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Abstract:

Clouds play a key role in the climate of the polar regions. Understanding cloud properties and their climate feedback therefore is paramount to ensure the realism of future climate projections. Despite the importance of the cloud effects, climate models struggle to accurately represent clouds, particularly over the ice sheets. The recent availability, however, of cloud radar (Cloudsat) and lidar (CALIPSO) satellite observations have enlarged the observational dataset of clouds in the polar regions. Together with an extensive ground-based remote sensing instrument set at the Summit station in Greenland and the Princess Elisabeth station in Antarctica, these data provide new opportunities for studying clouds and their radiative forcing. This study aims at validating Cloudsat and CALIPSO cloud measurements for the Summit and Princess Elisabeth stations, based on various cases including clear-sky conditions, typical low-level stratus clouds, clouds with liquid, ice or mixed phase and precipitating as well as non-precipitating clouds. Validation consists of comparing the satellite observations with ceilometer, micropulse lidar, millimeter cloud radar and radiosonde measurements (Greenland) and ceilometer and micro rain radar measurements (Antarctica). Results show an overall good agreement between satellite and ground-based observations, with sensitivity of both ground-based and satellite-based lidars to optically thin ice clouds, while radar images provide information about optically thick clouds which the lidars cannot penetrate. Optically thin ice clouds are more difficult to detect than liquid-containing clouds. A newly developed ceilometer algorithm, specifically adapted to polar regions, detects the base height of the lowest hydrometeor layers, which provides a quantitative basis for validating satellite observations. A successful validation will lead to the construction of a cloud climatology over the ice sheets, ultimately enabling us to investigate cloud-climate feedback mechanisms.