Assimilation of TROPOMI SO2 Layer Height Data in the CAMS System

The Copernicus Atmosphere Monitoring Service (CAMS), operated by the European Centre for Medium-Range Weather Forecasts (ECMWF) on behalf of the European Commission, provides daily analyses and 5-day forecasts of atmospheric composition, including forecasts of volcanic sulfur dioxide (SO2) in near-real time (NRT). CAMS currently assimilates total column SO2 retrievals from the GOME-2 and TROPOMI instruments which give information about the location and strength of volcanic plumes. However, the operational TROPOMI and GOME-2 retrievals do not provide any information about the height of the volcanic plumes and therefore some prior assumptions need to be made in the CAMS data assimilation system about where to place the resulting SO2 increments in the vertical. Currently, the SO2 increments are placed in the mid-troposphere, around 550 hPa. While this gives good results for the majority of volcanic emissions, it will clearly be wrong for eruptions that inject SO2 at very different altitudes, in particular for exceptional events where part of the SO2 reaches the stratosphere. A new algorithm, developed by DLR for GOME-2 and now being adapted to TROPOMI in the frame of the ESA-funded Sentinel-5P Innovation–SO2 Layer Height Project (S5P+I: SO2LH), the Full-Physics Inverse Learning Machine (FP_ILM) algorithm, retrieves SO2 layer height from TROPOMI in NRT in addition to the SO2 column. CAMS is testing the assimilation of these data, making use of the NRT layer height information to place the SO2 increments at a better altitude. We present results from assimilation tests with the TROPOMI SO2 layer height data for several volcanic eruptions and show that making use of the additional layer height information leads to improved SO2 forecasts.