*2.2. SEVIRI Radiometer*

The Spinning Enhanced Visible and InfraRed Imager (SEVIRI) radiometer [77] is the main instrument onboard of Meteosat Second Generation (MSG). The MSG is a geostationary satellite located at about 36,000 km above the Earth surface at 0◦N, 0◦E. SEVIRI is a passive microwave instrument collecting radiation from a target area and focusing it on detectors sensitive to 12 different bands of the electromagnetic spectrum. The twelve channels are distributed among visible part of electromagnetic spectrum (channels VIS 0.6 μm and VIS 0.8 μm), near-infrared (channel NIR 1.6 μm), infrared (channels IR 3.9 to IR 13.4 μm—for a total of eight channels) and High Resolution Visible (channel HRV 0.75 μm). The SEVIRI nominal time resolution is 15 min, of which twelve minutes are allocated to collect images, while the remaining three minutes are used for calibration, retrace, and stabilization. The SEVIRI spatial resolutions ranges from 1 km for the HRV channel to 3 km for VIS-NIR-IR channels at sub-satellite point (i.e., at 0◦N, 0◦E). The spatial resolution decreases moving away from the sub-satellite point (e.g., over the study area, the Italian peninsula, it is around 4 km for the VIS-NIR-IR channels).

SEVIRI measures the radiation emitted by a target located along the radiometer field of view (i.e., the total radiation emitted by clouds). Depending on the considered channel, the amount of measured radiation is representative of different cloud characteristics. While the measurement in the VIS channels gives an indication about the optical depth of the cloud, the measurement in the IR channels are generally indicative of different cloud properties. In this study, we focused only on three IR channels, namely channels 5, 6, and 9. Channels 5 and 6 are centered in the emission spectrum of the water vapor (WV) at wavelengths at 6.25 and 7.35 μm, respectively, giving an indication about the cloud optical depth other than to determine the water vapor distribution in two distinct layers of the atmosphere. The IR 10.8 μm channel provides continuous observation of the cloud top temperature. For these channels, the final output of SEVIRI is the brightness temperature (TB) that is defined as the temperature of a black body, which emits the same amount of radiation as observed.
