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  • View in gallery

    Scatter diagrams for the Eady index estimated for 300- and 1000-hPa horizontal winds and potential temperatures averaged over an area within a 1000-km radius from the cyclone center when observing the maximum deepening rate vs maximum deepening rate for (a) OJ, (b) PO–L, and (c) PO–O cyclones. Boldface lines are regression lines corresponding to equations for the correlation coefficient (r) given in the upper-left panels. Squares show the extreme cases, and triangles show the standard cases.

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    (left) Surface cyclone tracks (solid lines, crosses represent 6-h interval positions, closed circles show the position at every 0000 UTC), and (right) time series of central sea level pressure between the formation (triangle) and the disappearance (square) from the GANAL dataset; shown are the extreme (a),(b) OJ, (c),(d) PO–L, and (e),(f) PO–O cases. Stars show the minimum sea level pressure. Arrows show the maximum deepening rate. Broken lines show the results of the CNTL runs, and dotted lines show results of the DRY runs.

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    Synoptic weather chart for the extreme OJ case. (left) Sea level pressure (solid line; contour intervals are 4 hPa), potential temperature at 850 hPa (broken line; contour intervals are 2 K), and horizontal gradient of potential temperature at 850 hPa (shading). Here, L shows the surface cyclone center. (right) The geopotential height (solid line; contour intervals are 120 m), potential vorticity (broken line; contour intervals are 1 PVU = 10−6 m−2 s−1 K kg−1), and wind speed (shading) at 300 hPa at (a), (b) 1200 UTC 26 Feb, (c), (d) 1200 UTC 27 Feb, and (e), (f) 1200 UTC 28 Feb 1999.

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    As in Fig. 3, but for the extreme PO–L case: (a), (b) 1800 UTC 9 Feb, (c), (d) at 1800 UTC 10 Feb, and (e), (f) 1800 UTC 11 Feb 1998.

  • View in gallery

    As in Fig. 3, but for the extreme PO–O case: (a), (b) 0000 UTC 30 Dec 1997, (c), (d) 0000 UTC 31 Dec 1997, and (e), (f) 0000 UTC 1 Jan 1998.

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    (left) Sea level pressure (thin solid line; contour intervals are 4 hPa), potential temperature at 850 hPa (broken line; contour intervals are 4 K), vertically integrated rainwater (thick solid line; 0.2 and 2 mm are contoured), and precipitable water (shading). (right) Geopotential height (solid line; contour intervals are 120 m), potential vorticity anomaly from 2-day averages (shading), and horizontal wind speed (boldface line; contour intervals are 10 m s−1 and are drawn for speeds >50 m s−1) at 300 hPa at (a), (b) T = 12 h, (c), (d) T = 24 h, and (e), (f) T = 36 h of the extreme OJ CNTL run. Here, L is the position of the surface cyclone center, and the A–A′ line is the cross section in Fig. 12a.

  • View in gallery

    As in Fig. 6, but for the extreme PO–L CNTL run.

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    As in Fig. 6, but for the extreme PO–O CNTL run.

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    As in Fig. 6, but for the extreme OJ DRY run.

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    Schematic illustration of the initial arrangement of air parcels for the backward trajectory analysis. The L represents the surface cyclone center at the maximum deepening rate.

  • View in gallery

    Major trajectories for the extreme (a) OJ CNTL, (b) OJ DRY, (c) PO–L CNTL, (d) PO–L DRY, (e) PO–O CNTL, and (f) PO–O DRY runs. Sea level pressure (solid line; contour intervals are 4 hPa) and precipitable water (shading) at T = 24 h are shown at the bottom of each cube. The colors of the trajectories indicate four groups by Ward’s clustering method (see text for details).

