Impact of surface inhomogeneity on solar radiative transfer under overcast conditions

[1] The goal of this study was to assess the ability of the Moderate-Resolution Transmittance 4 (MODTRAN-4) code to simulate high-resolution shortwave (SW) fluxes given detailed and complete input information under overcast conditions. The study underlines the impact of surface inhomogeneity on the closure of SW radiative transfer. It also leads to a method of estimating surface spectral areal-mean albedo from downwelling solar transmittance measurements. The investigation made use of ample Atmospheric Radiation Measurement (ARM) field data collected by a suite of instruments, including broadband and narrowband radiometers and spectrometers, cloud radar and lidar, microwave radiometer, atmospheric sounding instruments, and satellite data. Furnishing the MODTRAN-4 code with observed atmospheric, cloud, and surface parameters generates spectral solar transmittance at the surface and reflectance at the top of the atmosphere (TOA). The transmittances were compared with the Rotating Shadowband Spectroradiometer measurements and showed significant discrepancies in the near-infrared (NIR) region, the bulk of which was attributed to the use of unrepresentative surface spectral albedos. A field campaign was undertaken to collect surface albedo data for a wide variety of land cover types near the ARM Central Facility. The sampled data were combined with thematic mapper/Landsat-based land cover classification data to map surface spectral albedo. Substitution of the derived areal-mean spectral albedo into the MODTRAN-4 model eliminates major discrepancies in the NIR, and also leads to good agreements with surface solar broadband fluxes and TOA satellite spectral reflectance. On the basis of these findings, one may use downwelling spectral transmittance data, together with detailed cloud and atmospheric information, to estimate surface effective areal-mean albedo. The estimated values agree well with those derived from the ground survey data. Following the method, a data set of effective areal-mean spectral albedo throughout a year was obtained.