Earth Planets Space, Vol. 51 (Nos. 7, 8), pp. 485-498, 1999
Kenshi Goya and Saburo Miyahara
Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, Hakozaki 6-10-1, Fukuoka 812-8581, Japan
(Received August 4, 1998; Revised January 28, 1999; Accepted April 9, 1999)
Abstract: Generation of internal gravity waves (IGWs) by tropospheric convections and vertical propagation of the generated IGWs throughout the middle atmosphere are simulated by a non-hydrostatic compressible nonlinear two-dimensional numerical model. The present simulation demonstrates that (i) IGWs are generated by the tropospheric dry convections, (ii) zonal mean wind is decelerated by critical layer absorption of the generated IGWs in the upper tropospheric shear zone, (iii) secondary IGWs are radiated by the critical layer instability, and (iv) the secondary IGWs break down and accelerate zonal mean winds in the upper middle atmosphere. The detailed analyses show that (1) eastward propagating IGWs generated by the convection in tropospheric westerly with small wavelength of the order of 10 km and short period of the order of 10 min are dominant. It is found that the dominant waves are selected by a filtering effects of the prescribed westerly. (2) Several secondary IGWs with smaller horizontal wavelength than the primary IGW are radiated. The secondary IGWs propagate vertically in the form of wavepackets and break in the upper middle atmosphere due to local convective instability because of the exponential growth of the wave amplitudes with height. In the breaking region, the observed and theoretically predicted universal power law of the wind fluctuation, which states the m-3 dependence for the power spectra versus the vertical wavenumber mdue to the wave saturation and breakdown, is also realized in the present model simulation.