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Eigo Tochimoto
and
Hiroshi Niino

Abstract

To clarify the effects of the horizontal shear of the jet stream on the structure and environment of extratropical cyclones that are accompanied by tornado outbreaks (OCs) and those that are not (NOCs), two idealized numerical experiments are performed. The experiments (OC-CTL and NOC-CTL) adopt the basic states taken from the corresponding composites of reanalysis data (JRA-55), except that the humidity field in both cases is taken from the OC composite.

The simulated cyclone in OC-CTL exhibits a more meridionally elongated structure and stronger low-level wind in the southeast quadrant of the cyclone center, resulting in larger values of storm relative environmental helicity (SREH) than those in NOC-CTL. These results are consistent with the characteristics of the cyclones found for OCs and NOCs in the authors’ composite study. The distributions of surface-based convective available potential energy (SBCAPE) show no notable differences between OC-CTL and NOC-CTL, while those of CAPE based on the most unstable air parcel (MUCAPE) show some differences.

A sensitivity experiment without moist processes such as condensation heating and evaporative cooling shows that the differences in the cyclone structure and environmental parameters between OCs and NOCs can be qualitatively explained by the dry dynamics. However, inclusion of moist processes results in notably larger differences.

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Eigo Tochimoto
and
Hiroshi Niino

Abstract

The structural and environmental characteristics of extratropical cyclones that cause tornado outbreaks [outbreak cyclones (OCs)] and that do not [nonoutbreak cyclones (NOCs)] are examined using the Japanese 55-year Reanalysis dataset (JRA-55). Composite analyses show differences between OCs and NOCs: for OCs, storm relative environmental helicity (SREH) and convective available potential energy (CAPE) are notably larger, and the areas in which these parameters have significant values are wider in the warm sector than they are for NOCs. The larger CAPE in OCs is due to larger amounts of low-level water vapor, while the greater SREH is due to stronger low-level southerly wind.

The composite analyses for environmental fields defined by 20-day means suggest that environmental meridional flows have the potential to advect large amounts of warm and moist air northward, creating atmospheric instability in the troposphere that contributes to the occurrence of a tornado outbreak. A piecewise potential vorticity (PV) diagnosis shows that low- to midlevel PV anomalies are the main contributor to the difference in the low-level winds between OCs and NOCs, whereas upper-level PV anomalies make only a minor contribution.

An examination of the structures of the extratropical cyclones and the upper-level jet stream suggests that the difference in the low-level winds between OCs and NOCs is due to differences in the structure of the jet stream. The OCs develop when the jet stream displays larger anticyclonic shear. This causes a more meridionally elongated structure of OCs, resulting in stronger low-level winds in the southeastern quadrant of the cyclones.

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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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Atsushi Mori
and
Hiroshi Niino

Abstract

The basic processes of time-dependent nonlinear horizontal convection caused by differential bottom cooling in a two-dimensional stratified Boussinesq fluid are investigated both theoretically and numerically. The fluid is initially at rest, and horizontal convection is induced by suddenly cooling one half of the bottom boundary.

It is shown that three distinct flow regimes exist according to the nondimensional elapsed time and the nondimensional stratification parameter: diffusion regime, gravity current regime, and gravity wave regime. In each regime, existence of a self-similar solution is predicted theoretically, and its realizability is confirmed by a numerical experiment. The vertical length scale of the circulation in each regime is given by the diffusion length scale. The horizontal length scales of the circulation in the diffusion, gravity current, and gravity wave regimes are determined by the diffusive spread, horizontal propagation of the gravity current, and horizontal propagation of the gravity wave, respectively.

The presence of a self-similar solution for each flow regime gives a useful perspective about the development process and dynamics of horizontal convections in the atmosphere. For example, the formation process of a steady heat/cool island circulations is understood as a merging of two horizontal convection initiated from both ends of the heat/cool island. In fact, this interpretation is proved to give correct estimates for the vertical scale of the circulation as well as the time required for the formation of the steady heat/cool island circulation.

