Development and applications of a cloud regime dataset over Europe using satellite observations

Within the framework of the present dissertation, a cloud regime dataset was developed over Europe using satellite observations. The climatology of the “Cloud Regime dAtAset based on the CLAAS-2.1 climate data record” (CRAAS) is thoroughly investigated and the derived cloud regimes are used in a number of applications. The cloud regimes are generated over Europe on a 1°x1° spatial resolution and extend over a 14-year period (from 2004 until 2017) with a high temporal resolution of 15-minutes. In particular, the cloud optical thickness (COT) and cloud top pressure (CTP) products of CLAAS-2.1 are used in order to compute 2D histograms. Then, the k-means clustering algorithm is applied to the generated 2D histograms for the derivation of the cloud regimes. Eight cloud regimes are identified, which assist in providing a detailed description of the climate of the cloud properties over Europe. Three of these cloud regimes are associated with high-level clouds and, in particular, with cirrus, cirrostratus and dense cirrus from deep convection and frontal systems. An additional cloud regime is connected to alto- and nimbo-type clouds, while a fifth one is related to mid-level clouds. Two cloud regimes are found to represent low-level clouds, specifically stratocumulus, shallow cumulus and fog. The final cloud regime is associated mainly with fair weather clouds and joint cloud histograms with no distinguishable structure. The spatiotemporal variability and changes of the CRAAS cloud regimes are investigated over the European region from 2004 until 2017. The last cloud regime is found to be the most dominant, in terms of frequency of occurrence, reaching values up to 85.5% over regions of lower latitude. The annual and diurnal cycles are also dominated by the last cloud regime, while features of the main representative cloud type of each cloud regime can be observed as well. The biggest decrease in the frequency of occurrence, -0.65% during the time period of the study, is observed for the cloud regime representing the alto- and nimbo-type clouds. In contrast, the largest increase, +0.70% during the time period of the study, is noted for the cloud regime associated with shallow cumulus and fog. The majority of the statistically significant trends are observed over lower latitudes, focusing mostly over the Mediterranean. An intercomparison against a similar classification, based on cloud optical properties from the International Satellite Cloud Climatology Project (ISCCP), showed that it is important to distinguish between regionally and globally derived cloud regimes. Furthermore, in order to identify relations between large-scale weather patterns and the cloud regimes, co-occurrences between the cloud regimes and the Objective Weather Type Classification are studied. Finally, the combination of the cloud regimes with vertical profiles from ground based measurements is used for a detailed description of cloud structures and their temporal evolution in frontal systems