Volcanic sulfur dioxide plume forecasts based on UV satellite retrievals for the 2011 Grímsvötn and the 2010 Eyjafjallajökull eruption

The sulfur dioxide plumes released by the eruptions of the Icelandic volcanoes Eyjafjallajökull in May 2010 and Grímsvötn in May 2011 were studied using satellite observations from the Second Global Ozone Monitoring Experiment (GOME‐2), Ozone Monitoring Instrument (OMI), and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography and modeled with the Integrated Forecasting System of the European Centre for Medium‐Range Weather Forecasts (ECMWF). The retrievals of SO 2 total columns (TCSO2) were (i) used to estimate emission rate and injection height of the two eruptions and (ii) assimilated with ECMWF's four‐dimensional variational data assimilation algorithm to obtain initial conditions for subsequent forecasts. The OMI retrievals provided the highest plume observation values, and GOME‐2 had the best coverage. The emission parameters were estimated by comparing TCSO2 observations with an ensemble of test tracers injected at different heights. The applied methodology led to emission estimates of 0.25 Tg over 20 days in May 2010 and 0.32 Tg over 2 days in May 2011. The SO 2 analyses produced by assimilating GOME‐2 TCSO2 retrievals captured the plume maxima well but exaggerated the plume area. The injection height estimate was used during the assimilation to determine the height of the assimilated plume. Plume forecasts were evaluated by means of hit‐rate and plume‐size statistics for different TCSO2 thresholds. Plume forecasts using either the emission parameters or the SO 2 analyses as initial conditions agreed reasonably with the observations, but using both led to the best forecast performance. The initialization with SO 2 analysis fields improved, in particular, the forecast of the Grímsvötn plume after the end of the eruption. The developed forecast and assimilation system can be applied for near–real‐time forecasting of volcanic SO 2 plumes.