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The extratropical transition (ET) of tropical cyclones often has an important impact on the nature and predictability of the midlatitude flow. This review synthesizes the current understanding of the dynamical and physical processes that govern this impact and highlights the relationship of downstream development during ET to high-impact weather, with a focus on downstream regions. It updates a previous review from 2003 and identifies new and emerging challenges and future research needs. First, the mechanisms through which the transitioning cyclone impacts the midlatitude flow in its immediate vicinity are discussed. This “direct impact” manifests in the formation of a jet streak and the amplification of a ridge directly downstream of the cyclone. This initial flow modification triggers or amplifies a midlatitude Rossby wave packet, which disperses the impact of ET into downstream regions (downstream impact) and may contribute to the formation of high-impact weather. Details are provided concerning the impact of ET on forecast uncertainty in downstream regions and on the impact of observations on forecast skill. The sources and characteristics of the following key features and processes that may determine the manifestation of the impact of ET on the midlatitude flow are discussed: the upper-tropospheric divergent outflow, mainly associated with latent heat release in the troposphere below, and the phasing between the transitioning cyclone and the midlatitude wave pattern. Improving the representation of diabatic processes during ET in models and a climatological assessment of the ET’s impact on downstream high-impact weather are examples for future research directions.

Extratropical transition (ET) is the process by which a tropical cyclone, upon encountering a baroclinic environment and reduced sea surface temperature at higher latitudes, transforms into an extratropical cyclone. This process is influenced by, and influences, phenomena from the tropics to the midlatitudes and from the meso- to the planetary scales to extents that vary between individual events. Motivated in part by recent high-impact and/or extensively observed events such as North Atlantic Hurricane Sandy in 2012 and western North Pacific Typhoon Sinlaku in 2008, this review details advances in understanding and predicting ET since the publication of an earlier review in 2003. Methods for diagnosing ET in reanalysis, observational, and model-forecast datasets are discussed. New climatologies for the eastern North Pacific and southwest Indian Oceans are presented alongside updates to western North Pacific and North Atlantic Ocean climatologies. Advances in understanding and, in some cases, modeling the direct impacts of ET-related wind, waves, and precipitation are noted. Improved understanding of structural evolution throughout the transformation stage of ET fostered in large part by novel aircraft observations collected in several recent ET events is highlighted. Predictive skill for operational and numerical model ET-related forecasts is discussed along with environmental factors influencing posttransition cyclone structure and evolution. Operational ET forecast and analysis practices and challenges are detailed. In particular, some challenges of effective hazard communication for the evolving threats posed by a tropical cyclone during and after transition are introduced. This review concludes with recommendations for future work to further improve understanding, forecasts, and hazard communication.

Objective cyclone tracking applied to a 30-yr reanalysis dataset shows that cyclone development in the summer and autumn seasons is active in the tropics and extratropics and inactive in the subtropics. To understand this geographically bimodal distribution of cyclone development associated with tropical and extratropical cyclones quantitatively, the direct relationship between cyclone types and their environments are assessed by using a parameter space of environmental variables [environmental parameter space (EPS)]. The number of cyclones is analyzed in terms of two different factors: the environmental conditions favorable for cyclone development and the area size that satisfies the favorable condition. The EPS analysis is mainly conducted for two representative environmental parameters that are commonly used for cyclone analysis: potential intensity for tropical cyclones and baroclinicity for extratropical cyclones. The geographically bimodal distribution is attributed to the high sensitivity of the cyclone development to the change in the environmental fields from tropics to extratropics. In addition, the bimodal distribution is partly attributed to the rapid change in the environmental fields from tropics to extratropics. The EPS analysis also shows that other environmental parameters, including relative humidity and vertical velocity, may enhance the contrast between the tropics (extratropics) and subtropics, whereas they are not essential for determining cyclone types. The relationship between cyclones and their environments is found to be similar between the hemispheres in the EPS, although the geographical distribution, particularly the longitudinal uniformity, is markedly different between the hemispheres.

Tropical cyclones (TCs) in the western North Pacific are examined to present climatology of their extratropical transition (ET) during the period of 1979–2004. Two parameters of the cyclone phase space (CPS) are calculated using the Japanese 25-year Reanalysis (JRA-25) dataset as indices of ET for the TCs in the best track data from the Japan Meteorological Agency. The onset and the completion of ET are defined as the beginning of an increase in thermal asymmetry and the evolution from the warm to the cold-core structure, respectively. Forty percent of all TCs are assumed to complete ET. The mean transition period from the ET onset until ET completion is estimated to be about 18 hrs, although 16.8% of all ET storms are transformed into the cold-core structure before the increase in thermal asymmetry. Meridional fluctuation in the location of ET completion by season attains its peak in August, while the peak of ET events occurs in September-October with regards to the ratio of ET frequency to all TCs, the transition period, and intensity at the time of ET completion. The background environment of ET events are also examined using the monthly mean of the JRA-25 and sea surface temperature of the centennial in situ observation-based estimates (COBE). Both air-sea thermal contrast and the tropospheric vertical shear are greater in autumn than in the Baiu season and summer. The results suggest that, whereas a TC can translate into higher latitudes with a warm-core structure in an environment of weak vertical shear and small air-sea contrast in summer, it tends to complete ET in lower latitudes in an environment of strong vertical shear and large air-sea contrast in autumn-winter. These characteristics of ET affect the seasonal variation in the structure of landfalling TCs on the main islands of Japan.

