1. Introduction
Explosively developing extratropical cyclones, which are called meteorological “bombs” (Sanders and Gyakum 1980), frequently occur over the northwestern Pacific Ocean and the northwestern Atlantic Ocean during cold seasons (Sanders and Gyakum 1980; Roebber 1984; Wang and Rogers 2001). Since the explosive cyclones induce strong winds, heavy rainfall and snowfall, and high waves, they often cause serious damage to human activities, infrastructure, and agricultural production in the surrounding regions.
Previous studies have pointed out that the development of explosive cyclones is influenced not only by baroclinic instability, but also by diabatic heating processes (e.g., Kuo et al. 1991b; Reed et al. 1993b; Yoshida and Asuma 2004; Kuwano-Yoshida and Asuma 2008; Fu et al. 2014). Using numerical simulations, Kuo et al. (1991b) revealed that latent heat release contributed to the intensification of an explosive cyclone over the northwestern Atlantic Ocean. They also suggested that extratropical cyclogenesis should be regarded as moist baroclinic instability with nonlinear interactions between the baroclinic dynamics and the diabatic processes. Kuwano-Yoshida and Asuma (2008) performed numerical simulations of several explosive cyclones over the northwestern Pacific Ocean with and without latent heat release, indicating that latent heat release played a significant role in facilitating the rapid intensification of cyclones. They also clarified that the cyclone type that appeared and rapidly developed over the northwestern Pacific Ocean, the so-called Pacific Ocean–ocean (PO–O) cyclones, was more reinforced by the effect of latent heating than were other types. This may be because the PO–O cyclones occur under moister environments. From the viewpoint of forecasting explosive cyclone development, Kuwano-Yoshida and Enomoto (2013) demonstrated that the underestimation of latent heat release in a numerical model is a primary factor in PO–O cyclone forecasting errors.
Over the northwestern Pacific Ocean and northwestern Atlantic Ocean, the Kuroshio/Kuroshio Extension and the Gulf Stream (i.e., western boundary currents) supply a large amount of heat and moisture to the midlatitude atmosphere (e.g., Kelly et al. 2010; Kwon et al. 2010). Several previous studies have shown that the supply of heat and moisture contributed to the rapid development of extratropical cyclones through decreased atmospheric stability and increased latent heating in the cyclone (Nuss and Kamikawa 1990; Kuo et al. 1991a; Neiman and Shapiro 1993; Reed et al. 1993b; Takayabu et al. 1996; Booth et al. 2012). Reed et al. (1993b) highlighted an explosive cyclone developing along the Gulf Stream and suggested that airmass modification by the warm current led to intensification of the cyclone. Nuss and Kamikawa (1990) compared two cyclones—an explosive cyclone and a nonexplosive cyclone developing along the Pacific coast of Japan during March 1986—in terms of atmospheric circulation fields and surface heat fluxes from the ocean. They revealed that surface energy fluxes from the Kuroshio/Kuroshio Extension under the updraft region of the cyclones were better maintained for the explosive cyclone than for the nonexplosive cyclone during its development stages. Takayabu et al. (1996) also pointed out that the energy supply from the Kuroshio/Kuroshio Extension is an important factor in the rapid intensification of extratropical cyclones. From a climatological viewpoint, in the vicinity of the northwestern Pacific Ocean, a large number of explosively developing cyclones tend to concentrate in the Kuroshio/Kuroshio Extension (Gyakum et al. 1989; Chen et al. 1992; Yoshiike and Kawamura 2009; Iizuka et al. 2013), suggesting the warm currents’ significant role in the rapid growth of cyclones.
As noted by the above-cited investigators, the moisture supply from the Kuroshio/Kuroshio Extension exerts a substantial influence on the rapid intensification of explosive cyclones through diabatic heating processes. However, our understanding is still limited with respect to how water vapor that evaporates from the warm currents is transported into the cyclone system and how it facilitates rapid cyclone development through latent heat release. Since the latent heat release around the cyclone center is closely related to the mesoscale structures of the cyclone, a further understanding of its structures is also required.
