The development of volcanic ash cloud layers over hours to days due to turbulence layering

TL;DR

This paper investigates how turbulence layering in the atmosphere causes volcanic ash clouds to develop multilayered structures over hours to days, emphasizing the role of atmospheric turbulence in particle distribution.

Contribution

It introduces a dispersion model that explicitly incorporates observed turbulence layering to explain the formation of multilayered volcanic ash clouds.

Findings

Turbulence layering modulates vertical particle motion in ash clouds.

Larger particles (>100 μm) are less affected by turbulence.

Particles in the 10-100 μm range are influenced by turbulence intensity.

Abstract

Volcanic ash clouds often become multilayered and thin with distance from the vent. We explore one mechanism for 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 development of multilayered ash clouds by modulating vertical particle motion. The largest particles, generally $> 100 \mu$m, 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 $\mu$m, 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. For smaller particles, mostly in the submicron range, the more rapid diffusion in the turbulent layers causes these particles to ``wash out'' quickly.