A Study of the Life Cycle of Cirrus Clouds Based on METEOSAT-SEVIRI IR Data
Huber, Isabelle; Kox, Stephan; Margarita, Vazquez-Navarro

Clouds have an enormous impact on Earth's climate. Nevertheless, current climate models do not represent cloud physics well and suffer from a lack of validation data. We provide such validation data for cirrus clouds through the retrieval and analysis of cloud parameters like optical depth, top altitude, ice water path, and effective radius over the whole life cycle of cirrus clouds in a high spatial (~3km x 3km) and high temporal resolution (5min to 15min).

We present the Cirrus cloud Tracking Algorithm (CiTA) that tracks different kinds of cirrus clouds from their formation to their dissolution. CiTA uses data provided by the Cirrus Optical properties derived from CALIOP and SEVIRI algorithm (COCS) that is based on a backpropagation neural network. COCS retrieves cirrus optical thickness, top altitude, ice water path and effective radius from the thermal infrared channels of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) aboard the METEOSAT Second Generation satellites (MSG) through a neural network trained by coincident measurements of Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard NASA's Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations mission (CALIPSO). The COCS data are chosen as input for CiTA because the tracking of clouds over the whole time of their life cycle demands a combination of the advantages of active instruments like CALIOP on polar-orbiting satellites and the advantages of geostationary passive sensors such as SEVIRI. Only geostationary sensors with their orbit and their infrared channels enable the retrieval of cloud parameters over the whole life cycle of the cloud during day and night. Polar-orbiting satellites with their repeat cycle of more than two weeks and their narrow ground track cannot retrieve information during the shorter-term life cycle processes of clouds.
Applying CiTA on COCS leads to insights into the physical processes that govern the formation and dissolution of cirrus clouds thereby providing validation data for cirrus cloud modeling. An analysis of the life cycle of different cirrus cloud types is presented for selected regions.

In a later stage the COCS and CiTA algorithms may be adapted to the instruments on ESA's Earth Clouds, Aerosols and Radiation Explorer mission (EarthCARE).