The main objectives of this study are 1) to investigate how the moisture that evaporates from the Kuroshio/Kuroshio Extension plays a vital role in the rapid intensification of an explosive cyclone and 2) to clarify how the moisture is transported into the cyclone center and then induces latent heat release in association with the fundamental structure of the cyclone system (such as the warm and cold conveyor belts and surface fronts). We pay special attention to an explosive cyclone that occurred over the northwestern Pacific Ocean in the middle of January 2013. The cyclone migrated along the southern periphery of the Kuroshio/Kuroshio Extension and was the most rapidly developing cyclone in recent years in the vicinity of the Kuroshio Extension (Fig. 1). It caused severe weather disasters, with heavy snowfall and exceptionally strong winds in Japan. To reproduce the detailed structures of the cyclone system and the associated air–sea interaction, we need to properly simulate the heat and water exchanges between the atmosphere and the ocean. Therefore, we used a high-resolution coupled atmosphere–ocean regional model in this study.
(a) Maps of 925-hPa equivalent potential temperature (shaded) and SLP (contours) provided by the Japanese 55-yr Reanalysis (Kobayashi et al. 2015) at early (0000 UTC 13 Jan 2013), developing (0000 UTC 14 Jan 2013), and mature (0000 UTC 15 Jan 2013) stages of the explosive cyclone highlighted in this study. The shaded interval is 10 K. The contoured interval is 4 hPa. (b) As in (a), but for simulation by CReSS–NHOES.
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
In this paper, section 2 contains a description of the model design and validation of the model simulation. Section 3 is dedicated to a discussion of possible factors of cyclone development. Section 4 presents examinations of the trajectories associated with the cold and warm conveyor belts of the cyclone system and of the role of latent heating in cyclone intensity. A discussion and a summary are presented in sections 5 and 6, respectively.
2. Model design and validation of the simulation
a. Design of a coupled atmosphere–ocean regional model
To properly simulate an explosive cyclone, we used a high-resolution coupled atmosphere–ocean regional model, the Cloud Resolving Storm Simulator–Non Hydrostatic Ocean model for the Earth Simulator (CReSS–NHOES; Aiki et al. 2015). The coupled model consists of CReSS (Tsuboki and Sakakibara 2002, 2007) and NHOES (Aiki et al. 2006, 2011). The model domain is East Asia and the northwestern Pacific sector (12°–60°N, 110°–179.3°E), as seen in Fig. 2. The domain consists of 1386 × 960 grid points, and the horizontal grid size is 0.05° longitude by 0.05° latitude. The domain and the horizontal resolution of all models are the same. The initial time of the simulation is 1200 UTC 12 January 2013, which is about 2 days before the time of the cyclone’s maximum deepening rate. The integration period is 5 days. The time steps for CReSS and NHOES are 9 and 30 s, respectively. The coupling time step between CReSS and NHOES is 300 s.
Tracks of an explosively developing cyclone as identified by the CReSS–NHOES run (red line), the CReSS-only run (blue line), and the JMA Mesoscale Model (MSM) data (Japan Meteorological Agency 2013) with a spatial resolution of 0.05° × 0.0625° (green line). The details of the CReSS-only run are introduced in section 5a. The location of the cyclone when its deepening rate reached the maximum is also shown with a circle. The magnitude of sea surface current (shaded) and sea surface temperature (contours) averaged from 1200 UTC 12 Jan to 2300 UTC 14 Jan 2013 is depicted. The shaded interval is 0.4 m s−1. The contoured interval is 2 K.
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
In this study, a bulk parameterization of cold rain (Tsuboki and Sakakibara 2007) is used as a cloud microphysical process. The microphysics scheme used in our simulation includes both prognostic equations for the mixing ratio of water vapor, cloud ice, rain, snow, and graupel, and the number concentrations of cloud ice, snow, and graupel. A 1.5-order turbulent kinetic energy closure scheme (Tsuboki and Sakakibara 2007) is applied to subgrid-scale turbulence processes in CReSS. The atmospheric vertical coordinate is a terrain-following coordinate. There are 45 vertical layers, with the top level at a height of 22 500 m. The initial and lateral boundary conditions for CReSS are derived from the Japan Meteorological Agency (JMA) Global Spectral Model (GSM; Japan Meteorological Agency 2013) with a time resolution of 6 h and a spatial resolution of 0.5° × 0.5°. To provide the lateral boundary field at each time step, we linearly and temporally interpolate the 6-hourly GSM data into the corresponding boundary values at every time step.
In NHOES, the horizontal and vertical subgrid-scale turbulent processes are formulated by the Smagorinsky model (Smagorinsky 1963) and the turbulence closure model proposed by Furuichi et al. (2012), respectively. There are 100 oceanic vertical layers with a z coordinate. The vertical grid sizes are 2 m from the sea surface to a depth of 100 m and are stretched from a depth of 100 m to the bottom of the ocean. The initial and lateral boundary conditions for NHOES are provided by the Japan Coastal Ocean Predictability Experiment 2 (JCOPE2; Miyazawa et al. 2009).
