A Three-Dimensional Minimal Model of QBO-Like Oscillations in a Stratosphere–Troposphere Coupled System under a Radiative-Moist Convective Quasi-Equilibrium State

A self-sustained oscillation which is dynamically analogous to the equatorial quasi-biennial oscillation (QBO) was firstly obtained as a radiative-moist-convective quasi-equilibrium state in a minimal model of the stratosphere-troposphere coupled system by Held et al. (1993), and the robust feature of the oscillation was confirmed by Yoden et al. (2014). The minimal model is a two-dimensional cloud-system-resolving non-hydrostatic model without Coriolis effect, under a periodic lateral boundary condition to permit the oscillation of the zonal mean zonal wind. The QBO-like oscillation modulates organized features of moist-convective systems periodically through alternative appearance of squall-line-type or back-building-type precipitation patterns, and modulates the spectral features of the vertical flux of horizontal momentum by tilted convection and upward- and downward-propagating gravity waves (Nishimoto et al., 2016). Downward influence of the QBO-like oscillation on moist convection in the two-dimensional minimal model framework was further investigated by Bui et al. (2017), by controlling the vertical shear of the zonal mean zonal wind near the surface with a nudging technique. It was demonstrated that the QBO-like oscillation modulates the moist convection via two mechanisms related to the vertical shear. Large values of the shear near the surface enhance the longevity and intensity of the moist convective systems by separating the updraft and downdraft. On the other hand, large values of shear near the cloud top tend to disrupt the convective structure and lead to weakening moist convection, although this mechanism seems to be secondary.

In this study, we present the first result of a three-dimensional minimal model that produces a self-sustained QBO-like oscillation in the radiative-moist convective quasi-equilibrium state. The domain is a rectangular with a shorter meridional dimension of 160 km compared to the zonal dimension of 640 km. After the initial transition time of 200 or 300 days, an oscillation with a period of about 300 days is established in the stratosphere in both of the zonal mean zonal wind and the zonal mean meridional wind. The penetration of the oscillation into the troposphere still occur in both zonal and meridional oscillations, although the meridional tropospheric oscillation is a little weaker. There is a synchronization of the zonal mean zonal wind and meridional wind, which is shown as an anti-clockwise skewed spiral with height in a hodograph of the domain averaged horizontal wind vectors in the stratosphere, and its anti-clockwise rotation with time by changing the skewed shape of the spiral periodically.

There are modulations of the domain-averaged tropospheric temperature anomaly and smoothed precipitation with irregular periods of about 100 or 150 days (nearly a half of the QBO-like oscillation), in which the heavier smoothed precipitation is associated with the positive temperature anomaly. Fine temporal sampling of every five minutes reveals irregular recurrence of three types of precipitation patterns; fast-moving back-building, and squall-line types of precipitation, moving over the domain in a day or so, and quasi-stationary isolated precipitation cluster for a substantial period. The quasi-stationary isolated precipitation periods correspond to the period of heavier domain-averaged precipitation. There is a strong correlation between the vertical shear of horizontal wind near the surface and the domain-averaged smoothed precipitation. Zonal-height sections of meridionally averaged clouds, water vapor, precipitating rain, and stream lines for a snap shot in each type of precipitation patterns show clear contrast in the distribution of shallow clouds and water vapor in non-precipitating areas; no shallow clouds over non-precipitating areas in a quasi-stationary isolated precipitation period, whereas shallow clouds in non-precipitating areas over all domain in fast-moving back-building and squall-line types of precipitation.

References

Bui, H.-H., E. Nishimoto, and S. Yoden, 2017: Downward influence of QBO-like oscillation on moist convection in a two-dimensional minimal model framework. J. Atmos. Sci., 74, 3635-3655, https://doi.org/10.1175/JAS-D-17-0095.1

Held, I. M., R. S. Hemler, and V. Ramaswamy, 1993: Radiative-convective equilibrium with explicit two-dimensional moist convection. J. Atmos. Sci., 50, 3909-3927.

Nishimoto, E., S. Yoden, and H. Bui, 2016: Vertical momentum transports associated with moist convection and gravity waves in a minimal model of QBO-like oscillation. J. Atmos. Sci., 73, 2935-2957, https://doi.org/10.1175/JAS-D-15-0265.1

Yoden, S., H.-H. Bui, and E. Nishimoto, 2014: A minimal model of QBO-like oscillation in a stratosphere-troposphere coupled system under a radiative-moist convective quasi-equilibrium state. SOLA, 10, 112-116, doi:10.2151/sola.2014-023.