Application of weather radar to the basaltic phreatomagmatic eruption in Grímsvötn in 2004, comparison with models of plume transport

Weather radar data has been used to monitor volcanic plumes but comparison of these data with direct measurements of plume height have been few, at least for basaltic eruption. The phreatomagmatic eruption in Grímsvötn in 2004 was monitored by weather radar and some measurements of plume height were also made from aircraft. The main phase of the eruption lasted for 33 hours and an almost continuous record was obtained by the radar, located in Keflavík, 260 km to the west of the volcano. During this period the eruption plume reached 6-10 km height relative to the vent. Comparison of the radar measurements of plume height and direct measurements from aircraft suggests that atmospheric super-refraction can cause bending of the radar beam towards the Earth. This leads to a systematic overestimate of plume height by the radar. Visual inspection indicates that apparent oscillation in plume height indicated by the radar record, is greatly exaggerated. The radar beam width increases with distance and is about 4 km over Grímsvötn, resulting in a 2 km uncertanty in plume height determination, and the discrete stepping in elevation angles of the radar is the most likely cause of the apparent fluctuation of 2-4 km in plume height. These shortcomings due to long distance errors underpin the need for the installation of a second weather radar in North-East Iceland, to provide adequate cover for eruptions within Vatnajökull. The measured tephra mass of the eruption can be compared with predictions from theoretical and empirical models relating plume height and mass transport. These models predict 2-4 times greater mass flux than observed in the erupion. Although the error margins are considerable, this suggests that these models overestimate the transport by basaltic phreatomagmatic eruptions.