To explicitly treat the influence of surface waves, CReSS–NHOES systems include the wave model proposed by Donelan et al. (2012). The time step for the wave model is 15 s. Since the surface waves change the roughness of the near surface, their effects are expected to significantly improve simulation of the cyclone through appropriate estimations of the energy transfer between the atmosphere and the ocean (e.g., Doyle 1995; Powers and Stoelinga 2000; Zhang et al. 2006), although validation of the wave model is beyond the scope of this study.
b. Validation of the simulation
Time series of the central pressure of a cyclone derived from the CReSS–NHOES run (red line), the CReSS-only run (blue line), and the JMA MSM data (green line). The time of the maximum deepening rate of the cyclone is indicated by a closed circle.
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
3. Key factors in the development of an explosive cyclone
In this section, we present major factors in the development of an explosive cyclone simulated by CReSS–NHOES. Synoptic-scale circulation fields and the associated moisture supply from the Kuroshio/Kuroshio Extension during the development stage of the cyclone are examined in section 3a. In section 3b, we will describe how moisture that evaporates from the warm currents intrudes into the cyclone system.
a. Synoptic circulation field and moisture supply from warm currents
While the surface cyclone develops, an upper-level trough accompanied by a high Ertel’s potential vorticity (PV) approaches the cyclone from its western side (Fig. 4a). Thus, it seems that a typical coupling between an upper-level disturbance and a surface cyclone (e.g., Hoskins et al. 1985; Takayabu 1991; Shapiro et al. 1999) serves as one of the primary factors in cyclone development. In addition, a northward moisture flux prevails on the eastern side of the cyclone at 0000 UTC 14 January and 1500 UTC 14 January (the time of the maximum deepening rate) (Fig. 4a). As will be revealed in section 4, the salient northward moisture flux is intimately related to a warm conveyor belt (WCB; e.g., Carlson 1980; Browning and Roberts 1994; Schemm et al. 2013; Madonna et al. 2014; Pfahl et al. 2014). Since the latent heat release due to water vapor condensation results in rapid intensification of the explosive cyclone (e.g., Kuo et al. 1991b; Reed et al. 1993b; Kuwano-Yoshida and Asuma 2008), we infer that such moisture transport associated with the WCB from lower latitudes also contributes to the cyclone’s development as a second primary factor.
(a) Maps of Ertel’s potential vorticity (PV) on the 320-K surface of potential temperature (shaded), vertically integrated moisture flux (vectors), and SLP (contours) simulated by CReSS–NHOES at 0900 UTC 13 Jan, 0000 UTC 14 Jan, and 1500 UTC 14 Jan 2013 (the time of the maximum deepening rate). Note that the 320-K surface corresponds roughly to the 450-hPa (300 hPa) level at 30°N (40°N) around 135°E. The shaded interval is 2 PVU. The reference arrow is 1200 kg m−1 s−1. Fluxes of less than 100 kg m−1 s−1 are suppressed. The contoured interval is 4 hPa. (b) As in (a), but for surface turbulent latent heat flux (shaded) and 10-m horizontal wind (vectors). The shaded interval is 200 W m−2. The reference arrow is 30 m s−1. Winds of less than 10 m s−1 are suppressed.
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
When the cyclone is rapidly growing, moisture is abundantly supplied from the ocean along the Kuroshio/Kuroshio Extension (Fig. 4b). At the time of the maximum deepening rate, the horizontal SLP gradient and the surface wind speed are enhanced on the northwestern side of the cyclone center. The resultant strong winds are expected to locally facilitate surface evaporation from the warm currents. Additionally, when an extratropical cyclone is developing, it is well known that a cold conveyor belt (CCB), which is characterized by cold and dry easterlies, is organized on the northern side of the cyclone center (e.g., Carlson 1980; Schultz 2001; Schemm and Wernli 2014). Since the cyclone highlighted in this study migrated along the southern periphery of the Kuroshio/Kuroshio Extension during its development stage (Fig. 2), it is conceivable that the corresponding CCB is just located over warm currents and facilitates active evaporation from the ocean. As will be shown in section 4, the CCB is well organized along the northern periphery of the cyclone around the time of the maximum deepening rate, eventually reinforcing the moisture supply from warm currents. We would like to stress that the large amounts of moisture imported into the vicinity of the cyclone center are not only subtropical in origin but also are the origin of the midlatitude warm currents. The moisture that evaporates from the warm currents may be a key factor in the explosive development of the cyclone. We will discuss how these two separate moisture supplies contribute to the cyclone development using trajectory analyses and a PV inversion technique in section 4.
