Terminal settling velocity measurements of volcanic ash during the 2002-2003 Etna eruption by an X-band microwave rain gauge disdrometer

This is the first report in the scientific literature of direct measurement of the terminal settling velocity of volcanic particles during an eruption. Field measurements using a continuous wave X-band disdrometer were carried out at Mt. Etna on 18 and 19 December 2002, when the explosive activity produced a 4 km high volcanic plume. These data allow the estimation of the intensity of the fallout and the measurement of the terminal settling velocities of the volcanic particles in real-time. The main results are: ( 1) the tested instrument detected coherent falling volcanic particles from 0.2 to 1 mm diameter; ( 2) measured terminal settling velocities were in agreement with both experimental and theoretical methods; ( 3) however, the measured velocities were clustered around few discrete values, rather than a range of velocities as would be expected if the particles were falling simultaneously and discretely. This new methodology has many new applications for local hazard mitigation and improved understanding of fallout processes.