The evolution of a breaking mesospheric bore wave packet

All-sky CCD observations of mesospheric gravity waves have been made from Halley Station Antarctica (75.5 degrees S, 26.7 degrees W) as part of a collaborative research program between British Antarctic Survey, U. K. and Utah State University, USA. A mesospheric bore event was observed in the nightglow emissions over a period of several hours on the 27th of May, 2001. Two dimensional S-Transform (ST) analysis is applied to the airglow images of this bore event. This local spectral technique allows one to calculate the wave parameters as a function of time and space. It is observed that the horizontal phase speed and wavelength decrease over time as the amplitude attenuates. Simultaneously with this wave event the background wind experiences a large acceleration in the direction of the wave propagation. Mesospheric bore theory calculations are used to estimate the bore duct depth and it is shown that as the wave packet evolves, the bore duct collapses (decreasing in its vertical extent). As the bore duct shrinks, the wave's group velocity decelerates, the amplitude attenuates, and the wave dissipates.