1. Introduction
Explosive development of extratropical cyclones occurs frequently around the Kuroshio/Kuroshio Extension in the northwestern Pacific Ocean and the Gulf Stream in the northwestern Atlantic Ocean during northern winters (e.g., Sanders and Gyakum 1980; Gyakum et al. 1989; Chen et al. 1992; Wang and Rogers 2001; Lim and Simmonds 2002; Yoshida and Asuma 2004; Yoshiike and Kawamura 2009; Iizuka et al. 2013). Both regions are characterized by strong near-surface baroclinicity (e.g., Hoskins and Valdes 1990; Hotta and Nakamura 2011; Seiler and Zwiers 2016). Such an environmental condition favors cyclogenesis through interaction between the upper-level and surface disturbances (e.g., Hoskins et al. 1985; Takayabu 1991; Shapiro et al. 1999). Additionally, latent heat release owing to water vapor condensation plays a vital role in facilitating the rapid intensification of cyclones (e.g., Kuo et al. 1991b; Reed et al. 1992; Davis et al. 1993; Ahmadi-Givi et al. 2004; Kuwano-Yoshida and Asuma 2008). Previous studies indicate that the moisture and sensible heat supply around warm currents is one of the factors responsible for the enhanced latent heating in extratropical cyclones (e.g., Davis and Emanuel 1988; Nuss and Kamikawa 1990; Kuo et al. 1991a; Reed et al. 1993; Gyakum and Danielson 2000; Booth et al. 2012; Kuwano-Yoshida and Minobe 2017).
To clarify how the moisture and sensible heat supply from the warm currents affect the latent heating, the structures of synoptic-scale flows that characterize extratropical cyclones need to be considered, particularly the warm conveyor belt (WCB) and the cold conveyor belt (CCB). The WCB is a northward flow with warm and moist air that prevails in the warm sector of cyclones (e.g., Carlson 1980; Whitaker et al. 1988; Browning and Roberts 1994). The CCB is an easterly flow accompanied by cold and dry air that passes beneath the warm-frontal zone of cyclones (e.g., Carlson 1980; Whitaker et al. 1988; Schultz 2001). Booth et al. (2012) examined the roles of the WCB in a cyclone’s growth and investigated how surface moisture and heat fluxes influence the development of extratropical cyclones in the Gulf Stream region. They postulated that the surface moisture and heat supply under the warm sector produces increased latent heating in the cyclones via the WCB, thereby strengthening the extratropical cyclogenesis.
Another view is the active role of the CCB in the rapid development of cyclones over warm currents. Hirata et al. (2015, hereafter H15), showed that the CCB facilitates surface evaporation from the Kuroshio/Kuroshio Extension and imports the evaporated moisture into the cyclone’s center. This subsequently leads to intensification of the cyclone through the increased latent heating around the bent-back front, which results in further development of the CCB. H15 proposed that such a positive feedback process contributes to the rapid development of an extratropical cyclone over the warm currents, which is illustrated in their Fig. 18. We hereafter call this feedback process CCB–latent heating (CCB–LH) feedback process for convenience. Furthermore, Hirata et al. (2016, hereafter H16), indicated that sea surface temperature (SST) variations in the vicinity of the Kuroshio Extension have the potential to deform the inner structures of explosive cyclones through this feedback process.
Because the magnitude of sensible heat fluxes from the Kuroshio/Kuroshio Extension is always smaller than that of latent heat fluxes during the growth phase of the cyclone, H15 did not consider roles of sensible heat supply from those currents in the CCB–LH feedback process. However, the sensible heat supply from the ocean acts to decrease the static stability in the lower troposphere (e.g., Kuo et al. 1991a; Neiman and Shapiro 1993; Reed et al. 1993). Moreover, the increase in temperature in the atmospheric boundary layer by sensible heating may lead to a rise in water vapor content because the saturation mixing ratio also increases. If these two possible effects of the sensible heat supply are significant in the CCB–LH feedback process, we need to revise the concept of the feedback process proposed by H15.
The main objectives of this study are 1) to examine the relative roles of the sensible heat supply from the Kuroshio/Kuroshio Extension in the CCB–LH feedback process and 2) to revise the CCB–LH feedback process if necessary. To address these issues, this study focuses on an extratropical cyclone with rapid development over the Kuroshio/Kuroshio Extension on 14 January 2013, which is the same cyclone analyzed in H15. Additionally, we conducted a control experiment and three sensitivity experiments with respect to surface heat fluxes using a regional cloud-resolving model. To quantitatively evaluate the influence of the sensible heat supply from the Kuroshio/Kuroshio Extension on the cyclone growth, an experiment is performed without sensible heat fluxes from the warm currents. In addition, two other sensitivity experiments, one without latent heat fluxes and one without both sensible and latent heat fluxes from the currents, are also conducted to validate the CCB–LH feedback process.
Section 2 presents the design of the numerical experiments and validation of the control experiment. Section 3 addresses the influences of the sensible heat and moisture supply from the Kuroshio/Kuroshio Extension on the cyclone’s intensity and the mesoscale structures around the cyclone’s center. Section 4 examines the significant roles of the sensible heat supply from these currents in the CCB–LH feedback process. A discussion and a summary are provided in sections 5 and 6, respectively.
