Comparison of Modeled and Measured Spectral Shortwave Irradiance Using CLAMS Data

One of the goals of The Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) campaign was to collect a comprehensive dataset to support clear sky (cloud-free) shortwave closure experiments, a test to determine and improve the level of agreement between state of the art measurements and calculations of the transfer of incoming and reflected solar radiation through the Earth-atmosphere system. Such experiments permit an unambiguous test of the accuracy with which top of atmosphere satellite radiance observations can be inverted to retrieve information describing properties of the Earth and atmosphere such as atmospheric aerosols and the surface and atmospheric radiation budget, both targeted during CLAMS to validate EOS TERRA satellite data products being derived from the CERES, MISR and MODIS instruments. In this paper we compare calculations of shortwave spectral flux with measurements made from the NASA Langley OV-10 aircraft. The OV-10 instrument compliment includes uplooking and downlooking Analytical Spectral Devices spectroradiometers measuring spectra from 350-2250 nm with an effective resolution of 8-12 nm interpolated to 1 nm. The data are calibrated and tied to an accepted TOA solar spectrum using high mountain top Langley techniques in spectral regions with little absorption. A NIST lamp technique is applied to pin the remaining spectra to the mountain top calibration. Several popular radiative transfer models are employed to calculate spectral fluxes suitable for comparison with the aircraft measurements. The models tested allow various degrees of flexibility for treating the lower boundary condition and incorporating aerosol properties. In this presentation we first emphasize the comparison of the downwelling spectral irradiances at higher altitudes with little aerosol loading above. We then test the common treatment of ocean BRDF and standard aerosol models making comparisons in the upwelling stream and under higher aerosol conditions. Ultimately, our goal is to perform comparisons using CLAMS measurements of aerosols and ocean optics as inputs. This study represents early efforts to develop a methodology to incorporate CLAMS data, as they become available, into radiative transfer calculations to close on shortwave radiative fluxes.