A new multispectral cloud retrievalmethod for ship-based solar transmissivity measurements

Within the German Leibniz-network OCEANET project, ship-based lidar and microwave remote sensing as well as spectral zenith radiance observations with the COmpact RAdiation measurements System (CORAS) were performed. During three cruises latitudes between 50 degrees N and 50 degrees S were covered. A new spectral retrieval method to derive the cloud optical thickness.. and the droplet effective radius r(eff) using CORAS measurements is developed. The method matches CORAS measurements of ratios of spectral transmissivity at six wavelengths with modeled transmissivities. This retrieval is fast and accurate and thus suitable for operational purposes. The new approach circumvents ambiguities of existing cloud retrievals and reduces the influence of measurement uncertainties. It is applied to homogenous and heterogeneous liquid water and cirrus clouds. In boundary layer liquid water clouds, the retrieved effective radius was more variable, whereas in the cirrus it was rather constant. Furthermore, the liquid water path LWP was derived and compared to data from a microwave radiometer. The new retrieval tends to overestimate LWP for thick liquid water clouds but slightly underestimate LWP for thin clouds. The presented method cannot be applied to mixed-phase clouds. The maximum retrieval of tau and r(eff) for liquid water clouds is 80 in tau and 30 mu m in r(eff), respectively; for cirrus clouds the limitations of the retrieval are 10 in tau and 60 mu m in r(eff).