Wind measurement and momentum balance in the southern mesosphere.

Abstract

This thesis makes use of diverse data sets in order to develop an understanding of the dynamics of the upper mesosphere in the region near Birdlings Flat, New Zealand. Medium frequency radar data from the Birdlings Flat site were obtained for the period between May 1993 and December 1996. When analysed, the data reveal two distinct annually varying patterns of circulation, one of which corresponds to seasonal behaviour characteristic of the upper mesospheric, while the other is part of the lower thermospheric climatology. Time-series of wind data in the upper mesosphere also reveal the presence of a feature, recurring each winter, which appears to be the product of breaking gravity waves acting on the mesospheric jet. Wind data from the Birdlings Flat radar were compared with wind measurements made by the High-Resolution Doppler Imager aboard the Upper Atmosphere Research Satellite. Data from the two instruments were compared on short time-scales and are discussed with reference to a number of physical parameters, especially proximity of measurement, data reliability and the local hour of observation. Although the small number of satellite overpasses limit the reliability of the conclusions of this comparison, proximity was found to be the most relevant of the three features. Data from the two instruments were also compared on longer time-scales with the intention of avoiding tidal contamination in the comparison. Two approaches to this problem are discussed. In one, the overpasses used in the individual comparisons are binned together according to season, while in the other, the longitudinal restriction was lifted from the satellite instrument, and a sampling period corresponding to a full 24 hours of local time was obtained for both the radar and satellite instruments. This approach gave at least as good agreement as the previous methods. Radar wind data are also compared with wind data derived from the Improved Stratospheric And Mesospheric Sounder for the month of May 1992. The comparison between the two data sets, in conjunction with other material from this thesis, forms the basis of a study of the effects of gravity wave breaking on the momentum budget of the winter mesosphere in the mid-latitude Southern Hemisphere. Even when the maximum likely uncertainty is added to the magnitude of the monthly mean radar winds, there is still a significant discrepancy between observed and derived winds. This appears as unresolved-scale forcing terms in the atmospheric momentum equations. The unresolved-scale term in the zonal momentum equation is due primarily to the effects of breaking gravity waves, and was calculated to be D = 80 ± 40 ms-1d-1. The large and highly variable value of the unresolved-scale term in the meridional momentum equation, E = 88 ± 120 ms-1d-1 is more complicated, but is likely to incorporate a large contribution from the inter-play of various small-scale curvature terms.