  • View in gallery

    Vertical cross section of the projected trajectories rising near the surface cyclone center (thin line, corresponding to blue lines in Fig. 11), the potential vorticity anomaly from 2-day averages (shading), the potential temperature (medium line; contour intervals are 8 K), and the wind speed (thick line; contour intervals are 10 m s−1 and are drawn for speeds >50 m s−1) at T = 24 h for the extreme (a) OJ CNTL along A–A′ in Fig. 6, (b) OJ DRY along B–B′ in Fig. 9, (c) PO–L CNTL along C–C′ in Fig. 7, (d) PO–L DRY along D–D′ in Fig. 13, (e) PO–O CNTL along E–E′ in Fig. 8, and (e) PO–O DRY along F–F′ in Fig. 14.

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    As in Fig. 6, but for the extreme PO–L DRY run.

  • View in gallery

    As in Fig. 6, but for the extreme PO–O DRY.

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    Upper potential vorticity anomaly averaged between 200 and 500 hPa from 2-day averages (shading) and vertically integrated rainwater (solid line; mm) at T = 24 h for (a) the extreme OJ CNTL, (b) the standard OJ CNTL, (c) the extreme PO–L CNTL, (d) the standard PO–L CNTL, (e) the extreme PO–O CNTL, and (f) the standard PO–O CNTL runs.

  • View in gallery

    Sea level pressure (thin solid line; contour intervals are 4 hPa), potential temperature at 850 hPa (broken line; contour intervals are 4 K), vertically integrated rainwater (thick solid line; 0.2 and 2 mm are contoured), and precipitable water (shading) for (a) the standard OJ CNTL, (b) the standard OJ DRY, (c) the standard PO–L CNTL, (d) the standard PO–L DRY, (e) the standard PO–O CNTL, and (f) the standard PO–O DRY runs at T = 24 h.

  • View in gallery

    Geopotential height (solid line; contour intervals are 120 m), potential vorticity anomaly from 2-day averages (shading), and horizontal wind speed (thick line; contour intervals are 10 m s−1 and are drawn for speeds >50 m s−1) at 300 hPa for (a) the standard OJ CNTL, (b) the standard OJ DRY, (c) the standard PO–L CNTL, (d) the standard PO–L DRY, (e) the standard PO–O CNTL, and (f) the standard PO–O DRY runs at T = 24 h.

  • View in gallery

    The 48-h backward trajectories of air parcels from the vertically integrated vapor flux convergence maximum points at 500, 600, 700, 850, and 925 hPa [circles; colors shows altitudes of air parcels (hPa)] at the time of maximum deepening, and 48 h before and after that. Precipitable water amount averaged over 96 h (contours) and precipitation amount from GPCP composited near the cyclone during its lifetime (shading). White line and open circles show cyclone track (star, maximum deepening; triangle, 48 h before maximum deepening; square, 48 h after maximum deepening) for the extreme (a) OJ, (b) PO–L, and (c) PO–O cases.

  • View in gallery

    Vertically integrated vapor flux (arrows) and PE (color shading) composited near the cyclone during its lifetime. Cyclone tracks are the same as in Fig. 18: the extreme (a) OJ, (b) PO–L, and (c) PO–O cases.

  • View in gallery

    Fig. A1. (top) Sea level pressure (thin solid line; contour intervals are 4 hPa), potential temperature at 850 hPa (broken line; contour intervals are 4 K), vertically integrated rainwater (thick solid line; 0.2 and 2 mm are contoured), and precipitable water (shading). (bottom) Geopotential height (solid line; contour intervals are 120 m), potential vorticity anomaly from 2-day averages (shading), and horizontal wind speed (thick line; contour intervals are 10 m s−1 and are drawn for speeds >50 m s−1) at 300 hPa for (a), (d) CNTL, (b), (e) 15-km grid without cumulus parameterization, and (c), (f) 5-km grid without cumulus parameterization at T = 24 h. Here, L is the position of the surface cyclone center.

