Integrated Water Vapor retrieval under cloudy sky conditions from SWIR satellite measurements in the context of C3IEL space mission project.

We present a retrieval algorithm based on the optimal estimation method for estimating the integrated water vapor above clouds using shortwave infrared radiance observations (Peroni et al., 2025). Water vapor plays a crucial role in cloud formation and evolution, particularly in convective systems where exchanges between clouds and their surrounding environment strongly modulate the local variability of atmospheric humidity. Improved knowledge of the water vapor distribution above and around clouds is therefore essential for better understanding cloud/water vapor interactions and for constraining Large-Eddy Simulations and weather prediction models.

The retrieval algorithm is developed in the framework of the Cluster for Cloud evolution, ClImatE and Lightning (C3IEL) space mission, scheduled for launch in 2028. C3IEL aims to advance our understanding of convective cloud dynamics by providing observations of three-dimensional cloud development velocities, electrical activity, and the water vapor distribution above and around clouds.

Results obtained for idealized atmospheric conditions with vertically homogeneous cloud profiles demonstrate the feasibility of retrieving the integrated water vapor above clouds from three shortwave infrared radiances. Absolute retrieval errors are found to be below 2 kg.m⁻² for optically thick clouds or for integrated water vapor contents below 20 kg.m⁻², and below 1 kg m⁻² for very thick clouds (COT > 150). For more realistic cases, from the ECMWF-IFS dataset, the retrieval performs well for water clouds, with RMSE generally below 1 kg.m⁻². Retrieval accuracy is found to mainly depend on cloud vertical penetration, with degraded performance for optically thin and low-level clouds (COT < 50 and cloud top height < 2 km).

For very low water vapor contents encountered mainly above high deep convective clouds, the algorithm tends to systematically overestimate the retrieved values due to an overestimation of the cloud extinction profile in the upper cloud layers within the inversion model. These results demonstrate the strong potential of shortwave infrared observations for retrieving integrated water vapor above clouds and provide guidance for further improvements of the retrieval algorithm in preparation for the C3IEL mission.