State-of-the-art Total Column Water Vapour Retrievals for Improved Characterization of Pre-convective Environments

Observations of total column water vapour (TCWV) play an integral role in advancing our understanding and nowcasting of convective initiation, convective cloud development and associated severe weather phenomena. Since most water vapour is accumulated in the lower levels, recently, TCWV has gained even more research interest and applications in nowcasting and storm intensification research.

We present a TCWV retrieval that is sensitive to the lowest level moisture which is based on satellite observations in the near-infrared (NIR) and thermal infrared (TIR). The NIR part is based on the Copernicus Sentinel-3 OLCI Water Vapour product (COWa) retrieval framework for Sentinel 3 Ocean and Land Colour Imager (OLCI) measurements at the rho-sigma-tau water vapour absorption peak (900nm) and provides clear-sky daytime TCWV fields at 300 m resolution. The TIR part is based on the split window technique at 11 and 12 micron and is applied to measurements from the the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on Meteosat Second Generation (MSG) and provides clear-sky TCWV fields at a spatial resolution of several km every 15 minutes.

Both algorithms and their combination will be adaptable to any sensor with bands in these spectral regions, this includes the new Meteosat Third Generation Flexible Combind Imager (MTG-FCI). For work towards this adaptation, the synergy of measurements from OLCI and the Sea and Land Surface Temperature Radiometer (SLSTR) are used as a stand-in. First processed scenes show that the combination of NIR and TIR provide a significant improvement over the use of only TIR or NIR-measurements over land and water surfaces, respectively.

As a first application, the spatial information from the OLCI TCWV fields is combined with the temporal information from the SEVIRI TCWV time series to characterize TCWV variability in pre-convective environments in Germany. To this end, several years of both TCWV fields and Nowcasting and Very Short Range Forecasting Satellite Application Facility (NWCSAF) convective cloud products have been processed, combined and analyzed. Future TCWV retrievals from MTG-FCI will further improve spatio-temporal resolution and availability and advance accompanying applications, such as detection of CI and assimilation in Numerical Weather Prediction models.