Once released, the ash plume can drift over great distances and cause disruptions to air traffic, even thousands of kilometres from its source. This was the case in September 1992 following the eruption of Mount Spurr in Alaska. Given the immediate threat posed by volcanic ash, predicting its movement and dispersion in the atmosphere is an important challenge that must be answered in real time. The numerical simulation of atmospheric dispersion is used increasingly to assess the state of the environment. The current version constitutes the core of a sophisticated computer system designed to produce quick and real-time dispersion simulations in a forecast or diagnostic mode.
Advection is calculated using the semi-Lagrangian method.
The model simulates wet and dry scavenging and provides estimates of wet and dry deposition. The virtual source is expressed as a 3D Gaussian function. An experimental version with 25 vertical levels is also available. The operational procedures reside on a front- end workstation and are accessible to the duty meteorologist through an Xwindow menu. It can be executed at any time and the results are usually available between 30 and 60 minutes from the time of notification.