Cloud properties retrieved from airborne measurements of transmitted and reflected shortwave spectral radiation
Wednesday, 17 December 2014
We present a comparison of retrieved cloud properties based on transmitted and reflected light for a cloud sampled during the recent Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS). Transmitted light interacts with cloud particles throughout the vertical extent of the cloud, while reflected light, commonly used for satellite remote sensing of clouds, is more influenced by the top-most cloud particles. To quantify cloud properties from transmitted shortwave radiation, a new retrieval utilizing spectrally resolved measurements is employed. 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 aircraft by using the Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) instrument. The 4STAR instrument was deployed on an airborne platform during SEAC4RS alongside the Solar Spectral Flux Radiometer (SSFR). The cloud optical thickness and effective radius from the retrieval based on transmitted shortwave radiation are compared to cloud properties obtained from above the cloud by using reflected shortwave radiation measured with SSFR and with the enhanced MODIS Airborne Simulator (eMAS). We present the difference in calculated surface cloud radiative forcing when using input of cloud properties obtained from retrievals based on transmitted instead of reflected light.