2. Experimental design and validation of the control experiment
a. Design of the Cloud Resolving Storm Simulator
To conduct numerical experiments of the explosive cyclone, the Cloud Resolving Storm Simulator (CReSS) was used (Tsuboki and Sakakibara 2007; Tsuboki 2008). The basic design of the CReSS model was same as that in H15 and H16, and the following text is derived from there with minor modifications. A bulk parameterization of cold rain (Tsuboki and Sakakibara 2007) was applied to the cloud microphysical processes in this model. The microphysical scheme used in the present experiments included both the prognostic equations for the mixing ratio of water vapor, cloud water, cloud ice, rain, snow, and graupel and the number concentrations of cloud ice, snow, and graupel. We applied a 1.5-order turbulent kinetic energy closure scheme (Tsuboki and Sakakibara 2007) as subgrid-scale turbulence processes. The model domain was East Asia and the northwestern Pacific sector (12°–60°N, 110°–179.3°E) (as shown in Fig. 2b). The horizontal grid size was 0.05° longitude × 0.05° latitude, and the vertical coordinate was a terrain-following coordinate. A total of 45 vertical layers were included; the height of the top level was 22 500 m (approximately 37 hPa). The time steps of terms related to sound waves and other terms were 4.5 and 9.0 s, respectively. The integration period was from 0600 UTC 13 January to 0000 UTC 15 January 2013, including the period of the cyclone’s rapid development. We chose 0600 UTC 13 January as the initial time because the experiment with that initial time successfully simulated the rapid development of the cyclone. The initial and lateral boundary conditions were derived from the Japan Meteorological Agency (JMA) Global Spectral Model (GSM) data (Japan Meteorological Agency 2013) with a time resolution of 6 h and a spatial resolution of 0.5° longitude × 0.5° latitude. We used the Japan Coastal Ocean Predictability Experiment 2 (JCOPE2; Miyazawa et al. 2009) with a spatial resolution of 1/12° longitude × 1/12° latitude as SST data. The SST field was assumed to be constant throughout the integration period because daily SST variations over the northwestern Pacific Ocean were very small during that period (not shown).
We first carried out a control experiment (CNTL run). Figure 1 shows the spatial distributions of surface turbulent sensible and latent heat fluxes along with sea level pressure (SLP) during the cyclone’s development stage in the CNTL run. The area enclosed by the green line in Fig. 1 corresponds to the oceanic region where both fluxes were similarly enhanced during that stage. To examine the roles of those fluxes in the rapid intensification of the cyclone, we performed three experiments in which sensible heat fluxes, latent heat fluxes, and both sensible and latent heat fluxes were excluded from the ocean area enclosed by the aforementioned green line during the integration period. For convenience, these experiments are hereafter referred to as runs with no sensible heat flux (NSH), no latent heat flux (NLH), and no sensible/latent heat fluxes (NSLH).
(a)–(c) Maps of surface turbulent sensible heat flux (shading) and SLP (contours) simulated by the CNTL run at 0000, 0900, and 1800 UTC 14 Jan 2013. The shading interval is 250 W m−2. The contour interval is 5 hPa. The ocean area enclosed by the green line corresponds to an area in which the surface heat fluxes were removed in the sensitivity experiments. (d)–(f) As in (a)–(c), but for surface turbulent latent heat flux (shading).
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
b. Validation of the control run
(a) Time series of the central pressure of an explosive cyclone derived from the CNTL run (red line) and JMA MSM data (green line). The time of the maximum deepening rate of the cyclone is indicated by a closed circle. (b) Tracks of the cyclone as identified by the CNTL run (red line) and MSM data (green line). The location of the cyclone at its maximum deepening rate is also shown by a circle.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
The reproducibility of the cyclone in the CNTL run was better than that in the H15 simulation using a high-resolution coupled atmosphere–ocean regional model, CReSS–Non Hydrostatic Ocean model for the Earth Simulator (NHOES) (Aiki et al. 2015). This is because the maximum deepening rate in the H15 simulation was 2.8 hPa h−1, which is lower than that in the CNTL run and the MSM. The design of the CNTL run was the same as that of the atmospheric model in H15 except for the initial time. The initial time of the simulation in the CNTL run was 18 h later than that in H15. Because the H15 simulation successfully reproduced the observed SST evolutions, we believe that the improved simulation in this study is attributable to the change in the initial time.
3. Impact of sensible heat and moisture supply from warm currents on an explosive cyclone
In this section, we investigate the cyclone’s sensitivity to the supply of sensible heat and moisture from the Kuroshio/Kuroshio Extension by comparing the CNTL run with the three flux sensitivity runs. Sections 3a and 3b highlight the cyclone’s intensity and the mesoscale structures around the cyclone center, respectively.
a. Intensity of the cyclone
Figure 3 shows a time series of the difference in the central pressure of the cyclone between each sensitivity run and the CNTL run. It turns out that the central pressure differences between the individual sensitivity runs and the CNTL run occurred from 0000 UTC 14 January when the cyclone’s center began to intrude into the area enclosed by the green line in Fig. 1. As the sensible heat and moisture supply from the Kuroshio/Kuroshio Extension became active in the CNTL run (Fig. 1), the differences in central pressure tended to increase between each sensitivity run and the CNTL run. At 1800 UTC 14 January, the central pressure difference of the NSH, NLH, and NSLH runs with respect to the CNTL run reached about 5, 12, and 15 hPa, respectively. After 1800 UTC 14 January, the central pressure differences were almost constant, although the difference decreased slightly after 2200 UTC 14 January in the NSLH run. The maximum deepening rates in the CNTL, NSH, NLH, and NSLH runs were 3.9, 3.4, 2.9, and 2.7 hPa h−1, respectively. These results imply that the sensitivity of the central pressure deepening owing to the sensible heat fluxes was smaller than that owing to the latent heat fluxes. However, it must be noted that the sensible heat supply contributed to the development of this cyclone. Previous studies also pointed out that sensible heat supply intensified extratropical cyclones (e.g., Kuo et al. 1991a; Langland et al. 1995; Booth et al. 2012).
