Validating Subglacial Volcanic Eruption Using Ground-Based C-Band Radar Imagery

The main phase of the moderately sized November 2004 eruption of the Grímsvötn volcano, located in the center of the 8100 km² Vatnajökull glacier, was monitored by the Icelandic Meteorological Office C-band weather radar in Keflavík, 260 km west of the volcano. The eruption plume reached a height of 6-10 km relative to the vent. The distribution of the most distal tephra was measured in the autumn of 2004, while the deposition on the glacier was mapped in the summers of 2005 and 2006. The tephra formed a well-defined layer on the glacier in the region north and northeast of the craters. The total mass of the tephra layer is quantitatively compared with the retrieved values, obtained from an improved version of the volcanic ash radar retrieval (VARR) algorithm. VARR was statistically calibrated with ground-based ash size distribution samples, taken at Vatnajökull, and by taking into account both antenna beam occlusion and wind-driven plume advection. The latter was implemented by using a space-time image phase-based cross-correlation technique. Accuracy of the weather radar records was also reviewed, noting that a large variability in the plume height estimation may be obtained using different approaches. The comparisons suggest that, at least for this subglacial eruption, the surface tephra mass, estimated by using the VARR inversion approach, is in a fairly good agreement with in situ measurements in terms of spatial extension, distribution, and amount.