b. Mesoscale structures of the cyclone and associated moisture transport
(a) Maps of the horizontal gradient of 850-hPa potential temperature (shaded) and SLP (contours) simulated by CReSS–NHOES at the same times as in Fig. 4. The shaded interval is 3.0 × 10−5 K m−1. The contoured interval is 4 hPa. (b) As in (a), but for a divergence of 950-hPa horizontal wind (shaded). The shaded interval is 2.0 × 10−4 s−1. (c) As in (a), but for a 700-hPa vertical velocity (shaded). The shaded interval is 0.2 m s−1. The illustrated blue rectangles show the domain of (d). (d) The magnified view of the apparent heat source 
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
To further clarify the mesoscale structures around the cyclone center, we made latitude–height (longitude–height) cross-sectional maps along the green (blue) lines illustrated in Fig. 5d, as indicated in Fig. 6. As shown in Figs. 6a and 6c, strong northerly and easterly winds accompanied by a low θe stimulate surface evaporation from the warm currents in the vicinity of the bent-back front. The northerly and easterly winds indicate a signature of the CCB. Particularly, the latent heat flux reaches a value of approximately 1200 W m−2 just north of the bent-back front (Fig. 6a). Around the front, the horizontal moisture flux convergence is enhanced in the lower troposphere, and the high 
(a),(b) Cross-sectional maps along lines A–A′ and B–B′ as illustrated in Fig. 5d. The line A–A′ (B–B′) meridionally crosses the bent-back front (the eastern warm front) at 1500 UTC 14 Jan 2013. (top) The latitude–height cross-sectional maps of 
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
On the other hand, strong southerly and westerly winds accompanied by relatively high 
4. Role of the cold and warm conveyor belts in cyclone development
From the results of the previous section, it appears that humid air intruding into the vicinity of the cyclone center in relation to the WCB is modified little by the Kuroshio/Kuroshio Extension; in contrast, the dry air related to the CCB can receive significant amounts of moisture from those warm currents. However, it is not certain how moisture that evaporates from the warm currents is actually imported into the vicinity of the cyclone center via the CCB. Therefore, in this section, we examine the behavior of the air parcels relevant to the two conveyor belts above using trajectory analyses. In sections 4a and 4b, we give the results of backward and forward trajectory analyses, respectively. In section 4c, we investigate how latent heat release associated with the conveyor belts exerts an influence on the cyclone intensity using a piecewise PV inversion technique. In particular, we highlight the time of the maximum deepening rate of the cyclone.
a. Backward trajectory analysis
In this subsection, we investigate how air parcels associated with latent heat release intrude into the cyclone center using a backward trajectory analysis. The initial positions of air parcels are put on the grids where 
(a) Horizontal distribution of 
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
It is obvious that the primary trajectories can be categorized into two paths (Fig. 8). One is the trajectory associated with the WCB (Fig. 8b), and the other is the trajectory related to the CCB (Fig. 8a). This feature suggests that the presence of the CCB and the WCB leads to organization of the bent-back front and the eastern warm front, respectively.
Trajectories of the air parcels that are initially located to the (a) west and (b) east of 146°E, which are calculated by a backward trajectory analysis. The colored lines denote the heights (m) of the air parcels. See text for details. The SLP distribution at 1500 UTC 14 Jan 2013 is also represented. The contoured interval is 4 hPa.
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
To clarify how the air parcels associated with the WCB and the CCB are modified by the Kuroshio/Kuroshio Extension, we examined the successive changes in the property of the parcels (Fig. 9). We chose two typical parcels that corresponded to the strongest 
(a) Trajectories of two selected typical air parcels transported by the cold conveyor belt (CCB) and the warm conveyor belt (WCB). Surface turbulent latent heat flux (shaded), SLP (contours), and 10-m horizontal winds (vectors) at 1500 UTC 14 Jan 2013 in the CReSS–NHOES run are also depicted. The shaded interval is 200 W m−2. The contoured interval is 10 hPa. The reference arrow is 40 m s−1. Winds of less than 10 m s−1 are suppressed. (b) Temporal changes in the properties of air parcels transported by the CCB. (top) The height of the parcel (black line). (bottom) The vapor mixing ratio (blue line), the potential temperature (red line), and the surface latent heat flux of the underlying ocean (shaded yellow). Symbols A–F correspond to the locations of the parcels. (c) As in (b), but for the air parcel transported by the WCB. Symbols G–L correspond to the locations of the parcels.
