Polar lows are intense meso-α-scale cyclones that develop over the oceans poleward of the main baroclinic zone. A number of previous studies have reported polar low formation over the Sea of Japan within the East Asian winter monsoon. To understand the climatology of polar lows over the Sea of Japan, a tracking algorithm for polar lows is applied to the recent JRA-55 reanalysis. The polar low tracking is applied to 36 cold seasons (October–March) from October 1979 to March 2015. The polar lows over the Sea of Japan reach their maximum intensity on the southeastern side of the midline between the Japanese islands and the Asian continent. Consistent with previous case studies, composite analysis demonstrates that the polar low development is associated with the enhanced northerly flow on the western side of a synoptic-scale extratropical cyclone, with the cold trough in the midtroposphere and with increased heat fluxes from the sea surface. Furthermore, the present climatological study has revealed two dominant directions of motion of the polar lows: southward and eastward. Southward-moving polar lows are steered by a strong northerly flow in the lower troposphere, which is enhanced on the western side of synoptic-scale extratropical cyclones, while the eastward-moving polar lows occur within a planetary-scale westerly flow in the midlatitudes. Thus, the direction of polar low motion reflects the difference in planetary- and synoptic-scale conditions.

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.

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.

Tropical transition (TT) is a cyclogenesis process in which a baroclinic disturbance is transformed into a tropical cyclone. Many studies have analyzed TT events over the North Atlantic. This study assesses TT processes from a possible subtropical cyclone to Tropical Storm Kirogi at a relatively high latitude over the western North Pacific in an environment of enhanced baroclinicity in August 2012. Analyses based on satellite observations, the JRA-55 reanalysis, and a simulation with 2.5-km horizontal grid spacing demonstrate three stages during the TT: the baroclinic, intermediate, and convective stages. Over the baroclinic stage, Kirogi had an asymmetric comma-shaped cloud pattern with convection in the northern and eastern parts of the cyclone. This convection is attributed to quasigeostrophic forcing and frontogenesis associated with advection of warm and moist air. Vorticity locally generated by this convection was advected to the cyclone center by cyclone-relative northerly flow. Kirogi also had a shallow warm-core structure due to the interaction with an upper-level cold trough extending from the midlatitudes. In the intermediate stage, the warm and moist air in the lower troposphere and the cold trough in the upper troposphere wrapped around Kirogi. In the convective stage, Kirogi attained characteristics of a typical tropical cyclone with convection concentrated near the cyclone center and a deep warm-core structure. These results demonstrate that baroclinic processes can directly trigger formation of a tropical storm at relatively high latitudes over the western North Pacific in a similar manner to that over the North Atlantic. Significance Statement: Tropical cyclogenesis is an important process for early identification of tropical cyclone hazards. Tropical transition is a tropical cyclogenesis process that is triggered by a subtropical or extratropical disturbance. It is unique to relatively high latitudes and has social importance particularly for midlatitude countries. There have been fewer studies on tropical transition over the western North Pacific than over the North Atlantic. This study demonstrates the dynamics of a distinct tropical transition event that led to the formation of Tropical Storm Kirogi (2012) at a relatively high latitude over the western North Pacific.

Tropical cyclones (TCs) undergoing extratropical transition (ET) in the midlatitudes have several structural differences from typical TCs. The link between specific structural changes and large-scale conditions during ET remains to be fully elucidated because processes can vary from one case to another, depending on various factors such as upper-tropospheric troughs and orography. To explore the fundamental evolution of ET, we conduct idealized simulations using a nonhydrostatic atmospheric model on the β plane with a horizontal grid spacing of 5 km and examine in detail the ET characteristics produced by the interaction between a northward-moving TC and a zonally uniform baroclinic zone. The control experiment reproduces the wind and precipitation patterns unique to ET and subsequent reintensification as an extratropical cyclone, which are in good agreement with observational studies. Two asymmetric processes govern the evolution and dynamics of structures during ET. Warm frontogenesis in the northeastern part of the cyclone is evident in the late ET period (frontal stage), which locally enhances wind and precipitation. In addition to this well-known frontal process, precipitation is enhanced in the slantwise convection in the northern part of the cyclone in the early ET period (prefrontal stage), where conditional symmetric stability is reduced in strong westerly vertical shear associated with the combination of the TC circulation and the westerly jet stream. Sensitivity experiments demonstrate that warm frontogenesis and reintensification are enhanced in the environment with stronger baroclinicity and that the slantwise convection north of the cyclone occurs even with spatially uniform baroclinicity. Significance Statement: Tropical cyclones undergoing extratropical transition in the midlatitudes have asymmetric structures different from typical tropical cyclones. Although understanding the structures unique to extratropical transition is important for disaster mitigation in midlatitude countries, their dynamics vary from one case to another depending on complicating factors such as preexisting weather disturbances and orographic effects. To explore the fundamental evolution and dynamics of extratropical transition, we conduct simplified model experiments of the interaction between a northward-moving tropical cyclone and a jet stream. This simplified experiment successfully reproduces several characteristics unique to extratropical transition reported in previous studies. We demonstrate that the causes of asymmetric wind and precipitation patterns change from the early to late transitioning stages.