Abstract
Volcanic ash clouds often become multilayered and thin with distance from the vent. We explore one mechanism for the development of this layered structure. We review data on the characteristics of turbulence layering in the free atmosphere, as well as examples of observations of layered clouds both near-vent and distally. We then explore dispersion models that explicitly use the observed layered structure of atmospheric turbulence. The results suggest that the alternation of turbulent and quiescent atmospheric layers provides one mechanism for the development of multilayered ash clouds by modulating vertical particle motion. The largest particles, generally μ>100 μm, are little affected by turbulence. For particles in which both settling and turbulent diffusion are important to vertical motion, mostly in the range of 10–100 μμm, the greater turbulence intensity and more rapid turbulent diffusion in some layers causes these particles to spend greater time in the more turbulent layers, leading to a layering of concentration. The results may have important implications for ash cloud forecasting and aviation safety.
Highlights
Volcanic ash is a multi-billion dollar economic hazard to aviation, as shown during the 2010 eruptions of Eyjafjallajökull, Iceland [1,2]
Particles as large as 1000 μm were affected by the turbulence layering [70], we focus on the results for representative 10 and 30 μm particles at 18 h, as these sizes were dominant in parts of the cloud for periods up to nearly a month (Section 2.1.2) [8], as their settling and dispersal behavior were similar to that for particles 30 μm, respectively, and as the 18-h time window is important to Volcanic Ash AdvisoryCenters (VAACs) ash cloud forecasting
We have presented data and models on near-vent and distal volcanic cloud morphology and ash loading
Summary
Volcanic ash is a multi-billion dollar economic hazard to aviation, as shown during the 2010 eruptions of Eyjafjallajökull, Iceland [1,2]. The International Airways Volcano Watch (IAVW), which seeks to safely separate aircraft from volcanic ash in flight, relies on detecting areas of ash and forecasting its future movement [3]. The IAVW acts through nine Volcanic Ash Advisory. In case of eruption that potentially affects flight safety, the VAACs issue both a written Volcanic Ash Advisory (VAA) and a Volcanic Ash Graphic (VAG), which state and show the analysis of the current position of an ash cloud and the forecast position to 18 h from present. Ash cloud positions are rendered as polygons for separate aircraft flight levels or altitude ranges, and flight paths are adjusted
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