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
b. Forward trajectory analysis
In section 4a, we stressed that the CCB is crucially responsible for the transport of moisture from the warm currents into the cyclone’s system. To further confirm the active role of the CCB in the importation of moisture from the warm currents, we examine in this subsection how the air parcels that are initially located over the Kuroshio Extension are transported into the vicinity of the cyclone center using a forward trajectory analysis. The initial time of the analysis was 0300 UTC 14 January. At that time, the parcel transported by the CCB, which is specifically highlighted in the backward trajectory analysis, was located on the grid (35°N, 152°E) (denoted by label B in Fig. 9a). Therefore, we put 32 × 24 parcels in the vicinity of that grid at a height of 500 m with a spatial resolution of 0.25° and integrated from 0300 UTC 14 January to 0900 UTC 15 January in 1-min intervals.
As the cyclone approaches the Kuroshio Extension, the air parcels start to be attracted to the cyclone by the CCB (Figs. 10a,b). In the meantime, the parcels are continuously modified by the moisture supply from the warm current. When the parcels reach the bent-back front, most of them are forced to ascend (Figs. 10c,d). These features are consistent with the results of Figs. 8 and 9. Afterward, part of these parcels heated by condensation is trapped in the vicinity of the cyclone center (Figs. 10e,f), being partly responsible for the transition to so-called warm-core seclusion (e.g., Shapiro and Keyser 1990; Reed et al. 1994; Hart 2003). In fact, warm-core seclusion occurs at 0000 UTC 15 January (not shown). Furthermore, it turns out that the surface turbulent latent heat flux from the Kuroshio Extension continuously prevails on the western side of the cyclone. Such moisture supplies may be expected to intensify the process of warm-core seclusion.
(a)–(f) Locations of air parcels (closed circles) calculated by a forward trajectory analysis at 0300 UTC 14 Jan, 0900 UTC 14 Jan, 1500 UTC 14 Jan, 2100 UTC 14 Jan, 0300 UTC 15 Jan, and 0900 UTC 15 Jan 2013. The colored circles indicate the heights (m) of the parcels. The surface turbulent latent heat flux (shaded), the SLP (contours), and 10-m horizontal winds (vectors) in the CReSS–NHOES run are also shown. The shaded interval is 250 W m−2. The contoured interval is 10 hPa. The reference arrow is 40 m s−1. Winds of less than 5 m s−1 are suppressed.
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
c. Piecewise PV inversion
In sections 4a and 4b, we indicated that latent heat release over the eastern warm front and the bent-back front related to the WCB and CCB, respectively. In this subsection, we examine how latent heating around these fronts plays an influential role in cyclone intensity using the piecewise PV inversion technique (e.g., Davis and Emanuel 1991; Davis 1992; Davis et al. 1993,1996).
The PV areas exceeding 1 PVU (1 PVU = 10−6 K kg−1 m2 s−1) appear around the eastern warm front and the bent-back front from the lower to middle troposphere at the time of the maximum deepening rate (Fig. 11). In particular, the 900-hPa PV over the bent-back front exceeds 5 PVU (Fig. 11a). It will be shown that these large values of PV are a function of latent heat release (Reed et al. 1992; Davis et al. 1993; Reed et al. 1993b; Ahmadi-Givi et al. 2004; Kuwano-Yoshida and Asuma 2008).
(a) Maps of 900-hPa PV (shaded) and SLP (gray contours) simulated by CReSS–NHOES at 1500 UTC 14 Jan 2013. The spatial resolution of the PV was coarsened to 0.5° × 0.5°. The shaded interval is 2 PVU. The contoured interval is 4 hPa. (b) As in (a), but for 750-hPa PV. (c) As in (a), but for 600-hPa PV.
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
The inversion technique developed by Davis and Emanuel (1991) is used to clarify how the PV associated with latent heating contributes to cyclone intensity. In this study, the time mean field is defined as a 5-day average from 1200 UTC 12 January to 1100 UTC 17 January. The perturbation field is defined as a deviation from the mean field. The vertical layers are divided from 950 to 100 hPa with an interval of 50 hPa. The homogeneous lateral boundary condition and the Neumann-type upper and lower boundary conditions, provided by the potential temperature at 100 and 950 hPa, are used in this study. The spatial resolution of the variables used in the integral was coarsened to 0.5° × 0.5° to improve the convergence of the inversion.
