Abstract
Large explosive eruptions can result in the formation of an umbrella cloud which rapidly expands, spreading ash out radially from the volcano. The lateral spread by the intrusive gravity current dominates the transport of the ash cloud. Hence, to accurately forecast the transport of ash from large eruptions, lateral spread of umbrella clouds needs to be represented within volcanic ash transport and dispersion models. Here, we describe an umbrella cloud parameterisation which has been implemented into an operational Lagrangian model and consider how it may be used during an eruption when information concerning the eruption is limited and model runtime is key. We examine different relations for the volume flow rate into the umbrella, and the rate of spreading within the cloud. The scheme is validated against historic eruptions of differing scales (Pinatubo 1991, Kelud 2014, Calbuco 2015 and Eyjafjallajökull 2010) by comparing model predictions with satellite observations. Reasonable predictions of umbrella cloud spread are achieved using an estimated volume flow rate from the empirical equation by Bursik et al. and the observed eruption height. We show how model predictions can be refined during an ongoing eruption as further information and observations become available.
Highlights
The role of the Volcanic Ash Advisory Centres (VAACs) is to issue advisories on the location and forecasted movement of volcanic ash within the atmosphere, in order to minimise risks to the aviation community
We describe here a parameterisation of lateral spreading within umbrella clouds that has recently been added to NAME and which is designed to be used in an operational context when information on the eruption may be limited and model runtime is key
To assess the performance of the umbrella cloud parameterisation for such small-scale eruptions, we study a period in early May when explosive activity began to increase again and the resulting ash cloud was well observed by satellites
Summary
The role of the Volcanic Ash Advisory Centres (VAACs) is to issue advisories on the location and forecasted movement of volcanic ash within the atmosphere, in order to minimise risks to the aviation community. VATDMs typically represent advection by the mean ambient wind, diffusion due to atmospheric turbulence, and gravitational settling of ash particles; these models often lack parameterisations of spreading within umbrella clouds. This omission limits their ability to accurately forecast ash clouds from large explosive eruptions. These case studies represent eruptions of differing scales and are well observed by satellite.
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