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Wataru Yanase
and
Hiroshi Niino

Abstract

Polar low dynamics in an idealized atmosphere in which baroclinicity, stratification, and average temperature are varied in the typically observed range is investigated using a 5-km-resolution nonhydrostatic model. The baroclinicity is found to be the most important factor that strongly controls the polar low dynamics.

When the baroclinicity is weak, a small, nearly axisymmetric vortex develops through a cooperative interaction between the vortex flow and cumulus convection. The surface friction promotes the vortex dynamics by transporting the sensible heat and moisture into the vortex center. The vortex development has a strong sensitivity to the initial perturbation.

As the baroclinicity is increased, most of the characteristics of polar low dynamics change smoothly without showing any significant regime shift. The vortex for an intermediate baroclinicity, however, moves northward, which is a unique behavior. This is caused by vortex stretching on the northern side of the vortex where intense convection produces a stronger updraft.

When the baroclinicity is strong, a larger vortex with a comma-shaped cloud pattern develops. The condensational heating, baroclinic conversion from the basic available potential energy, and conversion from the basic kinetic energy through the vertical shear all contribute to the vortex development, which depends little on the initial perturbation. The above relations between baroclinicity and vortex dynamics are proved to be robust in the typically observed range of stratification and average temperature.

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Mikio Nakanishi
and
Hiroshi Niino

Abstract

For the last decade, horizontal roll vortices have been often observed in hurricane boundary layers (HBLs). In this study, a large-eddy simulation is performed to explore the formation mechanism of the horizontal roll vortices and their significance in a near-neutrally stratified HBL at 40 km (R40) and 100 km (R100) from the center of the hurricane. Results are examined through turbulence statistics and empirical orthogonal function (EOF) analysis. The EOF analysis and budgets of turbulent kinetic energy demonstrate that an inflection-point instability in the radial velocity profile is responsible for the roll vortices with horizontal wavelengths of 1.5–2.4 km in the HBL both for R40 and R100. The roll vortices for R40 are nearly aligned with the gradient wind, while those for R100 are oriented slightly to the left of that wind. Also the horizontal distributions of velocity fluctuations suggest the presence of streaklike structures at horizontal intervals of several hundred meters near the ground surface. Internal gravity waves, Kelvin–Helmholtz waves, and entrainments occur above the HBL and are partly coupled with the roll vortices in the HBL, implying an enhancement of vertical transports of momentum and other quantities between the HBL and the free atmosphere.

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Wataru Yanase
and
Hiroshi Niino

Abstract

The development of cyclones, particularly in the Southern Hemisphere summer, is active in the tropics and extratropics but is inactive in the subtropics. To elucidate the influence of environmental fields on the cyclone development in the tropics, subtropics, and extratropics, idealized numerical experiments are conducted using a nonhydrostatic channel model. The experiments examine the development of a weak initial vortex within a zonally uniform environmental field that consists of five factors: the Coriolis parameter, zonal wind, potential temperature, relative humidity, and surface temperature difference between the ocean and atmosphere. The idealized experiments successfully reproduce the significant cyclone development in the tropical and extratropical environment as well as no cyclone development in the subtropical environment. This result confirms the dominant role of the environmental field in controlling the cyclone development. To clarify which environmental factor is responsible for the suppression of cyclone development in the subtropics, a series of sensitivity experiments is performed. A tropical cyclone cannot develop in the subtropics because of low temperature, strong stratification, and strong vertical shear compared to the tropics. On the other hand, an extratropical cyclone cannot develop in the subtropics because of the small Coriolis parameter and weak vertical shear compared to the extratropics. The relative humidity and surface temperature difference play only secondary roles. These results provide useful insights into the climatological distribution of various types of synoptic-scale cyclones.

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Hiroshi Niino
and
Nobuhiko Misawa

Abstract

The barotropic instability of horizontal shear flows is investigated by means of a laboratory experiment. Two kinds of basic flows with different velocity profiles am examined, one a free-shear layer and the other a jet. It is found that for both flows the stability is described by a single nondimensional parameter, a Reynolds number R=VL/v where V is the characteristic velocity of the basic flow, L=(E/4)¼ H is the characteristic length of the basic flow, v the kinematic viscosity, H the depth of the fluid layer, and E=vH 2)−1 the Ekman number, with Ω the angular velocity of the basic rotation.

