Infrared limb sounding of cirrus clouds: state of knowledge, recent progress, and future prospects

Cirrus clouds are the highest altitude clouds in the troposphere and play an important role in the climate system. They can either have a cooling or heating effect in radiation balance around of the planet, depending on which altitude and temperature they appear. Despite the importance of this type of clouds for the radiation budget there are still big gaps of knowledge regarding their micro and macro physical properties (e.g. particle sizes, ice water content, occurrence and coverage at the upper troposphere and lower stratosphere), especially for optically very thin cirrus in the tropopause region, which are difficult to detect even for active lidar measurements. Due to the long path length through the atmosphere and good vertical resolution passive infrared limb measurements are especially well suited to observe this type of clouds. The presentation will highlight the current status in infrared limb sounding and corresponding particle parameter retrievals with respect to recent and future space and airborne sensors (e.g. CRISTA, MIPAS, and IR limb-imaging instruments).