Time series of the differences in the cyclone’s central pressure for the NSH run (blue line), NLH run (green line), and NSLH run (red line) with respect to the CNTL run. The differences are calculated by subtracting the central pressure in the CNTL run from that in each sensitivity run.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
We next compare the cyclone’s intensity among the four runs from the perspective of near-surface wind speed in the vicinity of the cyclone’s center. Figure 4 displays the spatial distribution of 10-m horizontal wind, its magnitude, and the SLP in each run at 1800 UTC 14 January, when the wind speed around the cyclone center peaked in the CNTL run. In the CNTL run, the near-surface wind and horizontal SLP gradient were enhanced on the western side of the cyclone center. The wind speed and SLP distribution around the cyclone center in the CNTL run were characterized by a zonally asymmetric structure. Although the asymmetric structure in the NSH run resembled that in the CNTL run, it was less clear in the NLH and NSLH runs. The maximum values of the 10-m wind speed in the western quadrant of the cyclone center are about 47, 44, 39, and 31 m s−1 in the CNTL, NSH, NLH, and NSLH runs, respectively. The distribution of the near-surface wind west and north of the cyclone’s center corresponds to the CCB, which indicates that the wind speed along the CCB in the NSH run was less intense than that in the CNTL run. In addition, the CCB was much weaker in the NLH and NSLH runs. The area of strong wind southeast of the cyclone’s center, which may be related to the sting jet (e.g., Browning 2004; Slater et al. 2015), was identified in the CNTL and NSH runs but is hardly seen in the NLH and NSLH runs. The differences in the central pressure of the cyclone and the near-surface wind distributions among each run may be attributed to differences in mesoscale latent heating over the bent-back front (H15; H16), which will be shown in the following subsection.
(a)–(d) Maps of 10-m horizontal wind (vectors), its magnitude (shading), and SLP (contours) at 1800 UTC 14 Jan 2013 in the CNTL run, NSH run, NLH run, and NSLH run. The reference arrow is 35 m s−1. Winds of less than 5 m s−1 are suppressed. The shading interval is 5 m s−1; the contour interval is 5 hPa.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
b. Mesoscale structures in the vicinity of the cyclone’s center
Figure 5 illustrates the horizontal distributions of SLP and the latent heating rate owing to condensation integrated from the surface to 100 hPa at 1400 UTC 14 January, when the differences in the central pressure of the cyclone between each sensitivity run and the CNTL run increased. The frontal positions, which are estimated from the axis of the maximum horizontal gradient of 950-hPa equivalent potential temperature 
(a)–(d) Maps of latent heating rate owing to condensation integrated from the surface to 100 hPa (shading) and SLP (contours) at 1400 UTC 14 Jan 2013 in the CNTL run, NSH run, NLH run, and NSLH run. Latent heating rates less than 1 × 104 W m−2 are suppressed. The contour interval is 5 hPa. The light blue circles are placed on latitude with the maximum horizontal gradient of 950-hPa equivalent potential temperature between 144° and 149°E, with an interval of 0.5° longitude.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
To further compare the mesoscale structures around the bent-back front among the four runs, we examine the vertical distributions of latent heating and vertical wind. The top panel of Fig. 6 shows the latitude–height cross-sectional maps of latent heating rate, horizontal moisture flux convergence, and meridional and vertical winds along the green lines in Figs. 5a–5d. These green lines cross the grid where the vertically integrated latent heating rate was maximum in the vicinity of the bent-back front in each run. The bottom panel of Fig. 6 is the same as the top panel except for the vertical wind and the horizontal wind convergence. In the CNTL run, the horizontal convergences of moisture flux and wind were dominant near the surface at the bent-back front, and the updraft accompanied by latent heating was evident around 700 hPa over these convergence areas (Figs. 6a,e). These mesoscale structures over the bent-back front in the NLH run were less clear than those in the CNTL run (Figs. 6c,g). Although differences were also noted in the latent heating and updraft over the bent-back front between the NSH and CNTL runs (Figs. 6b,f), they were smaller than those between the NLH and CNTL runs. The latent heating and updraft were weakest in the NSLH run (Figs. 6d,h).
(a)–(d) Latitude–height cross-sectional maps of latent heating rate owing to condensation (shading), the horizontal moisture flux convergence (contours), and meridional and vertical winds (vectors) at 1400 UTC 14 Jan 2013 along lines A–A′, B–B′, C–C′, and D–D′, as illustrated in Fig. 5. Latent heating rates less than 10 K h−1 are suppressed. The contoured interval is 5 × 10−6 kg kg−1 s−1. Convergence of less than 5 × 10−6 kg kg−1 s−1 is suppressed. The reference arrows for the meridional and vertical winds are 40 and 1 m s−1, respectively. Meridional winds of less than 25 m s−1 are suppressed. (e)–(h) As in (a)–(d), but for vertical velocity (shading) and the convergence of horizontal wind (contours). Vertical velocity less than 1 m s−1 is suppressed. The contour interval is 5 × 10−4 s−1. Convergence of less than 5 × 10−4 s−1 is suppressed.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
Comparisons between the NLH and CNTL runs revealed that the moisture supply from the warm currents reinforces the latent heating over the bent-back front and facilitates the cyclone’s development, which is quite consistent with the results of H15. Moreover, the cyclone’s intensity and the associated latent heating over the bent-back front in the NSH run became weaker than those in the CNTL run. These results strongly suggest that the sensible heat supply from the warm currents plays additional roles in the CCB–LH feedback process.
