Titolo della tesi: Satellite remote sensing of volcanic particles: models, algorithms and case studies
This PhD thesis is part of the studies of the remote sensing of volcanic particles by satellite passive sensors. The work investigates the interaction of volcanic plumes and clouds with the atmosphere. Volcanic eruptions represent one of the most impressive and hazardous natural phenomena on our planet. Explosive eruptions can release large amounts of tephra into the atmosphere. The tephra come in different configurations based on their shape, size, and porosity, which, together with atmospheric conditions, determine how long they remain in the atmosphere. Tephra particles with a diameter < 2 mm are those that remain suspended in the atmosphere for longer. They represent a potential fatal risk for aviation, as they can erode engines turbines, and a potential threat for human health. The objective of this research project is to develop retrieval techniques based on the microphysical modelling of tephra. The microphysical modelling of tephra involves the study of dense spherical equivalent particles, but also non-spherical porous aggregates of ash with ice inclusions. These 3 years of research led to the development of a Radiative Transfer Model Algorithm (RTMA) designed to simulate the spectral response detected by satellite passive radiometers in the Thermal-Infrared (TIR) and Micro- Millimetre-Wave (MW-MMW) spectrum regions. The TIR signal (from 3 to 14 μm) is sensitive to finer ash particles and provide information about the mass loading (kg m-2) and size of particles < 20 μm mean diameter. The MW-MMW signals (from 1.57 to 3.36 mm) are more sensitive to coarser particles, and can be exploited to estimate the mass load (kg m-2) and size of particles > 0.25 mm mean diameter. In this work, I also investigated the interaction of volcanic clouds with wind fields to improve the retrieval of the volcanic cloud altitude. I used the RTMA to study different volcanic eruptions. Firstly, I applied the RTMA to the eruptions of Kelud in 2014 and Calbuco in 2015. The study of these two eruptions highlighted how the combination of TIR and MW-MMW signals can provide a more accurate characterisation of tephra in the atmosphere. I then applied the RTMA to the 2022 Hunga Tonga-Hunga Ha’apai eruption, that I used to study the impact of ash and ice aggregates on the TIR retrieval. The results show that, depending on the shape of the aggregate and the amount of ice inclusions, the estimated mass of fine ash can vary considerably (≈ 50\%). In addition, I used this eruption, together with the Etna eruption of 10th February 2022, to test two methods aiming at estimating the altitude of the volcanic clouds using wind fields. Moreover, I used the RTMA to estimate the amount of fine ash that is distally transported in atmosphere and I compared it with that found on the deposit, with the aim of constraining numerical ash dispersion models. Finally, I investigated the impact of tephra on satellite communication links, by simulating the signal attenuation and depolarisation at different communication bands.