We employed a conventional three-way partitioning of the PV anomalies (Korner and Martin 2000; Martin and Marsili 2002; Wu et al. 2011; Fu et al. 2014). In this method, the total PV perturbation field is separated into three layers. One is the upper layer (UL), between 700 and 100 hPa. The positive PV anomalies in the UL are set to 0.0 PVU whenever the relative humidity (RH) is greater than or equal to 70%. The PV anomalies in the UL are associated with the upper-tropospheric and lower-stratospheric air. The second is the interior layer (IL), between 900 and 400 hPa. In the IL, the PV anomalies are set to 0.0 PVU whenever the RH is less than 70%. In addition, the negative PV anomalies from 700 to 400 hPa with RH greater than or equal to 70% are set to 0.0 PVU to avoid the overlap between the UL and IL. The PV anomalies in the IL are closely related to latent heat release. The third is the surface layer (SL), which includes the 950-hPa potential temperature and PV anomalies from 900 to 850 hPa. In the SL, the PV anomalies are set to 0.0 PVU whenever the RH is greater than or equal to 70%. The PV anomalies in the SL are related to low-level baroclinity and boundary layer process. We ignore both moist processes in the UL and dry processes in the IL, which are considered acceptable in this case. The total geopotential field reconstructed by the PV anomalies in the three layers captures well the original field (not shown).
As shown in Fig. 12, most of the negative height anomalies at 900 hPa within the cyclone system are forced by anomalies in the SL and IL, whereas the contribution of UL forcing is minor. As for the cyclone center, the forcing from the IL, which is associated with latent heat release, accounts for about half of the total perturbation field.
(a) Maps of 900-hPa geopotential height anomalies (shaded) induced by the PV anomalies in the surface layer (SL) and the sum of 900-hPa geopotential height anomalies (contours) induced by the individual PV anomalies in the SL, the interior layer (IL), and the upper layer (UL) at 1500 UTC 14 Jan 2013. The shaded interval is 50 m. The contoured interval is 100 m. (b) As in (a), but for the PV anomalies in the IL. (c) As in (a), but for the PV anomalies in the UL.
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
To further investigate the relative contribution of the positive PV anomalies around the bent-back front and the eastern warm front to cyclone intensity, we divided the PV anomalies in the IL into two domains—one is the domain around the bent-back front (30.0°–35.0°N, 140.0°–145.5°E) and the other is the eastern warm front (30.0°–35.0°N, 146.0°–151.5°E)—and performed the inversion again. The PV-induced forcing around the bent-back front predominates in the vicinity of the cyclone center (Fig. 13a), whereas the forcing around the eastern warm front is relatively weak, although it may be responsible for the eastward extension of the low pressure area (Fig. 13b). Since the positive PV anomalies mainly result from latent heating, we confirm that latent heat release over the bent-back front related to the CCB is crucial to the cyclone’s intensity at the time of the maximum deepening rate. Although we do not discuss the interaction among the PV anomalies in each layer because it is beyond the scope of this study, Watanabe and Niino (2015, manuscript submitted to Mon. Wea. Rev.) address this issue with respect to the same cyclone highlighted in this study.
(a) Maps of 900-hPa geopotential height anomalies (shaded) induced by the PV anomalies in the IL within the domain (black rectangle) around the bent-back front and the 900-hPa geopotential height anomalies (contours) induced by the entire PV anomalies in the IL at 1500 UTC 14 Jan 2013. The shaded interval is 40 m. The contoured interval is 50 m. (b) As in (a), but for the domain (black rectangle) around the eastern warm front.
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
As we have indicated in this section, the predominance of the WCB and the CCB contributes to diabatic heating over the eastern warm front and the bent-back front, respectively. Since the air parcels transported by the WCB are already very humid in the subtropical region, they are modified little by midlatitude warm currents. In contrast, since air parcels associated with the CCB are quite dry, they are easily modified by a large amount of moisture from the warm currents. In addition, the CCB can efficiently transport moisture that evaporates from the warm currents into the vicinity of the bent-back front. The piecewise PV inversion analysis shows that latent heat release around the bent-back front facilitates the cyclone’s development through the production of positive PV in the lower troposphere. We emphasize that the significant role of the Kuroshio/Kuroshio Extension in the rapid intensification of an extratropical cyclone is played by the CCB rather than the WCB.
