Sentinel-4 Level-2 AC-SAF ATBD Total Column Water Vapour

Atmospheric water vapour is the most important natural greenhouse gas in the troposphere, accounting for more than 60% of the greenhouse effect (Clough and Iacono, 1995; Kiehl and Trenberth, 1997). Despite this importance, its roles in climate and its reactions to climate change are still difficult to assess. As the atmosphere warms, water vapour is expected to rise faster than the total precipitation amount, which is governed by the surface heat budget through evaporation (Trenberth and Stepaniak, 2003). This results in a positive water vapour feedback that further amplify the original warming effect (Colman, 2003; Soden et al., 2005; Soden and Held, 2006). On the other hand, clouds are known to have positive effects on cooling the earth's surface (Bellomo et al., 2014; Brown et al., 2016). However, the net cooling or warming effect of clouds in a continuous warming atmosphere is not yet well understood (Boucher et al., 2013). To investigate these complex interactions and evaluate climate models, continuous monitoring of the spatio-temporal variations of total column water vapour (TCWV) on a global scale is necessary (Hartmann et al., 2013). This document describes the Algorithm Theoretical Basis Document (ATBD) of the retrieval of total column water vapour from measurements of the Sentinel-4 payload onboard the Meteosat Third Generation-Sounding mission (MTG-S). The algorithm consists of two major steps. The first step is the retrieval of water vapour slant columns. The second step is the conversion of the water vapour slant columns to vertical columns. The purpose of the ATBD is to provide detailed mathematical and physical descriptions of the algorithm, along with discussions of algorithm inputs and outputs, data products, algorithm validation and error analysis. The ATBD also includes a summary description of the Sentinel-4 payload and the anticipated changes from heritage algorithms