Cloudiness retrieved from All-Sky camera and MSG satellite over Reunion Island and Antananarivo Madagascar

Abstract. To gain a deeper understanding of cloud variability over the Southwest Indian Ocean (SWIO) region, various measurement techniques can be used. We have used ground-based observations using all-sky cameras from the UV-Indien network provided by the Reuniwatt company, which capture sky images in the visible spectrum. Two algorithms, namely Elifan and Reuniwatt, were applied to analyze the camera images. Despite differences in the methodologies employed by each algorithm, we have found strong agreement between them, with a Bias of -5.48 %, a Root Mean Square Error (RMSE) of 6.48 %, and a correlation coefficient (r) of 0.99 for Saint-Denis. Ground-based measurements over the SWIO are insufficient, and the vast ocean coverage in this region makes spatial observation important for gathering information. Here we have used cloud products from the Meteosat Second Generation (MSG) satellites. The quality of the classification algorithm employed for camera and satellite plays a significant role in cloud analysis. Comparing camera and satellite observations is essential to ensure the complementarity of each measurement. We observed good consistency between the ground-based camera and satellite measurements (Bias=2.64 %, RMSE=21.43 %, and r=0.87) with Elifan, and (Bias=6.79 %, RMSE=25.70 %, and r=0.82) with Reuniwatt for the Saint-Denis site; and (Bias=6.48 %, RMSE=28.63 %, and r=0.78) for Antananarivo. In Antananarivo, during the dry season, heavy cloud cover (~50 %) is observed in the morning, gradually dissipating as the day progresses. Conversely, in the wet season, cloud cover varies between approximately 30 % and 60 % from December to April, with weaker cloudiness around noon in October and November. As for Saint-Denis, the morning skies are generally clear but become increasingly overcast throughout the day, reaching up to 80 % cloud cover during the wet season and 60 % during the dry season.