Three-dimensional structures of 70 tropical cyclones (TCs) that made landfall in the southern part of the main islands of Japan during 1979-2004 were examined by using the Japanese 25-year Reanalysis (JRA-25) dataset. These 70 TCs had been classified into 5 clusters C1-C5 in a previous study by means of the fuzzy c-means method based on the surface wind fields. A cyclone phase space analysis using three indicators exhibited that the average TC in each cluster has a thermal structure at its particular stage of the structural change in midlatitudes. The average C1 and C5 TCs, both of which were characterized by the area of strong winds concentrated near the cyclone center, were a relatively intense TC at the early stage of the extratropical transition (ET) and a weakening TC with a thermally symmetric structure, respectively. The C2 and C3 TCs, accompanied by the area of strong winds in the right semicircle with respect to the direction of the storm motion, were a mature TC and a thermally asymmetric, weak TC at the late ET stage, respectively. The C4 TC accompanied by strong winds on both right and left sides was a large, mature TC in August or, alternatively, a TC undergoing ET with a strong warm core in the months except for August. Composite analyses indicated that the average TC structure of each cluster is related to the environment characterized by other features such as a trough in the midlatitude westerly, the subtropical high and another TC.

Extratropical transition (ET) in the western North Pacific during 2001–02 is examined in terms of frontal evolution and its environment using a gridded global analysis dataset produced by the Japan Meteorological Agency. According to the best-track data created by the Regional Specialized Meteorological Center (RSMC) Tokyo-Typhoon Center, 23 out of 52 (44%) tropical cyclones (TCs) completed ET during these two years. These ET cases are classified into three categories in terms of the lower-tropospheric equivalent potential temperature distributions, characteristics of the TCs, and their environments. Eight TCs (35% of all ET cases) had temporary warm secluded frontal patterns and then occluded patterns at the completion of ET, which is defined as a seclusion–occlusion (SO) type. This occurs downstream of an intense upper-tropospheric trough interacting with a TC, which is then likely to move northward while keeping its tropical characteristics and have a large impact on relatively high latitudes including all the Japan islands. Three TCs (13%) were apparently absorbed into vigorous preexisting fronts in the southwest of midlatitude cyclones; this situation is defined as a cold advection (CA) type. A TC of the CA type is likely to lose its tropical characteristics rapidly in strong cold advection that is equatorward of a relatively straight upper-tropospheric jet stream. The other 12 TCs (52%) were organized into an open-wave frontal cyclone, which is defined as an open-wave (OW) type. This has characteristics of both SO and CA and is more related to the midlatitude baroclinic zone compared with the SO type.

Cyclone phases for 15 tropical cyclones (TCs) that made 17 landfalls in Japan during 2004-2006 are examined by using the gridded reanalysis datasets of the Japan Meteorological Agency. TCs with 9 landfalls south of 40°N from late June through early September had the structure of typical tropical cyclones, i.e., a thermally symmetric warm-core structure. TCs of 3 landfallings north of 40°N and one landfalling in western Japan in mid-June had the structure of a frontal cyclone, which suggests that the environmental baroclinicity would have contributed to the structure change of the cyclones. With the other 4 landfalls south of 40°N after mid-September, TCs were transitioning into extratropical cyclones with a thermally asymmetric warm-core structure, and were associated with significant features including frontal heavy rainfall and localized gusts related to the environmental baroclinicity. This suggests that precautions should be taken against a TC approaching Japan in mid- and late autumn associated with features different from those of a typical TC.

A statistical study was made for surface wind fields of tropical cyclones (TCs) at landfall on the main islands of Japan using the AMeDAS data. The analysis was performed for 70 TCs which made landfall on the southern part (south of 36°N) of the main islands during the period 1979-2004. The wind field within 200 km of each storm center was obtained by applying a composite analysis to hourly or tenminute observations for the four hours around landfall, with the direction of storm motion as the y-axis. Then the fuzzy c-means method (FCM), which is a kind of cluster analysis, was applied to classify the wind distribution. All but one of the resulting five patterns show stronger winds on the right side of the center than on the left, with different degrees of right-left asymmetry and the shapes of the strong wind area. The five patterns have some differences in the preferred season, central pressure, speed and direction of motion, and regions of landfall of storms. A supplementary analysis using the wind profiler data, available from 2001, reveals that the right-left asymmetry is roughly explained by the storm motion, although the storm-relative wind fields still have some asymmetry.