5. Discussion
a. Comparison between the CReSS–NHOES run and the CReSS-only run
In previous sections, we pointed out that the moisture supply from the Kuroshio/Kuroshio Extension is also responsible for explosive cyclone development. To further verify that process, a sensitivity experiment that suppresses the moisture supply from the ocean may be useful. Hence, we performed another experiment (a CReSS-only run).
The CReSS-only run is designed based on the atmosphere model coupled with a one-dimensional ocean model. In the CReSS-only run, the ocean is divided into 47 vertical layers with a 0.6-m grid, and the sea temperature is predicated by a vertically one-dimensional thermal diffusion equation (Tsuboki and Sakakibara 2007). The depth of the oceanic mixed layer is about 100 m in the region where the corresponding cyclone passed (not shown), but the oceanic depth in the CReSS-only run is very shallow (28.2 m). As a result of such artificiality, the oceanic heat content in the CReSS-only run is considerably smaller than that of the mixed layer in the CReSS–NHOES run. Since surface heat fluxes are enhanced in the Kuroshio/Kuroshio Extension in the case of this study, the SST obviously decreases along those currents in the CReSS-only run.
Refer to Figs. 2 and 3 again. The track and moving speed of the cyclone are extremely similar between the CReSS–NHOES and the CReSS-only runs (Fig. 2). However, the SLP of the cyclone center in the CReSS–NHOES run tends to be lower than that in the CReSS-only run (Fig. 3). The maximum deepening rate in the CReSS–NHOES run (the CReSS-only run) is 2.8 (2.4). Figure 14 shows the time series of the difference in the central pressure of the cyclone between the two runs. The difference grows large after 1600 UTC 13 January and reaches about 6 hPa at 1800 UTC 14 January.
Differences in the central pressure of the cyclone on an hourly basis between the CReSS–NHOES and the CReSS-only runs (the former minus the latter).
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
The SST difference gradually increases along the Kuroshio/Kuroshio Extension with the growth of the cyclone (Fig. 15a). Corresponding to the SST difference, the surface evaporation becomes more active in the CReSS–NHOES run than in the CReSS-only run (Fig. 15b). At the time of the maximum deepening rate, the differences in the SST and the surface latent heat flux over the Kuroshio Extension exceeded 2 K and 210 W m−2, respectively. It is highly possible that the difference in the moisture supply from the ocean along the warm currents leads to the difference in cyclone development. When focusing on the SLP distribution, it turns out that the SLP difference becomes evident around the bent-back front. As shown in previous sections, since the moisture that evaporates from the warm currents exerts an influence on diabatic heating over the bent-back front, the distribution of the SLP difference is consistent with the results of sections 3 and 4.
(a) Difference maps in SST (shaded) and SLP (blue contours) between the CReSS–NHOES and the CReSS-only runs (the former minus the latter) at the same times as in Fig. 4. The shaded interval is 1 K. The contoured interval for the SLP difference is 2 hPa. The zero contour has been suppressed. The SLP distributions (black contours) of the CReSS–NHOES run are also shown. The contoured interval is 4 hPa. (b) As in (a), but for the difference in the surface turbulent latent heat flux (shaded). The shaded interval is 70 W m−2.
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
(a) Difference maps of apparent heat source 
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
b. Mesoscale structures around the cyclone center and associated feedback processes
In this section, we discuss the mesoscale structures around the center of an explosive cyclone and the influential role of feedback processes in the rapid intensification of a cyclone migrating along the warm currents. As indicated in section 3, the complicated structures of diabatic heating appear in the vicinity of the cyclone center at the time of the maximum deepening rate (bottom panel of Fig. 5d). At that time, locally strong diabatic heating areas are separated into northern and eastern parts of the cyclone center. The northern (eastern) heating areas are located over the bent-back front (the warm front in the forward sector of the cyclone moving eastward). In particular, heating over the bent-back front is extremely strong (Figs. 6a,c). When extratropical cyclones develop rapidly, mesoscale distributions of precipitation dominate over the warm and bent-back fronts (Neiman et al. 1993; Reed et al. 1993a; Kuo et al. 1996; Liu et al. 1997). These results strongly suggest that mesoscale convection over the fronts facilitates rapid intensification of the cyclone. In fact, looking at the observed precipitation pattern just after the time of the MSM-based maximum deepening rate using the Tropical Rainfall Measuring Mission (TRMM) 3B42 data product, the vicinity of the bent-back front is characterized by strong mesoscale precipitation (Fig. 17). In section 4, we established that, since the CCB dominates over the Kuroshio/Kuroshio Extension when the cyclone moves along the southern periphery of those warm currents, surface evaporation from the warm currents is enhanced along the CCB. The evaporated moisture is efficiently imported into the bent-back front by the CCB and gives rise to latent heat release around that front, eventually reinforcing development of the cyclone (Fig. 13a). In contrast, the WCB interacts little with the midlatitude warm currents because its air is very humid—although, of course, the WCB is also responsible for development of the cyclone (Fig. 13b).
