Satellite-Derived Trends in Cloud Cover over Bavaria

Cloud cover trends are highly relevant for the energy and health sectors, as clouds affect the radiation balance and thereby influence parameters such as air temperature and UV index. In particular, during heat waves, cloud-induced reductions of nocturnal cooling rates are of considerable interest. For effective climate adaptation and mitigation, cloud cover trends must be assessed at fine spatial scales and with sufficient temporal resolution to distinguish at least between day- and nighttime conditions. Within the Bavarian state-funded EO4CAM (Earth Observation Laboratory for Climate Adaptation and Mitigation) project, which aims to leverage spaceborne Earth observation and model data to support climate change adaptation and mitigation, we derive spatially resolved cloud cover trends over Bavaria from spaceborne observations between 2004 and 2019 for three-hourly time slots at monthly resolution.

The analysis is based on data from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) aboard Meteosat Second Generation (MSG). We apply a Mann-Kendall trend analysis to the Optimal Cloud Analysis Climate Data Record [1], which provides a homogeneous long-term record of cloud properties. The dataset has a temporal resolution of 15 minutes and a spatial resolution of 6x6 km² over Bavaria.

A generalization of cloud cover trends is precluded by their strong spatial, seasonal, and diurnal dependence. On the one hand, however, cloud fraction typically increases during daytime due to enhanced convective activity. On the other hand, the temporal evolution over the years within a given calendar month is similar across different daytime hours.

As an example, cloud cover over Bavaria at noon in August typically ranges between 60 and 80%, exceeding 80% in the Alpine region. Between 2004 and 2019, trends are predominantly negative across Bavaria, reaching values of up to -1.5 percentage points per year, with the strongest statistical significance observed in northern Bavaria. In contrast, cloud cover trends in the Alpine region remain largely neutral. A more detailed classification shows an increase in the number of days with low (<15%) and medium (15–85%) cloud fractions throughout Bavaria, accompanied by a decrease in days with high (>85%) cloud fraction. These changes are most pronounced in northern Bavaria.

[1] EUMETSAT. Optimal Cloud Analysis Climate Data Record (Release 1): MSG, 0°. 2022. doi: 10.15770/EUM_SEC_CLM_0049. url: https://user.eumetsat.int/catalogue/EO:EUM:DAT:0617/access.