Skirting the Issue

By Geoff Wadge

During a major explosive volcanic eruption a set of three main processes transfers mass and heat from the solid earth to the atmosphere. These three processes are: a gas thrust (jet) extending up from the volcanic vent, a middle tower of vigorously convecting gas and ash with air entrainment and topped by a widening region of diffusion and gravity flow near the level of neutral buoyancy. Each eruption tends to produce different plumes, reflecting the variable behaviour of the magma flux at the vent and the ambient atmosphere. Whilst Pliny the Younger provided the original description of a major volcanic explosion plume, the first physical model was produced by Morton et al (1956). Here we focus on a variant of the classic eruption plume – the skirt cloud.

Skirt Clouds
On 17 April 1979 a former colleague of mine took off in a small aircraft to observe up close the eruption of Soufrière volcano, St Vincent. One of the photographs he took (shown below) made the cover of Science. The convection tower, of about 2 km in diameter and seen here from a distance of about 5 km, is partly encased in a “skirt cloud” of unusual complexity. Such clouds had been observed during the era of subaerial nuclear weapons testing, but not from such a close vantage point!

Photograph – K C Rowley

So what is the mechanism leading to the formation of a skirt cloud, in particular this one? There is no definitive analysis that I’m aware of and not all plumes have skirts. Any explanation needs to take account of its location in the convection tower and the high angle layering created. Barr (1982) argued that this example was initiated from thin sub-horizontal layers with high moisture interleaved with drier layers like pileus clouds, the layering being formed by horizontal shearing of cumulus cloud. These sub-horizontal layers are then deformed by the development of the high velocity (> 50 m s-1) eruption column through the layer(s). The heat flux profile at the base of the convective plume should be annular and variations in vent geometry or pressure could help create multiple skirts, as seen here.

The 1979 eruption at Soufrière, St Vincent yielded some excellent science, though my bold photographer colleague was lost to science and he went on to become the current Prime Minister of Trinidad and Tobago.

REFERENCES

Barr, S., 1982. Skirt clouds associated with the Soufrière eruption of 17 April 1979. Science, 216, 1111-1112.

Morton B.R., Taylor, G., Turner, J.S., 1956. Turbulent gravitational convection from maintained and instantaneous sources. Proc. R. Soc. Series A, Mathematical and Physical Sciences, 234(1196), 1-23.

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