In this case study we compare cloud fractionalcover measured by radiometers on polar satellites (AVHRR)and on one geostationary satellite (SEVIRI) to ground-basedmanual (SYNOP) and automated observations by a cloudcamera (Hemispherical Sky Imager, HSI). These observationstook place in Hannover, Germany, and in Lauder, NewZealand, over time frames of 3 and 2 months, respectively.Daily mean comparisons between satellite derivations andthe ground-based HSI found the deviation to be 6 14%for AVHRR and 8 16% for SEVIRI, which can be consideredsatisfactory. AVHRR’s instantaneous differences aresmaller (2 22 %) than instantaneous SEVIRI cloud fractionestimates (8 29 %) when compared to HSI due to resolutionand scenery effect issues. All spaceborne observationsshow a very good skill in detecting completely overcast skies(cloud cover 6 oktas) with probabilities between 92 and94% and false alarm rates between 21 and 29% for AVHRRand SEVIRI in Hannover, Germany. In the case of a clearsky (cloud cover lower than 3 oktas) we find good skill withdetection probabilities between 72 and 76 %. We find poorskill, however, whenever broken clouds occur (probability ofdetection is 32% for AVHRR and 12% for SEVIRI in Hannover,Germany).In order to better understand these discrepancies we analyzethe influence of algorithm features on the satellite-baseddata. We find that the differences between SEVIRI and HSIcloud fractional cover (CFC) decrease (from a bias of 8 toalmost 0 %) with decreasing number of spatially averagedpixels and decreasing index which determines the cloud coveragein each “cloud-contaminated” pixel of the binary map.We conclude that window size and index need to be adjustedin order to improve instantaneous SEVIRI and AVHRR estimates.Due to its automated operation and its spatial, temporaland spectral resolution, we recommend as well that moreautomated ground-based instruments in the form of cloudcameras should be installed as they cover larger areas of thesky than other automated ground-based instruments. Thesecameras could be an essential supplement to SYNOP observationas they cover the same spectral wavelengths as thehuman eye.
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