Horizontal distribution of precipitation (shaded) provided by the TRMM 3B42 product with a horizontal grid size of 0.25° (Huffman et al. 2007) and SLP (contours) of JMA MSM data at 0900 UTC 14 Jan 2013. The shaded interval is 2 mm h−1. The contoured interval is 4 hPa.
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
On the basis of these results, we propose that a possible feedback process over the midlatitude warm currents is a crucial factor in the explosive growth of cyclones, as schematically presented in Fig. 18. Latent heat release at the bent-back front contributes to a decrease in the central pressure of a cyclone. The falling central pressure reinforces the CCB accompanied by cold and dry air. The enhanced CCB facilitates surface evaporation from the warm currents. The evaporated moisture is transported into the cyclone system by the CCB, leading to the intensification of moisture convergence and the resultant latent heat release at the bent-back front. The increased latent heat release causes the central pressure to decrease further. Over the northwestern Pacific Ocean, explosively developing cyclones tend to concentrate over the Kuroshio/Kuroshio Extension (Gyakum et al. 1989; Chen et al. 1992; Yoshiike and Kawamura 2009; Iizuka et al. 2013). We suggest that the aforementioned feedback process is partly responsible for this concentration.
Schematic diagram representing a positive feedback process in relation to the rapid intensification of an explosive cyclone over the Kuroshio/Kuroshio Extension.
Citation: Monthly Weather Review 143, 10; 10.1175/MWR-D-15-0016.1
6. Summary
We have examined how moisture that evaporates from the Kuroshio/Kuroshio Extension affects the rapid intensification of an explosive cyclone using a high-resolution coupled atmosphere–ocean regional model, CReSS–NHOES. This study highlighted an explosive cyclone that migrated northeastward along the southern periphery of those warm currents in the middle of January 2013. The major findings of the present study are briefly summarized as follows:
The evolutions of surface fronts of a cyclone simulated by CReSS–NHOES closely resemble the Shapiro–Keyser model. A bent-back front and the associated frontal T-bone become evident at the time of the maximum deepening rate.
At that time, the complicated structures of diabatic heating dominate around the cyclone center. The strong mesoscale heating areas are divided into two: the vicinity of the bent-back front just north of the cyclone center and the warm front in the forward sector of the moving cyclone. Backward trajectory analysis demonstrates that the presence of the cold conveyor belt (CCB) and the warm conveyor belt (WCB) leads to the definition of the bent-back front and the eastern warm front, respectively.
Since the air parcels transported by the WCB are very humid in a subtropical region, they can hardly be modified by midlatitude warm currents. In contrast, since the air parcels transported by the CCB are relatively dry, they can receive large amounts of moisture from the Kuroshio/Kuroshio Extension. The CCB plays a vital role in importing evaporated vapor into the cyclone center. The imported moisture converges at the bent-back front, eventually deepening the cyclone through the positive PV production due to latent heating. The intensification of the cyclone further enhances the CCB. Such a positive feedback process through the CCB plays an influential role in the rapid intensification of extratropical cyclones over warm currents.
To verify the significant effect of the moisture supply from the Kuroshio/Kuroshio Extension on cyclone development, we performed an additional experiment: a CReSS-only run, which was designed based on the atmosphere model coupled with a one-dimensional shallow ocean model. The difference in SST along the warm currents between the two runs leads to the difference in the SLP in the vicinity of the cyclone center through changes in the surface evaporation from the ocean and the latent heat release, confirming the primary importance of the moisture supply from the warm currents to cyclone development.
Acknowledgments
We wish to thank Kazuhisa Tsuboki, Hidenori Aiki, and Mayumi Yoshioka for their kind support and for the use of a coupled atmosphere–ocean regional model, CReSS–NHOES. We would also like to thank Tetsuya Kawano, Satoshi Iizuka, and Eigo Tochimoto for their many helpful suggestions and Chris Davis for his kindness and for the use of the original PV inversion program. Comments by the editor and two anonymous reviewers were extremely helpful. This work was supported by JSPS KAKENHI Grants 14J04241 and 25242038 and MEXT KAKENHI Grant 22106005.
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