Validation Approaches to Volcanic Explosive Phenomenology

Large-scale volcanic eruptions are inherently hazardous events, hence cannot be described by detailed and accurate in situ measurements. As a result, volcanic explosive phenomenology is poorly understood in terms of its physics and inadequately constrained in terms of initial, boundary, and inflow conditions. Consequently, little to no real-time data exist to validate computer codes developed to model these geophysical events as a whole. However, code validation remains a necessary step, particularly when volcanologists use numerical data for assessment and mitigation of volcanic hazards as more often performed nowadays. We suggest performing the validation task in volcanology in two steps as followed. First, numerical geo-modelers should perform the validation task against simple and well-constrained analog (small-scale) experiments targeting the key physics controlling volcanic cloud phenomenology. This first step would be a validation analysis as classically performed in engineering and in CFD sciences. In this case, geo-modelers emphasize on validating against analog experiments that unambiguously represent the key-driving physics. The second “geo-validation” step is to compare numerical results against geophysical-geological (large-scale) events which are described ?as thoroughly as possible? in terms of boundary, initial, or flow conditions. Although this last step can only be a qualitative comparison against a non-fully closed system event —hence it is not per se a validation analysis—, it nevertheless attempts to rationally use numerical geo-models for large-scale volcanic phenomenology. This last step, named “field validation or geo-validation”, is as important in order to convince policy maker of the adequacy of numerical tools for modeling large-scale explosive volcanism phenomenology.