Cloud Remote Sensing Using Spectral Features in Transmitted Shortwave Radiation

Spectral features in shortwave radiation transmitted through clouds are sensitive to changes in cloud properties including cloud optical thickness, effective radius, and thermodynamic phase. The absorption and scattering of light by liquid water and ice clouds result in shifts in spectral slopes, curvatures, maxima, and minima of cloud-transmitted radiance. These spectral features have been observed in measured and modeled transmittance and have been quantified by 15 parameters, representing a new framework to evaluate spectral features in shortwave radiation. These parameters are used to retrieve cloud properties from measurements of zenith radiance from the ground by using the Solar Spectral Flux Radiometer (SSFR) instrument and from aircraft by using the Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) instrument. A ground-based SSFR was operated between May 2012 and January 2013 in Boulder, Colorado, and the 4STAR instrument was deployed on airborne platforms during the recent Two-Column Aerosol Project (TCAP) conducted in February 2013 based in Cape Cod, Massachusetts, and the recent Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) that took place in August-September 2013, based in Houston, Texas. The cloud optical thickness and effective radius from the new retrieval, based on multiple spectral features, are compared to results from two other retrieval methods. By using multiple spectral features, we find a closer fit between measured and modeled spectra compared to two other retrieval methods, applied to sample liquid water and ice clouds.