Modelling ash cloud dispersion and the impact of ash aggregation during volcanic eruptions

Improvements for characterizing eruption processes are made commonly through field or remote-sensing measurements, lab experiments and numerical models. However, the main volcanological parameters inter-dependency makes challenging the assessment of tephra dispersion and sedimentation, from which mass eruption rate, total erupted mass, and Total Grain-Size Distribution (TGSD) are typically estimated. This thesis aims at better constraining Eruption Source Parameters (ESP) and in particular the TGSD, usually derived from field sample analysis. The estimation of very fine ash (i.e. < 30 μm) fraction, within the TGSD, commonly suffers from the lack of distal field data, especially for basaltic eruptions. Besides, particle-particle aggregation affects ash dispersal and deposition. Although numerical simulations can account for ash aggregation, they need an accurate TSGD as input. Here, I report the use of the FALL3D model with airborne and ground-based data to quantify i) the very fine ash and the effect on the results and ii) the occurrence of ash aggregation during ash transport. I focus on the integration of field and satellite data to better estimate the TGSD and the PM10 fraction especially. The methodology, which integrates field, ground-based and satellite measurements to improve the TGSD characterization, is applied first to the 23rd February 2013 Etna paroxysm, then to the 23rd November 2013 Etna eruption. Indeed, these eruptions benefited from south-westerly winds dispersing tephra towards the Puglia region (southern Italy; ~410 km from the source), allowing collection of field samples to very distal areas. Then, I studied ash aggregation processes characterizing the explosive eruption of La Soufrière Saint Vincent on 26th April 1979. During this event, a significant aggregate fraction was observed contributing to premature tephra fallout from the vent to Bequia Island (36 km southwards). I investigated the effect of various TGSD together with different aggregation schemes on the resulting tephra loading and ash dispersal.