4. Role of sensible heat supply in increasing latent heating over the bent-back front
The results of the previous section suggest that the surface sensible heat flux in the Kuroshio/Kuroshio Extension may have caused an increase in latent heating over the bent-back front, thus facilitating the cyclone’s development. To understand the active roles of the sensible heat supply in the CCB–LH feedback process, we further examine the influence of the sensible heat supply from the warm currents on static stability and water vapor content in the atmospheric boundary layer around the cyclone center in sections 4a and 4b, respectively.
a. Influence of sensible heat supply on static stability
Figure 7 shows the spatial distributions of 950-hPa 
(a)–(d) Maps of 950-hPa equivalent potential temperature (shading), its horizontal gradient (red contours), and SLP (gray contours) at 1400 UTC 14 Jan 2013 in the CNTL run, NSH run, NLH run, and NSLH run. The shading interval is 10 K, and the contour interval for the gradient is 0.2 K km−1. Gradients less than 0.2 K km−1 are suppressed. The contour interval for the SLP is 10 hPa. The closed circles are placed 1° north of the latitude with the maximum horizontal gradient of 950-hPa equivalent potential temperature between 145° and 149°E, with an interval of 1° longitude.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
Considering the near-surface 
(a)–(d) Vertical profiles of equivalent potential temperature at the locations of closed circles illustrated in Figs. 7a–d. The colors of the lines correspond to those of the circles plotted in Fig. 7.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
Figure 9 shows the spatial distributions of the 
(a)–(d) Maps of the difference in equivalent potential temperature between the 875- and 950-hPa levels (the former minus the later; shading), 950-hPa horizontal wind (vectors), and SLP (contours) at 1400 UTC 14 Jan 2013 in the CNTL run, NSH run, NLH run, and NSLH run. The shading interval is 1 K. The reference arrow is 35 m s−1. Winds of less than 5 m s−1 are suppressed. The contour interval is 5 hPa.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
At the end of this subsection, we examine the thermal structures and Petterssen (1936) frontogenesis near the surface around the bent-back front to understand how the convective instability around the CCB is released. Since convectively unstable areas were simulated only around the CCB in the CNTL and NSH runs as seen in Fig. 9, we focus only on these two runs. The left panel of Fig. 10 shows the horizontal gradient of 950-hPa potential temperature 
(a) Maps of the horizontal gradient of 950-hPa potential temperature (shading), 950-hPa horizontal wind (vectors), and SLP (contours) at 1400 UTC 14 Jan 2013 in the CNTL run. The shading interval is 1.0 × 10−1 K km−1. The reference arrow is 40 m s−1. The contour interval is 5 hPa. (b) Longitude–height cross-sectional maps of potential temperature (shading) and zonal and vertical winds (vectors) along the blue line illustrated in (a). The shading interval is 4 K. The reference arrows for the zonal and vertical winds are 40 m s−1 and 1 m s−1, respectively. (c) As in (a), but for the 950-hPa Petterssen frontogenesis (shading). The unit of frontogenesis is 10−5 K km−1 s−1. (d)–(f) As in (a)–(c), but for the NSH run.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
b. Influence of sensible heat supply on water vapor content
The sensible heat supply from the Kuroshio/Kuroshio Extension may also affect the distribution of the water vapor mixing ratio 
Figure 11a shows the trajectory of the air parcel along with the SLP at 1400 UTC 14 January in the CNTL run. The top panel of Fig. 11b displays the temporal changes in 
(a) Trajectory of an air parcel calculated by a backward trajectory analysis using data derived from the CNTL run. The colored line denotes the height (m) of the air parcel. The SLP distribution (contours) in the CNTL run at 1400 UTC 14 Jan 2013 is also represented. The contoured interval is 5 hPa. (b) Temporal changes in the properties of the air parcel. (top) The saturation water vapor mixing ratio (shaded light green), the water vapor mixing ratio (blue line), and the potential temperature (red line). (bottom) The surface sensible (pink line) and latent (orange line) heat fluxes of the underlying ocean. Symbols A–E correspond to the locations of the parcel shown in (a).
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
To further examine the influence of the sensible heat supply on the distribution of the 
(a),(b) Maps of 950-hPa temperature (shading) and SLP (contours) at 1400 UTC 14 Jan 2013 in the CNTL run and NSH run. The shading interval is 4 K, and the contour interval is 5 hPa. (c) Difference map of 950-hPa temperature (shading) between the NSH run and the CNTL run (the former minus the latter). The SLP distributions (contours) of the CNTL runs are also shown. The shading interval is 3 K, and the contour interval is 5 hPa. (d)–(f) As in (a)–(c), but for the 950-hPa saturation water vapor mixing ratio. The shading intervals are (d),(e) 2 g kg−1and (f) 1 g kg−1. (g)–(i) As in (d)–(f), but for the 950-hPa water vapor mixing ratio.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
Another remarkable difference between the two runs is evident in the distribution of cloud water in the near-surface layer. The top panel of Fig. 13 displays the spatial pattern of the cloud water mixing ratio at the 950-hPa level in the CNTL and NSH runs. The bottom panel shows the vertical distributions of the cloud water mixing ratio and potential temperature along the red lines in Figs. 13a and 13b. The cloud water hardly existed near the surface in the CNTL run (Figs. 13a,c). In contrast, a significant amount of cloud water was apparent at the near-surface layer around the CCB in the NSH run (Figs. 13b,d). As seen in Figs. 12d–f, the 
(a),(b) Maps of 950-hPa cloud water mixing ratio (shading) and SLP (contours) at 1400 UTC 14 Jan 2013 in the CNTL run and NSH run. The shading interval is 0.05 g kg−1, and the contour interval is 5 hPa. (c),(d) Latitude–height cross-sectional maps of cloud water mixing ratio (shading) and potential temperature (contours) at 1400 UTC 14 Jan 2013 along the red lines illustrated in (a),(b). The contoured interval is 3 K.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
5. Discussion
a. Revision of CCB–LH feedback process
H15 proposed that the CCB–LH feedback process is a dynamic process of the explosive development of an extratropical cyclone over the Kuroshio/Kuroshio Extension, as illustrated in their Fig. 18. In this feedback process, the vapor evaporating from the warm currents enhances the mesoscale latent heating over the bent-back front with the aid of the CCB, leading to the rapid intensification of a cyclone. As shown in section 3, the deepening of the central pressure of the cyclone (Fig. 3) and the magnitude of the near-surface wind along the CCB (Fig. 4) were suppressed in the NLH run compared with those in the CNTL run. Since the latent heating over the bent-back front in the NLH run was also reduced in comparison with that in the CNTL run (Figs. 5 and 6), it seems that this reduction in the heating resulted in the inhibition of the cyclone’s development in the NLH run. These observations increase the reliability of the CCB–LH feedback process proposed by H15.
