here for each cloud is assessed by comparing the UVI estimated using the novel CMF function to UVI ground-based observations during a four-year period in São Paulo, Brazil. Because of high temporal and spatial satellite data availability, the parametrized CMF in terms of Rch1 is well suited for incorporation into operational weather services, estimating UVI under cloudy conditions with high correlation (r > 0.8) and low errors (root mean square error <2.0) when compared to ground data. In addition, CMF parametrization provides a better IUV cloud effect than the CMF given in the literature. The findings shown here enable better UVI estimation under overcast skies over tropical climates, allowing for more realistic information regarding health and sun exposure to be communicated to the public.