Diurnal Wind Cycles Forcing Inertial Oscillations: A Latitude-Dependent Resonance Phenomenon

The latitudinal dependence of inertial oscillation (IO) in a diurnally-evolving atmospheric boundary layer (ABL) is examined using a large eddy simulation (LES). Previous studies that used LES were unable to simulate such an ABL on a time scale of several days due to high computational cost. By using an LES with a simple radiation scheme, the present study has succeeded in simulating the diurnal behavior of the ABL above the nocturnal stable layer as a function of the latitude. The reality of model simulations is confirmed by comparison with Wangara experiments. It is shown that a resonance-like amplification of the IO appears only at two latitudes where the respective inertial periods are 24 h and 12 h. A horizontal wind oscillation with strong dependence on latitude is observed during an entire day. The oscillation amplitude is maximized slightly above the nocturnal stable layer. It seems that this maximum corresponds to the nocturnal low-level jet, whose mechanism is explained in terms of IO. Thus, the IO shown in the present study includes the nocturnal jet as a structural component. It is also shown that a wave-like structure whose phases propagate downward with near-inertial frequency at each latitude is observed above the ABL at all latitudes. This feature is consistent with that of inertia-gravity waves propagating energy upward. Previous observational and model studies indicate the dominance of inertia-gravity waves with inertial frequencies in the middle and high latitudes in the lower stratosphere. Results of the present study suggest that the IO in the ABL is a possible source of such inertia-gravity waves.