Combined retrieval of Arctic liquid water cloud and surface snow properties using airborne spectral solar remote sensing

The passive solar remote sensing of cloud properties over highly reflecting ground is challenging, mostly due to the low contrast between the cloud reflectivity and that of the underlying surfaces (sea ice and snow). Uncertainties in the retrieved cloud optical thickness tau and cloud droplet effective radius r(eff);(C) may arise from uncertainties in the assumed spectral surface albedo, which is mainly determined by the generally unknown effective snow grain size r(eff;S). Therefore, in a first step the effects of the assumed snow grain size are systematically quantified for the conventional bispectral retrieval technique of tau and r(eff;C) for liquid water clouds. In general, the impact of uncertainties of r(eff;S) is largest for small snow grain sizes. While the uncertainties of retrieved tau are independent of the cloud optical thickness and solar zenith angle, the bias of retrieved r(eff;C) increases for optically thin clouds and high Sun. The largest deviations between the retrieved and true original values are found with 83% for tau and 62% for r(eff;C). In the second part of the paper a retrieval method is presented that simultaneously derives all three parameters (tau, r(eff;C), r(eff;S)) and therefore accounts for changes in the snow grain size. Ratios of spectral cloud reflectivity measurements at the three wavelengths lambda(1) = 1040 nm (sensitive to r(eff;S)), lambda(2) = 1650 nm (sensitive to tau), and lambda(3) = 2100 nm (sensitive to r(eff;C)) are combined in a trispectral retrieval algorithm. In a feasibility study, spectral cloud reflectivity measurements collected by the Spectral Modular Airborne Radiation measurement sysTem (SMART) during the research campaign Vertical Distribution of Ice in Arctic Mixed-Phase Clouds (VERDI, April/May 2012) were used to test the retrieval procedure. Two cases of observations above the Canadian Beau-fort Sea, one with dense snow-covered sea ice and another with a distinct snow-covered sea ice edge are analysed. The retrieved values of tau, r(eff;C), and r (eff;S) show a continuous transition of cloud properties across snow-covered sea ice and open water and are consistent with estimates based on satellite data. It is shown that the uncertainties of the trispectral retrieval increase for high values of tau, and low r(eff;S) but nevertheless allow the effective snow grain size in cloud-covered areas to be estimated.