Intercomparison of satellite- and ground-based cloud fraction over Switzerland (2000–2012)

Satellite data provide the opportunity for systematic and continuous observation of cloud cover over large spatial scales. In this paper, we describe the generation of two new high spatial resolution (0.05°) daytime cloud fraction data sets over Switzerland. The data sets are based on the Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask products. The data sets cover the period from March 1, 2000 to February 29, 2012 (Terra/MODIS) and July 1, 2002 to February 29, 2012 (Aqua/MODIS) and represent mid-morning and early-afternoon cloud cover over Switzerland. Time series clearly reflected seasonal variations in cloud fraction over Switzerland. A comparison with cloud fraction observations at four Synop stations (Chur, Locarno/Monti, Payerne, Zurich/Kloten) revealed an agreement of monthly mean mid-morning cloud fraction (MMCF) within ± 1 octa (i.e., 12.5%). Relative to Synop observations, MMCF was positively biased by 0.3–5.0%, except at Payerne (− 2.5%). Linear correlation coefficients ranged from 0.878 to 0.972. Results were similar for monthly mean early-afternoon cloud fraction (MACF). Cloud fraction was found to be higher in the early-afternoon when compared to mid-morning, except at Payerne and Zurich/Kloten in fall, which is explained by typical daytime cloud cover patterns in Switzerland. Analysis of daily mid-morning cloud fraction showed that largest discrepancies were observed in partly cloudy conditions, which is mainly explained by differences in observation times and observation geometry. Our results demonstrate that the newly processed cloud fraction data sets from the MODIS sensor can play an important role in complementing traditional Synop observations in support of systematic cloud cover monitoring within the National Climate Observing System (GCOS Switzerland).