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Shun-ichi I. Watanabe and Hiroshi Niino

Abstract

A polar mesocyclone (PMC) observed over the Japan Sea on 30 December 2010 was studied using a nonhydrostatic mesoscale numerical model with a horizontal resolution of 2 km. The numerical simulation successfully reproduced the observed life cycle of the PMC. The results of the numerical simulation suggest that the life cycle of the PMC may be divided into three stages: an early development stage, in which a number of small vortices appear in a shear zone; a late development stage, which is characterized by the merger of vortices and the formation of a few larger vortices; and a mature stage, in which only a single PMC is present. During the early development stage, vortices are generated in the shear zones of strong updrafts in discrete cumulus convection cells. In contrast, during the late development stage, the vortices develop as a result of barotropic instability in the shear zone. A cloud-free eye and spiral cloud bands accompany the mature stage of a simulated PMC. A warm core structure also forms at the center of the PMC on account of adiabatic warming associated with downdrafts. The structures in the PMC during the mature stage resemble those of a tropical cyclone. Sensitivity experiments, in which sensible and latent heat fluxes from the sea surface and condensational heating were switched on/off, demonstrate that condensational heating is critical to the development of the PMC at all stages, and that sensible and latent heat fluxes play secondary roles.

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Shun-ichi I. Watanabe, Hiroshi Niino, and Wataru Yanase

Abstract

Polar mesocyclones (PMCs) are mesoscale cyclonic vortices that develop poleward of the main polar front. This article reports on a new algorithm for the objective tracking of PMCs, including meso-β-scale vortices, which will facilitate the study of their climatology. The algorithm is based mainly on the vorticity field and consists of three parts: the identification of vortices, the connection of vortices at consecutive time steps, and discrimination between PMCs and synoptic-scale disturbances. The objective tracking method was applied to Mesoscale Analysis (MA) data provided by the Japan Meteorological Agency, which has a horizontal resolution of 5 km. The detected tracks of PMCs were confirmed by subjective analysis of the MA data and satellite images. The method used here to discriminate between PMCs and synoptic-scale disturbances differs from that used in previous studies, which is based on the difference between the sea surface temperature and the temperature at 500 hPa, but gives a consistent result. This objective tracking method was used to obtain the climatology of PMCs over the Sea of Japan, which were classified into three groups according to the regions where they attained their maximum intensity. In each region, the PMCs have different characteristics with respect to their direction of movement, size, and intensity, which are likely to be related to their environment or development mechanism.

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Shun-ichi I. Watanabe, Hiroshi Niino, and Wataru Yanase

Abstract

Polar mesocyclones (PMCs) are mesoscale cyclonic vortices that develop poleward of the main polar front. The Sea of Japan is one region where PMCs frequently occur during winter. In this paper, the general characteristics of the structure and environment of PMCs that form over the northeastern part of the Sea of Japan and move southward are examined using composite analysis and numerical simulation. The composite analyses show that the synoptic-scale environment of the PMCs is characterized by a zonal temperature gradient at lower levels and a trough accompanied by cold air at upper levels. These elements of the environmental field form a reverse shear. The mesoscale structure of the PMCs exhibits characteristics of baroclinic development, while it is also accompanied by condensational heating. The numerical simulation in which the composite fields are used for the initial and boundary conditions successfully reproduces a realistic PMC. In this numerical simulation, the mesoscale structures are almost smoothed out in the initial and boundary fields, indicating that the PMCs spontaneously form and develop when the large-scale environment becomes favorable. Sensitivity experiments in which moisture is removed demonstrate that condensational heating is crucial for the genesis and development of the PMC. The sensitivity experiments also show that the warm sea surface temperature in the Strait of Tartary and the Sea of Japan to the west of Hokkaido Island, and the topography of the Sikhote-Alin mountain region provide favorable conditions for the development of the PMCs.

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Shun-ichi I. Watanabe, Hiroshi Niino, and Wataru Yanase

Abstract

Polar mesocyclones occur frequently over the Sea of Japan during winter in association with cold air outbreaks from the Eurasian continent. In this study, the general characteristics of polar mesocyclones over the western part of the Sea of Japan are examined using composite analysis. The synoptic-scale environment associated with these mesocyclones is characterized by a negative sea level pressure anomaly to the east that causes a cold air outbreak at low levels. There is also a geopotential height trough moving eastward at upper levels. In the cold air outbreak, a convergence zone known as the Japan Sea polar airmass convergence zone (JPCZ), collocated with a thermal ridge, develops on the lee side of the mountains at the root of the Korean Peninsula. These polar mesocyclones are generated when the upper-level trough approaches the JPCZ from the west. However, the behavior of the JPCZ and the movement of the polar mesocyclones differ depending on the location of the upper-level trough. A piecewise potential vorticity inversion analysis revealed that the circulation associated with the upper-level trough modifies the low-level winds, which affects the direction of extension of the JPCZ as well as the genesis location and the movement of the polar mesocyclones.

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Akihiko Murata, Shun-ichi I. Watanabe, Hidetaka Sasaki, Hiroaki Kawase, and Masaya Nosaka

Abstract

Goodness of fit in daily precipitation frequency to a gamma distribution was examined, focusing on adverse effects originating from the shortage of sampled tropical cyclones, using precipitation data with and without the influence of tropical cyclones. The data used in this study were obtained through rain gauge observations and regional climate model simulations under the RCP8.5 scenario and the present climate. An empirical cumulative distribution function (CDF), calculated from a sample of precipitation data for each location, was compared with a theoretical CDF derived from two parameters of a gamma distribution. Using these two CDFs, the root-mean-square error (RMSE) was calculated as an indicator of the goodness of fit. The RMSE exhibited a decreasing tendency when the influence of tropical cyclones was removed. This means that the empirical CDF derived from sampled precipitation more closely resembled the theoretical CDF when compared with the relationship between empirical and theoretical CDFs, including precipitation data associated with tropical cyclones. Future changes in the two parameters of the gamma distribution, without the influence of tropical cyclones, depend on regions in Japan, indicating a regional dependence on changes in the shape and scale of the CDF. The magnitude of increases in no-rain days was also dependent on regions of Japan, although the number of no-rain days increased overall. This simplified approach is useful for analyzing climate change from a broad perspective.

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Wataru Yanase, Hiroshi Niino, Shun-ichi I. Watanabe, Kevin Hodges, Matthias Zahn, Thomas Spengler, and Irina A. Gurvich

Abstract

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.

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