In all these experiments, vertically propagating mesoscale gravity waves are generated in the exit region of the upper-tropospheric jet streaks near the tropopause level at the later stage of linear growth and the early stage of nonlinear growth of baroclinic waves. The large-scale flow patterns of these mesoscale gravity waves are consistent with previous observational and numerical studies of typical mesoscale gravity waves. The low-stratospheric gravity waves simulated in these experiments have horizontal wavelengths of 75-160 km and frequencies of 3-9 times of the Coriolis parameter. The intrinsic frequency of these gravity waves appears to be directly correlated with the growth rate of the baroclinic waves. In general, the faster the growth rate of the baroclinic waves, the higher the intrinsic frequency of the gravity waves. Similar frequencies of the gravity waves are found in experiments with similar near-linear growth rates of baroclinic waves even though the initial baroclinic jets have significantly different tropospheric static stability and tropopause geometry (and this different vertical and horizontal shears).
Diagnostics show that flow imbalance in terms of maximum residual of nonlinear balance equation increases noticeably near the tropopause region before the gravity waves are generated and continues to increase even after the gravity waves are generated. The growth rate of imbalance is also directly related to the growth rate of baroclinic waves and thus the frequency of gravity waves of primary interests. The sensitivity experiments and the imbalance diagnosis are consistent with balance adjustment hypothesis, which, as a generalization of geostrophic adjustment, is believed to be primarily responsible for the generation of the gravity waves in these baroclinic jet-front systems.
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