An operational model for wind-blown volcanic umbrella clouds

The plume of ash and gas released by large explosive volcanic eruptions rises to its neutral buoyancy level in the atmosphere, then spreads laterally to form an umbrella cloud. Density stratification of the atmosphere generates buoyancy forces in the cloud, which drive the outward spread as an intrusion. Although umbrella clouds are often modelled as circular axisymmetric structures, in practice they are usually influenced quite strongly by the meteorological wind, with spread in the upwind direction halted by the oncoming wind, and different rates of spreading in the downwind and crosswind directions. Here, we present a physically based shallow-layer intrusion model for wind-blown volcanic umbrella clouds, and derive a simple parametrization of non-axisymmetric umbrella cloud spreading based on this shallow-layer model. The simplified parametrization is quick to evaluate and so is suitable for use in operational Volcanic Ash Transport and Dispersion Models (VATDMs) that are used to predict ash hazard operationally. In contrast to previous parametrizations, in which there is assumed to be no interaction between a circular umbrella cloud and the meteorological wind, here the umbrella cloud is influenced by the wind and adopts a shape determined by the balance of buoyant spreading and downwind drag forces. We test our scheme within the UK Met Office 'NAME' dispersion model, and apply it to four diverse case studies of eruptions at Puyehue 2011, Pinatubo 1991, Ulawun 2019, and Calbuco 2015. We demonstrate that buoyant spreading is important even in plumes that are highly wind-blown, and obtain better descriptions of cloud spread and ash distribution than existing parametrizations based on an axisymmetric umbrella cloud dynamics.