Comparisons between the CNTL and NSH runs demonstrate that the cyclone’s intensity (Figs. 3 and 4) and the associated mesoscale updraft and latent heating over the bent-back front (Figs. 5 and 6) are more attenuated in the latter than in the former. Such differences also appeared between the NLH and NSLH runs (Figs. 3, 4, 5, and 6). It should be noted that the sensible heat supply from the warm currents caused the increase in latent heating over the bent-back front, which eventually accelerated the cyclone’s growth. Thus, it is probable that sensible heat supply plays additional roles in the CCB–LH feedback process as will be discussed in detail in the next paragraph.
We discuss the specific roles of the sensible heat supply from the warm currents in the CCB–LH feedback process and revise this feedback process as indicated in Fig. 14. The sensible heating by the warm currents facilitated convectively unstable conditions within the atmospheric boundary layer along the CCB (Figs. 7, 8, and 9). The intensified convective instability of the boundary layer was released by the frontogenesis-forced updraft on the western edge of the bent-back front (Fig. 10), reinforcing the ascending motion and the associated latent heat release. As another discernible role, the sensible heat supply from the warm currents increased the temperature and saturation mixing ratio of the near-surface saturated layer around the CCB, promoting an increase in its water vapor mixing ratio (Figs. 11 and 12). On the other hand, the sensible heat supply decreased the cloud water mixing ratio at the near-surface layer (Fig. 13). The increase in water vapor provided favorable conditions for the moisture to penetrate the bent-back front via the CCB, resulting in the enhanced latent heating. We thus conclude that the sensible heat supply from the warm currents plays additional roles in the CCB–LH feedback process through the two aforementioned effects, as shown in Fig. 14.
Schematic diagram representing the revised CCB–LH feedback process in relation to the explosive development of an extratropical cyclone over the Kuroshio/Kuroshio Extension. Dashed arrows denote the original version of the CCB–LH feedback process proposed by H15. Solid arrows account for the additional roles of sensible heat supply from the warm currents highlighted in this paper.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0063.1
It should be noted that the roles of the WCB are also crucial to extratropical cyclogenesis (e.g., Booth et al. 2012; Schemm et al. 2013; Schemm and Wernli 2014; Binder et al. 2016). However, this study highlighted the CCB’s roles because no systematic differences were found in the distributions of equivalent potential temperature and water vapor near the surface around the WCB among each run (Figs. 7 and 12). Since the surface heat fluxes around the CCB were essentially removed in the sensitivity runs (Fig. 1), those under the cyclone’s warm sector, which modify the air transported by the WCB (Booth et al. 2012), are almost the same in the four runs. Thus, the differences in the properties of the near-surface air around the WCB would not have been significant among the four runs.
b. Positive and negative effects of surface heat fluxes on the cyclone’s development
In the previous sections, we discussed the positive effects of surface heat fluxes on the cyclone’s development. However, these fluxes often have a negative impact on cyclogenesis, as shown in Table 2 of Reed et al. (1993). Previous studies have reported that surface heat fluxes exert a negative influence on extratropical cyclones developing over ocean areas north of warm currents (Kuo et al. 1991a; Reed and Simmons 1991; Heo et al. 2015). Kuo et al. (1991a) showed that when an explosive cyclone moves over the colder SST to the north of the Gulf Stream, downward surface sensible heat fluxes prevail within the warm sector and near the cyclone’s center, whereas upward fluxes occur behind the cold front and well ahead of the cyclone. They inferred that the vertical motion forced by such surface sensible heat fluxes is unfavorable for a cyclone’s intensification. In contrast, Reed et al. (1993) demonstrated that surface heat fluxes facilitate the growth of an explosive cyclone moving near and parallel to the Gulf Stream. It is noteworthy that the track of the cyclone highlighted in this study resembles that discussed in Reed et al. (1993) in terms of the geographical relationship between the cyclone and the currents.
The results of the previous studies and the present study suggest that the positive impact of the surface heat fluxes becomes evident when extratropical cyclones develop near warm currents, whereas a negative impact is induced when the cyclones grow over ocean areas to the north of the currents. The revised CCB–LH feedback process may account for such a discrepancy. According to the findings of our sequential studies, when the CCB of extratropical cyclones overlaps with the warm currents, the increased surface heat fluxes are expected to promote a cyclone’s intensification via the CCB–LH feedback process. The contrasting effects of the surface heat fluxes on the cyclone development may depend on the sensitivity of such an overlap, as suggested by H16. Further examinations of the significant influence of surface heat fluxes on other extratropical cyclone evolutions are required.
