Clouds cover about 70% of the Earth’s surface and play a key role in the energy and water cycle of our planet. The Global Energy and Water Exchanges (GEWEX) Cloud Assessment (Stubenrauch et al. 2013) has highlighted the value of cloud properties derived from space observations for climate studies and model evaluation.
Compared to other passive remote sensing instruments, the high spectral resolution of IR vertical sounders in particular provides reliable properties of cirrus with IR optical depth as low as 0.1, day and night.
The CIRS retrieval (Stubenrauch et al. 2017) has been applied to observations from the Atmospheric IR Sounder (AIRS) aboard the Aqua platform and from the IR Atmospheric Interferometers (IASI) aboard the Metop platforms, and is easily adaptable to any other IR sounder. The synergy between AIRS and IASI particularly allows to study the diurnal cycle of high-level clouds (Feofilov and Stubenrauch 2019).