Optimal estimation retrieval framework for daytime clear-sky total column water vapour from MTG-FCI near-infrared measurements

Abstract

A retrieval of total column water vapour (TCWV) from the new daytime, clear-sky near-infrared (NIR) measurements of the Flexible Combined Imager (FCI) onboard the geostationary satellite Meteosat Third Generation Imager (MTG-I, Meteosat-12) is presented. The retrieval algorithm is based on the differential absorption technique, relating TCWV amounts to the radiance ratio of a non-absorbing band at 0.865 µm and a nearby water vapour (WV) absorbing band at 0.914 µm. The sensitivity of the band ratio to WV amount increases towards the surface which means that the whole atmospheric column down to the boundary-layer moisture variability can be observed well.

The retrieval framework is based on an optimal estimation (OE) method, providing pixel-based uncertainty estimates. It builds on well-established algorithms for other passive imagers with similar spectral band settings. Transferring knowledge gained in their development onto FCI required new approaches. The absence of additional, adjacent window bands to estimate the surface reflectance within FCI's absorbing channel is mitigated using a principal component regression (PCR) from the bands at 0.51, 0.64, 0.865, 1.61, and 2.25 µm.