Evaluating the assimilation of S5P/Tropomi NRT SO2 columns and layer height data into the CAMS integrated forecasting system (CY47R1), based on a case study of the 2019 Raikoke eruption

The Copernicus Atmosphere Monitoring Service (CAMS), operated by the European Centre for Medium-Range Weather Forecasts on behalf of the European Commission, provides daily analyses and 5-day forecasts of atmospheric composition, including forecasts of volcanic sulphur dioxide (SO 2 ) in near-real time. CAMS currently assimilates total column SO 2 products from the GOME-2 instruments on MetOp-B and -C and the TROPOMI instrument on Sentinel-5P which give 15 information about the location and strength of volcanic plumes. However, the operational TROPOMI and GOME-2 data 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 SO 2 increments in the vertical. In the current operational CAMS configuration, the SO 2 increments are placed in the mid-troposphere, around 550 hPa or 5 km. While this gives good results for the majority of volcanic emissions, it will clearly be wrong for eruptions that inject SO 2 at very 20 different altitudes, in particular exceptional events where part of the SO 2 plume reaches the stratosphere. A new algorithm, developed by DLR for GOME-2 and TROPOMI and optimized in the frame of the ESA-funded Sentinel-5P Innovation–SO 2 Layer Height Project, the Full-Physics Inverse Learning Machine (FP_ILM) algorithm, retrieves SO 2 layer height from TROPOMI in NRT in addition to the SO 2 column. CAMS is testing the assimilation of these products, 25 making use of the NRT layer height information to place the SO 2 increments at a retrieved altitude. Assimilation tests with the TROPOMI SO 2 layer height data for the Raikoke eruption in June 2019 show that the resulting CAMS SO 2 plume heights agree better with IASI plume height data than operational CAMS runs without the TROPOMI SO 2 layer height information and that making use of the additional layer height information leads to improved SO 2 forecasts than when using the operational CAMS configuration. Including the layer height information leads to higher modelled TCSO 2 values in better 30 agreement with the satellite observations. However, the plume area and SO 2 burden are generally overestimated in the CAMS analysis also when LH data are used. The main reason for this overestimation is the coarse horizontal resolution used in the minimisations. By assimilating the SO 2 layer height data the CAMS system can predict the overall location of the Raikoke SO 2 plume up to 5 days in advance for about 20 days after the initial eruption, which is better than with the operational CAMS configuration (without prior knowledge of the plume height) where the forecast skill reduces much more 35 for longer forecast lead-times. 40 Global coverage with GOME-2 is achieved within 1.5 days. The GOME-2 measurements allow for the retrieval of ozone and a range of atmospheric trace gases, including SO 2 which is retrieved with the GOME Data Processor (GDP) developed by DLR and operationally provided by the EUMETSAT’s ACSAF that uses a DOAS method. GDP4.8 is used for GOME-2A and GOME-2B (with a fitting window from 315-326 nm) and GDP4.9 for GOME-2C (with a fitting window of 312-326 nm to include the strong SO 2 line at 313 nm). Input parameters for the DOAS fit include the absorption cross section of SO 2 and the absorption cross sections of interfering gases, ozone and NO 2 , and a 200 correction is made in the DOAS fit to account for the ring effect (rotational Raman scattering). An empirical interference correction is applied to the SO 2 slant column to reduce the interference from ozone To reduce the interference from ozone absorption, the retrieval includes the fitting of two pseudo ozone cross-sections following the of et (2010). As the case for the TROPOMI dataset, a volcano activity detection algorithm is used to identify elevated SO 2 values from volcanic eruptions. Such flags were implemented in GDP4.8 (see Table and further 205 improved in GDP4.9 to use the same flagging TROPOMI (see only