The experimentally-determined critical Reynolds number Rc and critical wavenumber kc show excellent agreement with those predicted by a linear stability theory in which both Ekman friction and internal viscosity are incorporated. It is found that the internal viscosity plays an important role in explaining the observed values of Rc and kc .

When R is larger than Rc , several organized eddies develop along the shear zone of the basic, flows. Them eddies are quite steady and stable. In general, the number of eddies decreases as R is increased. This tendency is opposite to that shown by the results of linear stability theory in which the wavenumber k of the most unstable wave increases with R. The number of eddies realized for a certain Reynolds number is not unique, i.e., several different configurations are stable for a fixed value of R. Domains on the R-k plane in which finite-amplitude waves are stable are determined both for the shear layer and the jet. Having determined the domains, we are able to simulate the hysteresis phenomena in wavenumber selection.

The result that stable eddies are realized for the jet contradicts the prediction of the weakly nonlinear theory (Niino, 1982a) in which only Ekman friction is considered. It is found, however, that this contradiction can be removed if both Ekman friction and internal viscosity are incorporated in the weakly nonlinear theory.

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Wataru Yanase
and
Hiroshi Niino

Abstract

A wide range of environments that prevail over the globe generate various types of cyclones such as tropical, extratropical, and hybrid cyclones. In this paper, idealized numerical experiments are used to explore a spectrum of cyclones ranging from the diabatic type to the baroclinic type in a parameter space consisting of three environmental factors: temperature, vertical shear, and planetary vorticity. The experiments reproduce not only typical dynamics of tropical and extratropical cyclones but also their modified dynamics, which are consistent with theoretical studies; tropical cyclones are suppressed by vertical shear, while extratropical cyclones are intensified by condensational heating. The experiments also reproduce hybrid cyclones in environments with high temperature and large baroclinicity. The hybrid cyclones show multiscale dynamics in which synoptic-scale baroclinic systems spawn smaller-scale tropical cyclone–like convective cores. The spectrum of cyclones is found to be nonmonotonic in the parameter space because of a two-sided effect of the vertical shear: moderate shear weakens a tropical cyclone by tilting the small-scale vortex to the downshear, while strong shear develops a large-scale vortex of an extratropical cyclone or a hybrid cyclone through warm-air advection from the south. The indices based on the energetics and the symmetric and asymmetric structures overview the different types of cyclones in the parameter space. These parameter sweep experiments provide useful information on what environment is favorable for cyclones, particularly for intermediate environments where cyclone mechanisms are yet to be fully defined.

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Eigo Tochimoto
and
Hiroshi Niino

Abstract

The environmental characteristics and formation process of a tornado spawned by a quasi-linear convective system (QLCS) over Kanto Plain, Japan, are examined using observations, a reanalysis dataset, and a high-resolution numerical simulation with a horizontal grid spacing of 50 m. The QLCS environment responsible for tornadogenesis was characterized by small convective available potential energy and large storm-relative environmental helicity due to strong vertical shear associated with a low-level jet. The strong low-level jet was associated with a large zonal pressure gradient between two meridionally aligned extratropical cyclones and a synoptic-scale high pressure system to the east. The numerical simulation reproduced the tornado in the central part of the QLCS. Before the tornadogenesis, three mesovortices developed that were meridionally aligned at 500-m height, and a rear inflow jet (RIJ) associated with relatively cold air originated from aloft and developed on the west side of the QLCS, while descending from rear to front. Tornadogenesis occurred in the southernmost mesovortex at the northern tip of the RIJ. This mesovortex induces strong low-level updrafts through vertical pressure gradient force. A circulation analysis and vorticity budget analysis for the mesovortex show that environmental crosswise vorticity in the forward inflow region east of the QLCS played a significant role in the formation of the mesovortex. The circulation analysis for the tornado shows that frictional effects contribute to the increase of circulation associated with the tornado. Moreover, environmental shear associated with horizontal and vertical shear of the horizontal wind also contribute to the circulation of the tornado.

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