6. Summary
We examined additional roles of the sensible heat supply from the Kuroshio/Kuroshio Extension in the CCB–LH feedback process, which has been proposed as a dynamic process of the rapid development of an extratropical cyclone over warm currents (H15). We performed a control experiment (CNTL run) and three sensitivity experiments (NSH, NLH, and NSLH runs) with respect to surface turbulent heat fluxes from the ocean by using a regional cloud-resolving model. Sensible heat fluxes, latent heat fluxes, and both sensible and latent heat fluxes from the warm currents were excluded in the NSH, NLH, and NSLH runs, respectively. The major findings of this study are summarized in the following points:
The deepening of the central pressure of the cyclone, the CCB’s development, and the associated latent heating over the bent-back front are inhibited in the NLH run compared with those in the CNTL run. These differences between the two runs support the validity of the CCB–LH feedback process.
The cyclone’s intensity and the associated mesoscale updraft and latent heating over the bent-back front were weaker in the NSH run than in the CNTL run. Such differences were also evident between the NSLH and NLH runs. The sensible heat supply from the warm currents played significant roles in the CCB–LH feedback process.
The sensible heat supply from the warm currents reinforced the convectively unstable conditions within the atmospheric boundary layer around the CCB. The enhanced convective instability was released by the updraft associated with the frontogenesis on the western edge of the bent-back front. Consequently, the ascending motion and the associated latent heating over the bent-back front became activated. In addition, the sensible heat supply led to an increase in the temperature and saturation mixing ratio of the near-surface saturated air along the CCB, facilitating an increase in its water vapor content. This increase in water vapor contributed to an increase in the moisture penetrating the bent-back front via the CCB, which strengthened the latent heating. These two effects of sensible heat supply played a secondary role in the CCB–LH feedback process.
A series of studies including H15, H16, and the present study has revealed that the revised CCB–LH feedback process can be a crucial factor for the rapid development of an extratropical cyclone over the Kuroshio/Kuroshio Extension. However, it is uncertain whether this feedback process is applicable in all cases and in other regions that are characterized by similarly warm ocean currents. Thus, numerical simulations are under way with respect to extratropical cyclones developing rapidly over the Gulf Stream. The results of the new research will be reported in a separate paper.
Acknowledgments
The authors wish to thank Akira Kuwano-Yoshida, Masaru Yamamoto, and Tetsuya Kawano for offering many helpful suggestions, and Satoki Tsujino for providing technical support. We thank Editor Ron McTaggart-Cowan for his kind support and valuable comments, and the two anonymous reviewers for their extremely helpful comments. The CReSS model was developed by the Institute for Space–Earth Environmental Research (ISEE), Nagoya University. The JMA GSM and MSM data are available at the website of the Research Institute for Sustainable Humanosphere (RISH), Kyoto University (http://database.rish.kyoto-u.ac.jp/index-e.html). The JCOPE-2 data were provided by JAMSTEC (http://www.jamstec.go.jp/jcope/htdocs/e/distribution/index.html). The computation was performed mainly by using the computer facilities at Research Institute for Information Technology, Kyushu University. This work was conducted by the joint research program of the ISEE and was supported by JSPS KAKENHI 17J04041, 14J04241, and 16H01846.
REFERENCES
Ahmadi-Givi, F., G. C. Craig, and R. S. Plant, 2004: The dynamics of a mid-latitude cyclone with very strong latent heat release. Quart. J. Roy. Meteor. Soc., 130, 295–323, https://doi.org/10.1256/qj.02.226.
Aiki, H., M. Yoshioka, M. Kato, A. Morimoto, T. Shinoda, and K. Tsuboki, 2015: A coupled atmosphere-ocean-surface-wave modeling system for understanding air-sea interactions under tropical cyclone conditions. Bull. Coast. Oceanogr., 52, 139–148.
Binder, H., M. Boettcher, H. Joos, and H. Wernli, 2016: The role of warm conveyor belts for the intensification of extratropical cyclones in Northern Hemisphere winter. J. Atmos. Sci., 73, 3997–4020, https://doi.org/10.1175/JAS-D-15-0302.1.
Booth, J. F., L. Thompson, J. Patoux, and K. A. Kelly, 2012: Sensitivity of midlatitude storm intensification to perturbations in the sea surface temperature near the Gulf Stream. Mon. Wea. Rev., 140, 1241–1256, https://doi.org/10.1175/MWR-D-11-00195.1.
Browning, K. A., 2004: The sting at the end of the tail: Damaging winds associated with extratropical cyclones. Quart. J. Roy. Meteor. Soc., 130, 375–399, https://doi.org/10.1256/qj.02.143.
Browning, K. A., and N. M. Roberts, 1994: Structure of a frontal cyclone. Quart. J. Roy. Meteor. Soc., 120, 1535–1557, https://doi.org/10.1002/qj.49712052006.
Carlson, T. N., 1980: Airflow through midlatitude cyclones and the comma cloud pattern. Mon. Wea. Rev., 108, 1498–1509, https://doi.org/10.1175/1520-0493(1980)108<1498:ATMCAT>2.0.CO;2.
Chen, S.-J., Y.-H. Kuo, P.-Z. Zhang, and Q.-F. Bai, 1992: Climatology of explosive cyclones off the East Asian coast. Mon. Wea. Rev., 120, 3029–3035, https://doi.org/10.1175/1520-0493(1992)120<3029:COECOT>2.0.CO;2.
Davis, C. A., and K. A. Emanuel, 1988: Observational evidence for the influence of surface heat fluxes on rapid maritime cyclogenesis. Mon. Wea. Rev., 116, 2649–2659, https://doi.org/10.1175/1520-0493(1988)116<2649:OEFTIO>2.0.CO;2.
