On the benefits of assimilating clear-sky radiances every 75 km globally at sub-hourly time scales

A refined 4D-Var assimilation system within DestinE allows us to assimilate the Meteosat-10/SEVIRI clear-sky radiances over Europe, as well as globally at a spatial scale of 75 km instead of the previous 125 km in the ECMWF Integrated Forecasting System (IFS). Higher resolution observations can potentially improve the analysis and therefore the prediction of extreme weather events over Europe, as well as globally. The effects of using higher resolution observations have been investigated with a detailed set of experiments and the impact on wind, temperature, and humidity has been evaluated. A broad range of experiments indicate that exploiting the higher spatial density clear-sky radiances leads to an improvement of humidity sensitive fields in short-range forecasts with the IFS as independently measured for example by instruments on low-Earth-orbiting satellites (IASI, CrIS, SSMIS, or ATMS). Due to a reduced displacement and representativeness error, these changes further lead to improvements in longer range forecasts as these errors would propagate upscale nonlinearly. Our experiments show an upscale propagation of initially very localised increments in the analysis fields of vertical wind, as well as humidity above the Pacific or the North Atlantic. Over the first 25 days of cycling, these incremental improvements from the 4D-Var system lead to an improvement in forecast scores of the IFS. Such a configuration with globally denser radiances will go into the next IFS Cycle 50r1. In the DestinE 4 km analysis, spatial error correlations are significantly reduced, e.g., for Meteosat-10/SEVIRI above Europe, highlighting the potential of high resolution data assimilation, as a reduction in spatially correlated errors leads to more accurate inital conditions, and globally improved forecasts up to 5 days ahead.

For the chosen configuration with spatially denser observations every 75 km globally at the sub-mesoscale, we focus on assimilating geostationary satellite observations at sub-hourly timescales every 10 minutes. For that purpose, we assimilate the pre-processed GOES-16-18/ABI observations by NOAA, as well as HIMAWARI-9/AHI by the Japanese Meteorological Agency (JMA), every 10 min, 20 min and 30 min. Exploring how to best assimilate relatively small spatial and temporal scales for these geostationary satellites, will allow us to approach a higher resolution for the whole MTG/FCI satellite series above Europe. Thereby, single cycle experiments with a 4 km global analysis reveal the impact of wind tracing in 4D-Var. In combination with the spatially and temporally denser observations, we further discuss the impact of diabatic heating on the role of establishing a meridional circulation that significantly improves wind, temperature and humidity over the southern oceans.