On the critical layer interaction of the stratospheric Kelvin waves

Stratospheric Kelvin waves are known to be absorbed by the background flow via mechanical and thermal damping and, to less extent, by the critical layer interaction. Critical layer interaction occurs when the Kelvin waves' phase speed approaches the background flow's speed. This study aims to depict the structure of the Kelvin waves while approaching the critical layer, where the phase speed of the wave matches the speed of the background flow. In the time domain, the wavelet filtering technique filters Kelvin waves at a specific location and phase speeds using ERA-I zonal wind. Linear regression yields the pattern of specific phase speed's Kelvin wave. Yet, the critical layer interaction of the Kelvin waves with the environmental flow could be studied by choosing a background environment in which its flow speed matches the wave's phase speed, which could be implemented using the varying-coefficient regression technique. We found that the in-phase relationship between the zonal wind and height, associated with the structure of the Kelvin waves, relaxes with the decreasing of the Doppler-shifted speed; then, at a further reduction of the Doppler-shifted speed, the Gill pattern appears. Furthermore, Kelvin waves were found to be absent under an environment of westerly shear.