Gas ash-particle separation and the role of particle aggregation in volcanic plumes
The eruption of Eyjafjallajoekull in 2010 demonstrated the widespread disruption that the spread of volcanic ash in the atmosphere can cause, and hence the need for reliable predictions of its dispersion. Many outstanding issues remain, including accurate source characterisation, and better understanding of the processes which spread volcanic ash downwind of the eruption.
The 2011 eruption of Grimsvotn demonstrated one such particular problem: it was found that ash rose predominantly to one height while sulphur dioxide rose to a significantly higher level. The wind shear at the time of this eruption resulted in most of the ash being advected downwind of the volcano in one direction while the sulphur dioxide was transported in another direction (Cooke et al., 2014). With no prior knowledge of this separation of phases, initialisation of the ash in a large-scale atmospheric dispersion model at the same level as the sulphur dioxide produced erroneous predictions of ash in places where little or no ash was observed. This projects seeks to examine this case in detail and use it as a starting point for a systematic study of how such phase separation takes place.
A complex numerical plume model will be used to characterize and quantify the fate of volcanic gases and particles in volcanic eruption plumes. The key process of the separation of volcanic gases and ash particles can be explained in a number of ways: the multiphase nature of the plume with different phases (ash and gas) travelling at different speeds; differential sedimentation of ash particles with significantly different sizes and, related to this, the aggregation that this may induce.
The eventual aim of the project is to improve and extend existing one-dimensional volcanic plume models that reflect better the multiphase nature of volcanic plumes and include processes such as sedimentation. Simple integral plume models already play an important role in initialising atmospheric dispersion models and hence their improvement will have clear practical benefits.
What the student will do:
This project will make use of the Active Tracer High-resolution Atmospheric Model (ATHAM). ATHAM has been specifically designed to simulate the characteristics of three-dimensional volcanic eruption plumes (Oberhuber et al., 1998; Herzog et al., 2003). Unique features of ATHAM are its dynamically and thermodynamically active tracers, as well as its ability to represent strongly divergent flows with large vertical accelerations. Active tracers differ in their vertical velocity relative to the mixture so that particle sedimentation and gas particle separation are described by the model. ATHAM contains a cloud microphysical module (Herzog et al., 1998). An extension of this module has been used to study ash aggregation (Textor et al., 2006).
The first part of the project will be to improve the ash aggregation module and allow for aggregation induced by charged ash particles. This will then enable us to study the effects of sedimentation and aggregation more fully. Outcomes will be used to improve models that are used at the Met Office for ash dispersion forecasting. The project will include multiple visit to the Met Office through an industrial CASE partnership.
Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.
APPLYING TO GEOGRAPHY
The Course Description is "PhD in Geography"; entering the word Geography in the Course Directory Search should bring this up.
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