Study of the lower stratosphere and troposphere exploiting the synergy of limb and nadir measurements: expected contribution of Earth Explorer 11 candidate mission CAIRT

The upper-troposphere - lower stratosphere (UTLS) is a very important region since climate is strongly sensitive to the atmospheric composition in this region, while the transport of air from the stratosphere to the troposphere has also an impact on the air quality, being a source of variability mainly in tropospheric ozone. However, the UTLS is difficult to explore from space. Instruments looking through the UTLS in the nadir direction do not have the capability to resolve vertical details, while limb-viewing instruments are limited by the presence of clouds in the upper troposphere. The Changing-Atmosphere InfraRed Tomography explorer CAIRT, one of the four candidates selected for phase 0 of ESA Earth Explorer 11, could be the first limb-sounder with imaging Fourier-transform infrared technology in space. If selected, CAIRT will measure limb radiances from 5 to 115 km altitude with a nominal horizontal sampling of about 50 by 50 km (possible subsampling for selected science targets) and a vertical sampling of 1 km. The spectral range will cover the region from 720 cm-1 to 2200 cm-1 (4.5 to 14 ?m in wavelength), with a spectral resolution of 0.1 cm-1. With a targeted across-track coverage of 500 km, near-global coverage is achieved within about two days. Tomographic retrievals will provide temperature and trace gas profiles at a much higher horizontal resolution and coverage than achieved from space so far, enabling us to address new science questions. Flying in loose formation with the Second Generation Meteorological Operational Satellite (MetOp-SG) will enable combined retrievals with observations by the New Generation Infrared Atmospheric Sounding Interferometer (IASI-NG), as well as from Sentinel-5 nadir sounder, resulting in consistent atmospheric profile information from the surface to the lower thermosphere. This will open new scientific opportunities to study trace gas transport and transformations in the lower stratosphere and troposphere. The paper will describe how the synergy of CAIRT, IASI-NG and Sentinel-5 measurements could help to improve the knowledge of the atmospheric composition in this region of the atmosphere. The Complete Data Fusion technique will be used to study the feasibility and improvements coming from the exploitation of this synergy.