Analysis of Ultra-fast Kelvin Waves Simulated by Kyushu University GCM

Many observation results show that zonal wavenumber 1 type ultra-fast Kelvin waves that periods are about 3-6 days are one of prominent global scale wave motions in the mesosphere and lower thermosphere region (the MLT region; e.g. Riggin et al., 1997). There are also some studies to investigate how this kind of waves is excited in the lower atmosphere and propagate vertically through atmospheric layers to the MLT region (e.g. Forbes, 2000). In this study, we focus on behaviors of ultra-fast Kelvin waves simulated be the Kyushu University General Circulation Model (the Kyushu-GCM) T42L25 version (Yoshikawa and Miyahara, 2005).

The Kyushu-GCM is a GCM that divide the atmosphere into 250 layers from surface to about 150 km height with the triangular-truncation at wavenumber 42. Physical processes in the troposphere such as moist convective heating that is thought to be an excitation source are included in this model. In this study, we apply composite analysis and space-time Fourier analysis in the wintertime dataset to figure out the horizontal and vertical structures of ultra-fast Kelvin waves in this model.

First, we extract out 2-3 day period zonal wavenumber 1 components of the zonal wind, meridional wind and geopotential height fields. Composite analysis on these three fields along 3 day eastward propagating phase lines shows that 3 day period with zonal wavenumber 1 component in the height region of 70-120 km have the structure of Kelvin waves.

Second, we apply a space-time Fourier analysis on the dataset to separate eastward and westward propagating components with different periods and zonal wavenumbers. By this analysis, it is shown that some prominent peaks appear in the period band of 2-4 days in the eastward zonal wavenumber 1 components above 70 km height region. It is also shown that in the 2-4 day period band components have the structure of Kelvin waves in the height region of 70-120 km.

In both the results of composite analysis and space-time Fourier analysis, it is found that the phase lines of the waves tilt toward east with increasing height which shows that the phase propagates downward and the energy propagates upward. Also, from the results of space-time Fourier analysis, it is found that each amplitude of geototential height field waves grows at the rate about exp(z/2H) in the height region of 20-100 km which shows that excitation sources of these waves may exist in the region lower than 20 km, and the waves propagate upward up to 100 km conserving the energy.

References:

Dennis M. Riggin, David C. Fritts, Toshitaka Tsuda, Takuji Nakamura, and Robert Vincent, Radar observations of a 3-day Kelvin wave in the equatorial mesosphere, Journal of Geophysical Research, Vol. 102, No. D22, 26, 141-26, 157, November 27, 1997.

Jeffrey M. Forbes, Wave coupling between the lower and upper atmosphere: case study of an ultra-fast Kelvin Wave, Journal of Atmospheric and Solar-Terrestrial Physics 62(2000), 1603-1621.

M. Yoshikawa, S. Miyahara, Excitations of nonmigrating diurnal tides in the mesosphere and lower thermosphere simulated by the Kyushu-GCM, Advances in Space Research, 35(2005), 1918-1924.