COMPARISON OF MODELED AND OBSERVED SPECTRAL FLUXES UNDER OVERCAST CLOUDY SKIES: COUPLING BETWEEN FLUXES AND SURFACE SPECTRAL ALBEDO

摘要

The objective of this study is to assess the ability of a state-of-the-art radiative transfer model to simulate shortwave (SW) spectral fluxes available from recent high quality spectral measurements in overcast cloudy sky conditions. In view of some reported discrepancies concerning cloud property retrievals and cloud absorption, this may be instrumental in identifying problems and critical factors influencing the agreement between theory and observation. The latest version of the MODTRAN4 radiative transfer code was used. Model-derived fluxes were compared to observations made by the Rotating Shadowband Spectroradiometer (RSS) data of high spectral resolution (360-1100nm, 1024 channels) and broadband data. The general location and strength of gaseous absorption lines in the spectrum of RSS measurements under cloudy conditions is reproduced very well. The behavior of spectral data at the surface level, apart from the absorption lines, may also be explained by varying input parameters within reasonable ranges. Strong coupling between downward fluxes and the surface spectral albedo was identified. This suggests a method for the retrieval of areal-mean surface albedo providing that all other necessary input parameters are known.