Studying the life cycle of clouds with geostationary satellites

Kurzfassung

Clouds play a very important role in the water cycle of the Earth. At the same time, they control the radiation budget of the Earth-atmosphere system, by reflection, absorption, and emission of solar and thermal radiation. Clouds are highly variable in time; their life cycle is poorly understood yet. The new Meteosat Second Generation (MSG) geostationary satellite with its main instrument SEVIRI (Spinning Enhanced Visible and InfraRed Imager), provides comprehensive spectral information over the Earth disk observed from its nominal position of 3.4°W 0°N with a fast repeat cycle of 15 minutes which is excellently suited for the remote sensing of the life cycle of clouds. SEVIRI comprises 11 calibrated spectral channels in the visible and infrared spectral ranges, with a spatial resolution of 3 km x 3 km at the subsatellite point. In addition, it is equipped with a broadband high resolution visible (HRV) channel with a ground sampling distance of about 1 km at the sub-satellite point. Using these data, we are able to quantitatively derive detailed macro- and microphysical properties of clouds at an unprecendented time resolution of 15 minutes, allowing for the first time to study the formation, evolution, and dissipation of clouds from space. The investigation of the temporal evolution of the droplet and particle radius of clouds has the potential to shed new light e.g. on the process of rain formation. The modification of the cloud micro- and macrophysical structure with time has a strong impact on radiative forcing and consequently on climate. We will show examples of the temporal evolution of the main cloud properties, optical thickness, phase, and effective radius. A particular attention will be given to cirrus clouds because - due to their high altitude close to the coldest point of the radiatively relevant atmosphere - their trapping of thermal radiation often over-compensates the reflection of solar radiation, thus leading to a considerable warming of the Earth-atmosphere system in particular during night time. The time step of 15 minutes allows conclusions about the origin of clouds, and helps to better quantify their effect on the radiation budget. While polar-orbiting systems only provide singular snapshots, the geostationary perspective allows to follow clouds from their formation to their dissipation, thus allowing to quantify the total effect on the radiation budget.