Diurnal variation of cloud cover over the Baltic Sea

Abstract

Instantaneous cloud cover over the Baltic Sea, estimated from satellite information, may differ by as much as several dozen percent between the day and night. This difference may result from both weather conditions and different algorithms used for the day and night. The diurnal differences in cloudiness measured by proprietary and operational systems were analysed as part of research on marine environmental assessment and monitoring. An optimised algorithm for 2017 was presented and supplemented with information from radiation modelling. The study showed that, in general, the average values of daily changes in cloud cover over the sea depend on the season, which generally corresponds to the length of the day and contrasts with the amount of cloudiness. The results were compared with available online data that met the night and day detection criteria, the climate model, and the climate index. The averaged analysis of seasonal changes showed that similar values of the satellite estimates are higher than those obtained from the climate model and the lidar estimation. The satellite estimates from SatBaltic showed the lowest uncertainty. The diurnal cycle was confirmed by all analysed systems. These results may indicate common physical mechanisms and a methodological reason for the uncertainty of satellite-based data. The results clearly showed the existing diurnal difference in the amount of cloud cover over the Baltic Sea and indicated that this difference is not always explained by the physical properties of the atmosphere. The probable causes for these uncertainties were identified and diagnosed.