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Ryosuke Shibuya, Kaoru Sato, and Mikio Nakanishi

Abstract

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 because of 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 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 wavelike 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.

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Ryosuke Shibuya, Kaoru Sato, Yoshihiro Tomikawa, Masaki Tsutsumi, and Toru Sato

Abstract

Multiple tropopauses (MTs) defined by the World Meteorological Organization are frequently detected from autumn to spring at Syowa Station (69.0°S, 39.6°E). The dynamical mechanism of MT events was examined by observations of the first mesosphere–stratosphere–troposphere (MST) radar in the Antarctic, the Program of the Antarctic Syowa MST/Incoherent Scatter (IS) Radar (PANSY), and of radiosondes on 8–11 April 2013.

The MT structure above the first tropopause is composed of strong temperature fluctuations. By a detailed analysis of observed three-dimensional wind and temperature fluctuation components, it is shown that the phase and amplitude relations between these components are consistent with the theoretical characteristics of linear inertia–gravity waves (IGWs).

Numerical simulations were performed by using a nonhydrostatic model. The simulated MT structures and IGW parameters agree well with the observation. In the analysis using the numerical simulation data, it is seen that IGWs were generated around 65°S, 15°E and around 70°S, 15°E, propagated eastward, and reached the region above Syowa Station when the MT event was observed. These IGWs were likely radiated spontaneously from the upper-tropospheric flow around 65°S, 15°E and were forced by strong southerly surface winds over steep topography (70°S, 15°E). The MT occurrence is attributable to strong IGWs and the low mean static stability in the polar winter lower stratosphere.

It is also shown that nonorographic gravity waves associated with the tropopause folding event contribute to 40% of the momentum fluxes, as shown by a gravity wave–resolving general circulation model in the lower stratosphere around 65°S. This result indicates that they are one of the key components for solving the cold-bias problem found in most climate models.

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Mikio Nakanishi, Ryosuke Shibuya, Junshi Ito, and Hiroshi Niino

Abstract

Diurnal variations of an atmospheric boundary layer from 0900 LST on day 33 to 0600 LST on day 34 of the Wangara experiment are studied using a large-eddy simulation (LES) model that includes longwave radiation and baroclinicity. The present study directs its particular attention to phenomena in a residual layer (RL). As the surface heat flux decreases, an inertial oscillation is initiated and is accompanied by a low-level jet (LLJ) at a height of approximately 200 m. The maximum wind speed of the LLJ exceeds 12 m s−1 at 0300 LST on day 34. After 2100 LST on day 33, the horizontal advection due to the LLJ under a large-scale horizontal gradient of temperature destabilizes the RL and consequently induces horizontal convective rolls, parallel to a vertical wind shear (VWS) vector, between heights of 400 and 1400 m. The VWS in the layer between the bottom of the convective rolls and the gradually growing LLJ maximum is intensified after midnight, and the gradient Richardson number falls below its critical value of 0.25 at a height of 400 m at 0130 LST on day 34. An empirical orthogonal function analysis demonstrates that Kelvin–Helmholtz (KH) vortices appear below the convective rolls and are coupled with them. This study suggests that horizontal convective rolls can occur in an RL because an LLJ often advects warmer air to the lower layer according to a large-scale gradient of temperature and that the rolls may coexist with KH vortices in a stable boundary layer because the LLJ gradually increases a VWS.

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