On the retrieval of cloud optical thickness from spectral radiances. An intercomparison of ground-based measurements in the Arctic

In polar regions, reliable measurements of cloud optical thickness (tau) are scarce due to limited site accessibility, short sunlit seasons, and high surface albedo (alpha), which enhances multiple scattering and hinders cloud detectability by satellite instruments. This work evaluates different ground-based techniques for estimating tau, with emphasis on retrievals sensitivity to high alpha values. Estimates of tau are obtained with measurements collected on the north-western coast of Greenland, at the Thule High Arctic Atmospheric Observatory (76.5 degrees N, 68.8 degrees W, 225 m a.s.l). We compare retrievals of tau obtained by means of ground-based zenith sky spectral radiance measurements in the 320-950 nm wavelength range as well as downward shortwave irradiance (DSI) and Liquid Water Path (LWP) measurements. tau values are calculated on two days characterized by total cloud cover (overcast) conditions, different broadband alpha's (similar to 0.1 and similar to 0.6), and liquid-bearing clouds. tau values range between 1 and 50 and their comparisons during the two case studies present some differences most likely due to the different alpha. In the high surface albedo case, spectral estimates of tau are well correlated with the LWP-based estimates (R-2 = 0.92) which in turn are well correlated with the DSI-based estimates (R-2 = 0.90). For thin clouds (tau < 7) all methods show also a good agreement, whereas for thick clouds (tau > 13) spectral tau's largely overestimate tau(LWP) which in turn overestimate tau(DSI) values. The comparison between LWP- and DSI-based tau retrievals for thick clouds does not improve in low albedo conditions with respect to the high albedo case, suggesting that their differences are not due to a different alpha and that, after additional case studies are analyzed, DSI-based tau could be useful also in high alpha conditions. The occasional tau estimates from MODIS are consistent with ground-based retrievals.