Davis, C. A., M. T. Stoelinga, and Y.-H. Kuo, 1993: The integrated effect of condensation in numerical simulations of extratropical cyclogenesis. Mon. Wea. Rev., 121, 2309–2330, https://doi.org/10.1175/1520-0493(1993)121<2309:TIEOCI>2.0.CO;2.
Gyakum, J. R., and R. E. Danielson, 2000: Analysis of meteorological precursors to ordinary and explosive cyclogenesis in the western North Pacific. Mon. Wea. Rev., 128, 851–863, https://doi.org/10.1175/1520-0493(2000)128<0851:AOMPTO>2.0.CO;2.
Gyakum, J. R., J. R. Anderson, R. H. Grumm, and E. L. Gruner, 1989: North Pacific cold-season surface cyclone activity: 1975–1983. Mon. Wea. Rev., 117, 1141–1155, https://doi.org/10.1175/1520-0493(1989)117<1141:NPCSSC>2.0.CO;2.
Heo, K.-Y., Y.-W. Seo, K.-J. Ha, K.-S. Park, J. Kima, J.-W. Choia, K. Juna, and J.-Y. Jeonga, 2015: Development mechanisms of an explosive cyclone over East Sea on 3–4 April 2012. Dyn. Atmos. Oceans, 70, 30–46, https://doi.org/10.1016/j.dynatmoce.2015.03.001.
Hirata, H., R. Kawamura, M. Kato, and T. Shinoda, 2015: Influential role of moisture supply from the Kuroshio/Kuroshio Extension in the rapid development of an extratropical cyclone. Mon. Wea. Rev., 143, 4126–4144, https://doi.org/10.1175/MWR-D-15-0016.1.
Hirata, H., R. Kawamura, M. Kato, and T. Shinoda, 2016: Response of rapidly developing extratropical cyclones to sea surface temperature variations over the western Kuroshio–Oyashio confluence region. J. Geophys. Res. Atmos., 121, 3843–3858, https://doi.org/10.1002/2015JD024391.
Hoskins, B. J., and P. J. Valdes, 1990: On the existence of storm-tracks. J. Atmos. Sci., 47, 1854–1864, https://doi.org/10.1175/1520-0469(1990)047<1854:OTEOST>2.0.CO;2.
Hoskins, B. J., M. E. McIntyre, and A. W. Robertson, 1985: On the use and significance of isentropic potential vorticity maps. Quart. J. Roy. Meteor. Soc., 111, 877–946, https://doi.org/10.1002/qj.49711147002.
Hotta, D., and H. Nakamura, 2011: On the significance of sensible heat supply from the ocean in the maintenance of mean baroclinicity along storm tracks. J. Climate, 24, 3377–3401, https://doi.org/10.1175/2010JCLI3910.1.
Iizuka, S., M. Shiota, R. Kawamura, and H. Hatsushika, 2013: Influence of the monsoon variability and sea surface temperature front on the explosive cyclone activity in the vicinity of Japan during northern winter. SOLA, 9, 1–4, https://doi.org/10.2151/sola.2013-001.
Japan Meteorological Agency, 2013: Outline of the operational numerical weather prediction at the Japan Meteorological Agency (March 2013). Japan Meteorological Agency, accessed 21 November 2017, http://www.jma.go.jp/jma/jma-eng/jma-center/nwp/outline2013-nwp/index.htm.
Kuo, Y.-H., R. J. Reed, and S. Low-Nam, 1991a: Effects of surface energy fluxes during the early development and rapid intensification stages of seven explosive cyclones in the western Atlantic. Mon. Wea. Rev., 119, 457–476, https://doi.org/10.1175/1520-0493(1991)119<0457:EOSEFD>2.0.CO;2.
Kuo, Y.-H., M. A. Shapiro, and E. G. Donall, 1991b: The interaction between baroclinic and diabatic processes in a numerical simulation of rapidly intensifying extratropical marine cyclone. Mon. Wea. Rev., 119, 368–384, https://doi.org/10.1175/1520-0493(1991)119<0368:TIBBAD>2.0.CO;2.
Kuwano-Yoshida, A., and Y. Asuma, 2008: Numerical study of explosively developing extratropical cyclones in the northwestern Pacific region. Mon. Wea. Rev., 136, 712–740, https://doi.org/10.1175/2007MWR2111.1.
Kuwano-Yoshida, A., and S. Minobe, 2017: Storm-track response to SST fronts in the Northwestern Pacific region in an AGCM. J. Climate, 30, 1081–1102, https://doi.org/10.1175/JCLI-D-16-0331.1.
Lackmann, G., 2011: Midlatitude Synoptic Meteorology: Dynamics, Analysis and Forecasting. Amer. Meteor. Soc., 345 pp.
Langland, R. H., R. L. Elsberry, and R. M. Errico, 1995: Evaluation of physical processes in an idealized extratropical cyclone using adjoint sensitivity. Quart. J. Roy. Meteor. Soc., 121, 1349–1386, https://doi.org/10.1002/qj.49712152608.
Lim, E.-P., and I. Simmonds, 2002: Explosive cyclone development in the Southern Hemisphere and a comparison with Northern Hemisphere events. Mon. Wea. Rev., 130, 2188–2209, https://doi.org/10.1175/1520-0493(2002)130<2188:ECDITS>2.0.CO;2.
Martin, J. E., 2006: Mid-Latitude Atmospheric Dynamics: A First Course. John Wiley and Sons, Ltd., 324 pp.
Miyazawa, Y., and Coauthors, 2009: Water mass variability in the western North Pacific detected in a 15-year eddy resolving ocean reanalysis. J. Oceanogr., 65, 737–756, https://doi.org/10.1007/s10872-009-0063-3.