台風0418 (2004年9月7～8日) と台風9119 (1991年9月27～28日) は互いによく似た経路で西日本を通り，ともに大きな強風被害をもたらした。これら2つの台風による風速分布の特徴を気象庁のデータを使って調べた。どちらの台風も風は中心の右側（南東側）で最も強かったが，台風9119は中心の左後方（西側）にも強風域を伴っていた。この強風域は強雨と低温を伴っており，中心後方の降雨帯に乾燥空気が侵入することによる蒸発冷却によって形成されたと考えられる。これに対して台風0418は中心後方の降雨が弱く，風速分布は軸対称な渦と移動速度との重なったものにほぼ近かったことが示された。

2000年9月11日から12日にかけて、東海地方を中心に多いところで500mm/48hを超える豪雨があった。このときの総観場とそれによる豪雨の維持機構を、現業観測データと気象庁全球客観解析を用いて診断した。 豪雨の最盛期には、下層で湿潤絶対不安定成層が観測された。これは対流不安定成層の持ち上げによって生成されたと考えられる。θeの水平移流は925hPaの正の移流と700hPaの負の移流が同程度の大きさで、これらが下層の対流不安定の維持に同程度に寄与していたと思われる。 対流不安定成層を持ち上げるforcingは、下層のgeostrophic frontogenesisで説明された。日本付近ではまず、南海上の台風と日本の北の低気圧と太平洋高気圧により、合流の変形によるfrontogenesisが生じた。そこへ40N付近を中層の擾乱が東進して傾圧帯へ接近し、北陸地方のジオポテンシャル高度低下とシアー変形によるfrontogenesisの増大に寄与した。シアーfrontogenesisの位置とタイミングは東海地方の豪雨の最盛期とよく一致していた。またジオポテンシャル高度の低下により南風が強まると、それがさらに対流不安定の増大やfrontogenesisに寄与した。

Extratropical transformation of Typhoon Vicki (9807) is diagnosed in relation to its environment, particularly with respect to interactions with upper tropospheric disturbances, using the gridded dataset of global analysis (GANAL) produced by the Japan Meteorological Agency. TY Vicki made landfall on Japan with its maximum intensity on 22 September 1998, causing damage with strong winds. Vicki lost the diabatically generated potential vorticity (PV) anomaly at 00UTC 23 September 1998, when it was supposed to complete the extratropical transformation. It still had a considerable diabatic PV anomaly during passage over Japan, which is consistent with the cyclone intensity. At 06UTC 22 September, Vicki was located in the lower-baroclinic zone, and strong horizontal divergence directly above the cyclone center was dissipated in the upper troposphere, which suggest that was already beginning extratropical transformation. Regarding the upper tropospheric environment, Vicki was moving northward to the right entrance of the jet streak, and a trough, i.e., mesoscale PV anomaly, approached to the west of Vicki. To the north of the cyclone, mid-tropospheric warm frontogenesis and the associated northward out-flow were induced. To the west of the cyclone, the upper-tropospheric frontogenesis associated with the jet entrance and the trough contributed to upper-tropospheric outflow from the cyclone and midtropospheric dry-air inflow. These contributed to organizing a wide rainfall region in the northern semicircle of the cyclone and to limiting the rainfall region in the southwest of the cyclone center. It is also suggested that interactions between upper- and mid-tropospheric outflow, and the midlatitude jet streak to the north of the cyclone contributed to a deepened layer of horizontal divergence that delayed the weakening of the cyclone during the start of the extratropical transformation.

The relationship between a synoptic-scale, upper-level cold vortex and a meso-α scale polar low (PL) over the Sea of Japan, and the role played by low-level baroclinicity in the development of the PL were investigated. We examined three PLs from the perspective of potential vorticity (PV) and energy budgets, using mesoscale analysis data with a horizontal resolution of ~11 km. During the development stage of each PLs, a meso-α scale upper-level PV (UPV) anomaly, which intrudes to around the 700–600 hPa level and which is just a part of a synoptic-scale PV anomaly accompanying the cold vortex, moved to the west side of a low-level PL. The PL was located in a wide region of low static stability in the lower troposphere. These conditions suggested that the synoptic-scale cold vortex, accompanied by a cold dome and the different scales of UPV anomalies, played a role in creating a favourable environment for the PL development. One of the common characteristics of the PL cases studied here was that, in contrast to the low-level PL, the upper-level disturbance associated with the meso-α scale UPV anomaly showed no development at all. This is likely because of the mobile UPV anomaly cut off from its source. Low-level baroclinicity, which varied with time, was an important factor in differentiating the PL shape and the energy sources necessary for PL development. Furthermore, the strength of baroclinicity, through its enhancement of mesoscale fronts and convection, affected diabatic processes, as well as baroclinic development. Keywords: mesoscale front, PV anomaly, energy budget analysis (Published: 23 October 2014) Citation: Tellus A 2014, 66 , 24694, http://dx.doi.org/10.3402/tellusa.v66.24694