The generation of internal waves by explosive volcanic eruptions

The Generation of Internal Waves by Explosive Volcanic Eruptions Peter G. Baines Dept. of Infrastructure Engineering University of Melbourne Australia Recent observations have shown that explosive volcanic eruptions can generate low frequency surface pressure signals that are observable with microbarographs, and can be interpreted as atmospheric internal waves (Baines & Sacks 2013). Some examples from the Caribbean island of Montserrat are shown in Figure 1. Suitably interpreted, it is possible that these observations may give information about the eruption in locations where other observations are not possible or available (such as in remote locations, or at night etc.). This requires quantitative understanding of the link between surface pressure signals and the properties of the eruption, particularly its strength. Figure 1. Surface barograph records of four observations of explosions of SVH volcano on Montserrat as recorded at station AIRS (a distance of 5 km). The arrows denote the approximate time of onset of the explosions (from Baines & Sacks 2013). This paper presents a new model of internal wave generation by volcanic eruptions, in which the forcing is governed by plume dynamics. The effect of an explosive volcanic eruption on its environment can be represented by the horizontal flow on the surface of a vertical cylinder that encloses and is centred on the volcanic plume. An explosive volcanic eruption contains hot solid material as well as gases, and a model of plumes with this property has been described by Woods (1988, 2010). For a typical eruption with no crosswind, the flow forced by the plume consists of inflow at low levels where environmental fluid is entrained into the rising plume, and outward flow at upper levels (over a restricted range of heights) due to spreading of the risen fluid around its level of neutral buoyancy. This flow field is time- dependent, but for an explosive eruption it may be approximated by a steady flow field of the form shown that develops rapidly, exists for a period of time of several minutes, and then