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Junshi Ito and Hiroshi Niino

Abstract

A mesoscale atmospheric numerical model is used to simulate two cases of Kármán vortex shedding in the lee of Jeju Island, South Korea, in the winter of 2013. Observed cloud patterns associated with the Kármán vortex shedding are successfully reproduced. When the winter monsoon flows out from the Eurasian continent, a convective mixed layer develops through the supply of heat and moisture from the relatively warm Yellow Sea and encounters Jeju Island and dynamical conditions favorable for the formation of lee vortices are realized. Vortices that form behind the island induce updrafts to trigger cloud formation at the top of the convective boundary layer. A sensitivity experiment in which surface drag on the island is eliminated demonstrates that the formation mechanism of the atmospheric Kármán vortex shedding is different from that behind a bluff body in classical fluid mechanics.

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Junshi Ito, Toshiyuki Nagoshi, and Hiroshi Niino
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Junshi Ito, Toshiyuki Nagoshi, and Hiroshi Niino

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A renowned local wind in Japan, “Hijikawa-Arashi,” is a thermally driven nocturnal gap wind accompanied by fog. The wind is visually identified by the fog along the valley of the Hijikawa River between the Ozu basin and the Seto Inland Sea during the early morning in autumn and winter. A fine-resolution numerical model is employed to reproduce the main observed features of Hijikawa-Arashi. A vertical resolution of 10 m or less at the lowest level is required to express the nocturnal radiative cooling of the land that is required for fog formation in the basin, and fine horizontal resolution is necessary to express a realistic valley through which the fog is advected to the sea. Multiple hydraulic jumps accompanied by supercritical flow occur because of the complex topography. Both moisture transport by the sea breeze during the daytime and evaporation from the land surface are important for accumulating moisture to produce the fog.

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Junshi Ito, Hiroshi Niino, and Mikio Nakanishi

Abstract

Dust devils are small-scale vertical vortices often observed over deserts or bare land during the daytime under fair weather conditions. Previous numerical studies have demonstrated that dust devil–like vertical vortices can be simulated in idealized convective mixed layers in the absence of background winds or environmental shear. Their formation mechanism, however, has not been completely clarified. In this paper, the authors attempt to clarify the vorticity source of a dust devil–like vortex by means of a large-eddy simulation, in which a material surface initially placed in the vortex is tracked backward and the circulation on the material surface is examined. The material surface is found to originate from downdrafts, which already have sufficient circulation. As the material surface converges toward the vortex, the vorticity is increased because of conservation of circulation. It is shown that a convective mixed layer is inherently accompanied by circulation, which is scaled by a product of the convective velocity scale and the depth of the convective mixed layer. This circulation is considered to be originally generated by tilting of baroclinically generated horizontal vorticity principally at middepths of the convective mixed layer.

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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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Junshi Ito, Hiroshige Tsuguchi, Syugo Hayashi, and Hiroshi Niino

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Record-breaking precipitation due to a back-building convective system occurred in Kyushu Island, Japan, on 5 July 2017. In this paper, a quasi-stationary back-building convective system is reproduced using a regional weather prediction model initialized with a single representative sounding in which the land–sea distribution around the observed convective system is crudely simplified. The simulated convective system and heavy precipitation are reasonably similar to observations. Horizontal resolution finer than 1 km is found to be necessary for reproducing the convective system. The area of heavy precipitation tends to shift downstream with finer horizontal resolution. The surface temperature contrasts at the northern and southern coastlines cause sea breezes and a stationary convergence line between them continuously triggers cumulus clouds. The horizontal convergence near the surface is further enhanced by preceding cumulus clouds that cause the latent heating aloft and generate a mesoscale surface pressure depression. Vertical shear of the environmental wind is also found to be important for organizing the convective system but veering of its wind direction and a cold pool are not essential.

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