A dynamical study of gravity waves and instabilities in the mesopause region at Maui, Hawaii.

摘要

Correlative observations of the mesopause region (80--105 km) made by a cluster of remote sensing instruments were collected at Maui, Hawaii (20.7°N, 156.3°W) during the Maui-MALT campaign. Using this unique dataset, a dynamical study was performed to investigate gravity waves and instabilities in the mesopause region.; An investigation of a "wall" wave on 12 August 2004 marks the first time such a bright event was captured by lidar. The observations indicate this event was a large-amplitude gravity wave with period of 4 hr and vertical wavelength of 20 km. The spectacular changes in temperature, airglow intensity and Na density accompanying the passage of the wall were due to the phase reversal of the wall wave.; A gravity wave breaking event on 28 October 2003, which is the first time direct observation of gravity wave breakdown into small-scale ripples, was examined. The data suggest that the gravity wave breaking was due to a dynamical instability. The characteristics of the ripples were consistent with their attribution to Kelvin-Helmholtz billows. It is proposed that superposition of the background wind shear and the wave-induced shear caused Ri 0.25, leading to wave breakdown and the subsequent generation of the ripples.; The characteristics of instabilities in the mesopause region were analyzed. Unstable conditions are observed between 85 and 100 km ∼90% of the time. The probabilities of convective and dynamical instabilities are 3% and 10%, respectively. A distinct correlation between atmospheric static stability and wind shear is identified for the first. The distribution of stability is organized as layered structures, which are modulated by the progression of tides/long-period gravity waves. Unstable regions are found within the layers of reduced stability, which often located above thin layers of large N2/strong wind shear associated with the mesospheric inversion layers. It is suggested that gravity wave-tidal interactions might explain the observed features.; A ray-tracing simulation was performed to explore whether the interactions between tides and oppositely-propagating waves could explain the observed double-layer structure of large N2/strong wind shear. It is found, using the GROGRAT model, that tidal modulation has two effects on the wave propagation: (1) wave breaking levels descend at a tidal phase. (2) breaking levels for oppositely-propagating waves are separated by ∼10 km. The model results are qualitatively very similar to the observations.