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Microwave remote sensing of volcanic ash clouds.

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This talk is part of the Centre for Atmospheric Science seminars series

Plinian and sub-Plinian volcanic explosive eruptions probably represent one of the most devastating natural events for the surrounding environment, endangering people’s lives and property. Explosive volcanic eruptions can significantly influence climate as well as cloud formation and global circulation through atmospheric transport. Volcanic ash clouds are also an increasing hazard to aviation safety because of growing airline traffic.

From the first rudimental visual inspections at the times of Pliny the

Elder, the human curiosity and its needs to find effective countermeasures to these extreme volcanic episodes has never ceased, even though their (fortunate) rarity makes the scientific research quite challenging.

Nowadays, visual inspections are accompanied by sophisticated measurements made by direct analysis and from remote sensors. Microphysical characterization of the Plinian volcanic ash plumes is usually carried out by analyzing tephra deposits and ash sedimentation at ground. This analysis may only give indirect information on the ash cloud composition as several processes can take place during the ash fallout. On the other hand, real-time monitoring of a volcano eruption is not always possible by conventional visual inspections due to the usual low optical visibility. Finally, airborne flights within ash plumes using sample probes, as done for water clouds, are considered too dangerous for the safety reasons previously mentioned. Remote sensing techniques represent a unique tool to be exploited for this scope. They allow observing the evolution of some key parameters of volcanic eruptions without a direct interaction between the measurement system and the target of the measure. Electromagnetic or acoustic waves are usually used to this aim. Among the available remote sensing techniques, satellite-based approaches, using multi-frequency radiometers with visible and infrared channels, have demonstrated to be a valuable supports to the monitoring of ash clouds. Moreover, measurements in the visible spectral window are not always available due to its solar illumination dependence and the optical thickness of volcanic clouds can severely impair the sounding of lower cloud layers.

In contrast with satellite methodologies, ground-based microwave scanning weather radars can gather three-dimensional information of ash-cloud scattering volumes with ranges up to several hundreds of kilometers, in all weather conditions, at a fairly high spatial resolution (hundreds of meters) and with a repetition cycle of few minutes. So far, these systems have been mainly used for meteorological operational forecasts and for observing of some small number of volcanic areas which can be monitored by previously installed instruments. There are also several open issues about microwave weather radar capabilities to detect and quantitatively retrieve ash cloud parameters.

After a basic introduction on remote sensing principles, the presentation will focus at illustrating and assessing the potential and limitation of microwave remote sensing of Plinian and sub-Plinian volcanic eruption. This will be done using examples from both ground based radar and satellite radiometers data, collected after the 2010 and 2011 eruptions in Iceland. Some quantitative estimates of ash category and ash concentration will be shown together with explanations of the algorithms used.

This talk is part of the Cambridge Volcanology series.

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