Density-driven transport in the umbrella region of volcanic clouds: Implications for tephra dispersion models

Large explosive volcanic eruptions can generate ash
clouds from rising plumes that spread in the atmosphere
around a Neutral Buoyancy Level (NBL). These ash clouds
spread as inertial intrusions and are advected by atmospheric
winds. For low mass flow rates, tephra transport is mainly
dictated by wind advection, because ash cloud spreading due
to gravity current effects is negligible (passive transport).
For large mass flow rates, gravity-driven transport at the
NBL can be the dominant transport mechanism. Conditions
under which the passive transport assumption is valid have
not yet been critically studied. We analyze the conditions
when gravity-driven transport is dominant in terms of
the cloud Richardson number. Moreover, we couple an
analytical model that describes cloud spreading as a gravity
current with an advection-diffusion model. This coupled
model is used to simulate the evolution of the volcanic
cloud during the climatic phase of the 1991 Pinatubo
eruption. Citation: Costa, A., A. Folch, and G. Macedonio (2013),
Density-driven transport in the umbrella region of volcanic clouds:
Implications for tephra dispersion models.

This work has benefited from funding provided
by the Italian Presidenza del Consiglio dei Ministri - Dipartimento
della Protezione Civile (DPC), agreement INGV-DPC 2012-2013. This
paper does not necessarily represent DPC official opinion and policies.
A.F. acknowledges funding by the Spanish project ATMOST (CGL2009-10244).

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