Atmospheric internal waves generated by explosive volcanic eruptions

Observations of microbarograph recordings on the island of Montserrat in the Caribbean have shown signals with periods of several minutes and amplitude of approximately 1 mb following explosive eruptions of the Soufriere Hills Volcano. These properties suggest that the explosions are causing the generation of atmospheric internal waves. Here an analysis of wave generation by sudden disturbances in a stratified atmosphere is developed, and the properties of these waves are described and compared with observations. Two types of forcing of the atmosphere by a volcanic explosion are considered: that due to the sudden addition of mass, and separately, of thermal energy. The pattern and distribution of these depend on the nature of the explosion, which has a timescale of the order of 1 min. The theoretical results resemble the effect of 'throwing a stone' into the atmosphere, producing transient waves that radiate radially away from the source. Near the source, these waves have frequency around 0.7-0.8N (where N is the buoyancy frequency) initially, and approach N with decreasing amplitude. The results of forcing due to added mass and thermal forcing are presented and compared for a variety of vertical forcing profiles, and these are compared with observations of surface pressure. The results suggest that forcing due to the injection of mass (mostly solid particles) is the principal factor in forcing the observed internal waves. The addition of thermal energy (heat) produces waves that have frequencies closer to N, and persist for much longer periods than those observed at the stations on Montserrat.