ALBERT

Description: Determination of shortwave (SW) and longwave (LW) radiation from space is fundamental in regions where few surface observations are available. In the Arctic this is particularly important due to the complex feedback mechanisms affecting climate in this region. Few sites located in the Arctic region provide surface radiation flux (SRF) measurements long enough to conduct systematic comparisons with satellite products capturing the seasonality of atmospheric and surface parameters.Long-term measurements of downward and upward SW (DSI and USI) and LW irradiances (DLI and ULI) are carried out at the Thule High Arctic Atmospheric Observatory (THAAO, 76.5° N, 68.8° W, https://www.thuleatmos-it.it), in North-Western Greenland. DSI and DLI measurements were started in 2009, and USI and ULI in 2016.In this analysis the four SRF components have been compared with monthly and daily Level 3 Clouds and the Earth’s Radiant Energy System (CERES) surface products , with the aiming of assessing the agreement between ground-based and satellite-derived radiation data.The comparison of THAAO measurements with the monthly CERES products shows a general good agreement, with a high correlation (R〈sup〉2〈/sup〉 about 0.98) and mean negative bias of 4.4 Wm〈sup〉-2〈/sup〉 (CERES underestimates in situ) for DSI, and a mean positive bias of 2.3 Wm〈sup〉-2〈/sup〉 for DLI, possibly associated to cloud effects. The upward components present lower correlations compared to the downward ones, in particular for USI, partly because THAAO is a coastal site, and the CERES data may be influenced by the different surface reflective and emissive properties of land, ice, and sea.

Description: Clouds are one of the less understood Earth's system components. In the Arctic, clouds play a fundamental role in many processes, and their characterization is crucial for the understanding of regional climate, ice melting, radiative budget, and related processes. Arctic cloud optical properties are measured from ground-based and space-borne instruments, but high surface reflectance values, a widespread condition at high latitudes but in general for snow and ice-covered regions, pose severe limitations to the application of many retrieval algorithms. These aspects have been investigated based on measurements made with a UV-VIS-NIR (300-950 nm wavelength range) spectrometer at the Thule High Arctic Atmospheric Observatory (76.5° N, 68.8° W, http://www.thuleatmos-it.it/) on the northwestern coast of Greenland. Continuous measurements are available for the 2022 season (March to September). In combination with radiation transfer simulations carried out with the libRadtran package, different retrieval algorithms for estimating the cloud optical thickness (COT) have been tested and compared. In addition to the spectral measurements, the information on the cloud base height provided by a ceilometer has been included to address the challenges posed by the high reflectance surfaces. Sensitivity studies on COT retrievals in different atmospheric and surface conditions will be presented, along with a case study.