In this study, downward influence of the stratospheric QBO-like oscillation to convection is further investigated with the same minimal model in two series of parameter sweep experiments; model top experiments and low-level nudging experiments. In the model top experiments in which deviations of all variables are nudged to zeros in the upper most layers of 5 km, QBO-like oscillations are obtained even in the low top experiments with oscillations of the mean zonal wind only in the troposphere (Fig.1). In the low-level nudging experiments, on the other hand, only the zonal mean component of the zonal wind is nudged to 0 m/s in a certain depth from the bottom boundary to investigate the impact of the vertical shear of the zonal mean zonal wind near the surface on the evolution of convective systems and the mean zonal wind oscillation. Two prominent cases of experiment are chosen for further composite analysis to reveal the convective structures in extremely heavy and light conditions of precipitation.
The results show that the QBO-like oscillation modulates the convection via two mechanisms related to the vertical shear of zonal mean zonal wind. Large values of shear near the surface enhance the longevity and intensity of the convective systems in the form of squall line type. On the other hand, large values of shear near the cloud top disrupt the convective structure and leads to a smaller amount of precipitation. The first mechanism seems to be dominated, as the second one is revealed only when the low-level shear is nudged to some certain levels.
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