Kelvin-Helmholtz instabilities in volcanic clouds and their effects on ash dispersal
Abstract
Occasionally, volcanic clouds show horizontal structures (or stripes) oriented perpendicular to the prevailing direction of the wind. This phenomenon has been observed by satellite images and reported at several volcanoes worldwide (e.g., Klyuchevskaya in Kamchatka and Eyjafjallajökull in Iceland) and it was particularly evident during the 2006 eruption at Mt. Etna. The purpose of this study is two-fold: first we provide a reasonable description of the mechanism driving such phenomena, demonstrating that, even though the ambient wind field exerts significant control on aerosol and ash dispersal, the volcanic plume itself modifies the surrounding atmosphere and creates conditions favorable to Kelvin-Helmholtz instabilities (KHI) which are visible as horizontal structures or stripes; second, we use a Lagrangian particle model (LPAC) to simulate the transport of both passive tracers as well as particles with mass, and show that Kelvin-Helmholtz instabilities may significantly affect the transport of volcanic products. Specifically we used a wind-particle coupled model to reproduce such phenomenon observed by the MODIS satellite within the ash cloud generated during the 2006 Mt. Etna eruption. An Eulerian fully-compressible non-hydrostatic atmospheric model (WRF) is used to generate the driving wind field. We propose that the volcanic cloud itself is necessary to create the observed instabilities because the suspended volcanic ash changes the radiative balance absorbing sun radiation and has a cooling effect at the top of the cloud, creating a strong vertical gradient in potential temperature at the altitude of the spreading plume. Radio-sounding observations of wind and temperature profiles have been modified to simulate the presence of the plume and then used in WRF to generate a high-resolution 3D wind field representing the region downstream of the volcano. Stationary waves are produced downwind of the volcano by the model, and by comparing our results with the satellite images, we see that the modeled amplitude and length scale of such waves are in good agreement with the observed ones. Model results support the proposition that volcanic clouds create the observed instabilities by demonstrating that both wind shear, as measured by radio-sounding, and strong vertical gradients in potential temperature (which are not observed away from the volcano) are required to drive the unstable modes. In addition, the Lagrangian particle model demonstrates how observable stripes in the volcanic cloud are generated by the interaction between the unstable velocity field and the solid particulates, and shows that the instabilities affect transport and settling characteristics of volcanic products.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V23C2862S
- Keywords:
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- 3320 ATMOSPHERIC PROCESSES Idealized model;
- 8409 VOLCANOLOGY Atmospheric effects