Neiman, P. J., and M. A. Shapiro, 1993: The life cycle of an extratropical marine cyclone. Part I: Frontal-cyclone evolution and thermodynamic air–sea interaction. Mon. Wea. Rev., 121, 2153–2176, https://doi.org/10.1175/1520-0493(1993)121<2153:TLCOAE>2.0.CO;2.
Nuss, W. A., and S. I. Kamikawa, 1990: Dynamics and boundary layer processes in two Asian cyclones. Mon. Wea. Rev., 118, 755–771, https://doi.org/10.1175/1520-0493(1990)118<0755:DABLPI>2.0.CO;2.
Petterssen, S., 1936: Contribution to the theory of frontogenesis. Geofys. Publ., 11 (6), 1–27.
Reed, R. J., and A. J. Simmons, 1991: Numerical simulation of an explosively deepening cyclone over the North Atlantic that was unaffected by concurrent surface energy fluxes. Wea. Forecasting, 6, 117–122, https://doi.org/10.1175/1520-0434(1991)006<0117:NSOAED>2.0.CO;2.
Reed, R. J., M. T. Stoelinga, and Y.-H. Kuo, 1992: A model-aided study of the origin and evolution of the anomalously high potential vorticity in the inner region of a rapidly deepening marine cyclone. Mon. Wea. Rev., 120, 893–913, https://doi.org/10.1175/1520-0493(1992)120<0893:AMASOT>2.0.CO;2.
Reed, R. J., G. Grell, and Y.-H. Kuo, 1993: The ERICA IOP 5 storm. Part II: Sensitivity tests and further diagnosis based on model output. Mon. Wea. Rev., 121, 1595–1612, https://doi.org/10.1175/1520-0493(1993)121<1595:TEISPI>2.0.CO;2.
Sanders, F., and J. R. Gyakum, 1980: Synoptic-dynamic climatology of the “bomb.” Mon. Wea. Rev., 108, 1589–1606, https://doi.org/10.1175/1520-0493(1980)108<1589:SDCOT>2.0.CO;2.
Schemm, S., and H. Wernli, 2014: The linkage between the warm and cold conveyor belts in an idealized extratropical cyclone. J. Atmos. Sci., 71, 1443–1459, https://doi.org/10.1175/JAS-D-13-0177.1.
Schemm, S., H. Wernli, and L. Papritz, 2013: Warm conveyor belts in idealized moist baroclinic wave simulations. J. Atmos. Sci., 70, 627–652, https://doi.org/10.1175/JAS-D-12-0147.1.
Schultz, D. M., 2001: Reexamining the cold conveyor belt. Mon. Wea. Rev., 129, 2205–2225, https://doi.org/10.1175/1520-0493(2001)129<2205:RTCCB>2.0.CO;2.
Seiler, C., and F. W. Zwiers, 2016: How well do CMIP5 climate models reproduce explosive cyclones in the extra tropics of the Northern Hemisphere. Climate Dyn., 46, 1241–1256, https://doi.org/10.1007/s00382-015-2642-x.
Shapiro, M., and Coauthors, 1999: A planetary-scale to mesoscale perspective of the life cycles of extratropical cyclones: The bridge between theory and observations. The Life Cycles of Extratropical Cyclones, M. Shapiro and S. Gronas, Eds., Amer. Meteor. Soc., 139–186.
Slater, T. P., D. M. Schultz, and G. M. Vaughan, 2015: Acceleration of near-surface strong winds in a dry, idealized extratropical cyclone. Quart. J. Roy. Meteor. Soc., 141, 1004–1016, https://doi.org/10.1002/qj.2417.
Takayabu, I., 1991: “Coupling development”: An efficient mechanism for the development of extratropical cyclones. J. Meteor. Soc. Japan, 69, 609–628, https://doi.org/10.2151/jmsj1965.69.6_609.
Tsuboki, K., 2008: High-resolution simulations of high-impact weather systems using the cloud-resolving model on the Earth Simulator. High Resolution Numerical Modeling of the Atmosphere and Ocean, K. Hamilton and W. Ohfuchi, Eds., Springer, 141–156.
Tsuboki, K., and A. Sakakibara, 2007: Numerical Prediction of High-Impact Weather Systems—The Textbook for Seventeenth IHP Training Course in 2007. HyARC, Nagoya University, Japan, and UNESCO, 273 pp.
Wang, C.-C., and J. C. Rogers, 2001: A composite study of explosive cyclogenesis in different sectors of the North Atlantic. Part I: Cyclone structure and evolution. Mon. Wea. Rev., 129, 1481–1499, https://doi.org/10.1175/1520-0493(2001)129<1481:ACSOEC>2.0.CO;2.
Whitaker, J. S., L. W. Uccellini, and K. F. Brill, 1988: A model-based diagnostic study of the rapid development phase of the Presidents’s Day cyclone. Mon. Wea. Rev., 116, 2337–2365, https://doi.org/10.1175/1520-0493(1988)116<2337:AMBDSO>2.0.CO;2.
Yoshida, A., and Y. Asuma, 2004: Structures and environment of explosively developing extratropical cyclones in the northwestern Pacific region. Mon. Wea. Rev., 132, 1121–1142, https://doi.org/10.1175/1520-0493(2004)132<1121:SAEOED>2.0.CO;2.
Yoshiike, S., and R. Kawamura, 2009: Influence of wintertime large-scale circulation on the explosively developing cyclones over the western North Pacific and their downstream effects. J. Geophys. Res., 114, D13110, https://doi.org/10.1029/2009JD011820.
