Quantifying the Effect of Wind on Volcanic Plumes: Implications for Plume Modeling

The considerable effects that wind can have on estimates of mass eruption rates (MERs) in explosive eruptions based on volcanic plume height are well known but difficult to quantify rigorously. Many explicitly wind-affected plume models have the additional difficulty that they require the use of centerline heights of bent-over plumes, a parameter not easily obtained directly from observational data. We tested two such models by using the time series of varying plume heights and windspeeds of the 2010 eruption. The mapped fallout and photos taken during this eruption allow us to estimate the plume geometry and to empirically constrain input parameters for the two models tested. Two strategies are presented to correct the difference in maximum plume height and centerline height: (a) based on plume radius, and (b) by using the plume type parameter Pi, which quantifies the relative influence of buoyancy and cross-wind on the plume dynamics, to discriminate weak, intermediate and strong plumes. The results indicate that it may be more appropriate to classify plumes as either wind-dominated, intermediate or buoyancy-dominated, where the relative effects of both wind and MER define the type. The analysis of the Eyjafjallajokull data shows that the MER estimates from both models are considerably improved when a plume-type dependent centerline-correction is applied. For one model, we varied the wind entrainment coefficient beta. For this particular eruption, we find that the best value for beta lies between 0.28 and 0.36, unlike previous suggestions that set this parameter to 0.50. Plain Language Summary When a volcano explosively erupts, hot magma fragments (called "ash") are expelled into the atmosphere. Hot ash, gas and steam form mighty columns, which are called "ash plumes." Ash in the air is dangerous for planes. For volcanologists it is therefore very important to estimate as quickly as possible how much of ash is pushed into the air. We cannot directly measure the amount, but there are mathematical equations (called "models") that help us to estimate it. These equations require the top height of the plume as main input. If it is very windy, however, ash plumes are bent to the side, and we cannot simply use the top plume height as input anymore. Instead, we need to apply some correction, so that the model still can be used. Here, we present and examine such correction strategies. As test case, we use an Icelandic eruption that lasted 39 days and took place under different wind conditions.