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Numerical Study of Explosively Developing Extratropical Cyclones in the Northwestern Pacific Region

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  • 1 Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan
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Abstract

Numerical simulations of six explosively developing extratropical cyclones in the northwestern Pacific Ocean region are conducted using a regional mesoscale numerical model [the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5)]. Cyclones are categorized according to the locations where they form and develop: Okhotsk–Japan Sea (OJ) cyclones originate over the eastern Asian continent and develop over the Sea of Japan or the Sea of Okhotsk, Pacific Ocean–land (PO–L) cyclones also form over the Asian continent and develop over the northwestern Pacific Ocean, and Pacific Ocean–ocean (PO–O) cyclones form and develop over the northwestern Pacific Ocean. Two cases (the most extreme and normal deepening rate cases for each cyclone type) are selected and simulated. Simulations show that the extreme cyclone of each type is characterized by a different mesoscale structure and evolutionary path, which strongly reflect the larger-scale environment: an OJ cyclone has the smallest deepening rates, associated with a distinct upper-level shortwave trough, a clear lower-level cold front, and a precipitation area that is far from the cyclone center; a PO–L cyclone has moderate deepening rates with high propagation speeds under zonally stretched upper-level jets; and a PO–O cyclone has the strongest deepening rates associated with large amounts of precipitation near its center. Sensitivity experiments involving the latent heat release associated with water vapor condensation show that PO–O cyclones rarely develop without a release of latent heat and their structures are drastically different from the control runs, while OJ cyclones exhibit almost the same developments and have similar structures to the control runs. These tendencies can be seen in both extreme and normal deepening rate cases. These results reveal that the importance of latent heat release to explosive cyclone development varies among the cyclone types, as is reflected by the cyclone origin, frontal structure, moisture distribution, and jet stream configuration.

* Current affiliation: Earth Simulator Center, Japan Agency for Marine–Earth Science and Technology, Yokohama, Japan

+ Current affiliation: Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus, Okinawa, Japan

Corresponding author address: Akira Kuwano-Yoshida, Earth Simulator Center, Japan Agency for Marine–Earth Science and Technology, Showa-machi, Kanazawa-ku, Yokohama City, Kanagawa 236-0001, Japan. Email: akiray@jamstec.go.jp

Abstract

Numerical simulations of six explosively developing extratropical cyclones in the northwestern Pacific Ocean region are conducted using a regional mesoscale numerical model [the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5)]. Cyclones are categorized according to the locations where they form and develop: Okhotsk–Japan Sea (OJ) cyclones originate over the eastern Asian continent and develop over the Sea of Japan or the Sea of Okhotsk, Pacific Ocean–land (PO–L) cyclones also form over the Asian continent and develop over the northwestern Pacific Ocean, and Pacific Ocean–ocean (PO–O) cyclones form and develop over the northwestern Pacific Ocean. Two cases (the most extreme and normal deepening rate cases for each cyclone type) are selected and simulated. Simulations show that the extreme cyclone of each type is characterized by a different mesoscale structure and evolutionary path, which strongly reflect the larger-scale environment: an OJ cyclone has the smallest deepening rates, associated with a distinct upper-level shortwave trough, a clear lower-level cold front, and a precipitation area that is far from the cyclone center; a PO–L cyclone has moderate deepening rates with high propagation speeds under zonally stretched upper-level jets; and a PO–O cyclone has the strongest deepening rates associated with large amounts of precipitation near its center. Sensitivity experiments involving the latent heat release associated with water vapor condensation show that PO–O cyclones rarely develop without a release of latent heat and their structures are drastically different from the control runs, while OJ cyclones exhibit almost the same developments and have similar structures to the control runs. These tendencies can be seen in both extreme and normal deepening rate cases. These results reveal that the importance of latent heat release to explosive cyclone development varies among the cyclone types, as is reflected by the cyclone origin, frontal structure, moisture distribution, and jet stream configuration.

* Current affiliation: Earth Simulator Center, Japan Agency for Marine–Earth Science and Technology, Yokohama, Japan

+ Current affiliation: Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus, Okinawa, Japan

Corresponding author address: Akira Kuwano-Yoshida, Earth Simulator Center, Japan Agency for Marine–Earth Science and Technology, Showa-machi, Kanazawa-ku, Yokohama City, Kanagawa 236-0001, Japan. Email: akiray@jamstec.go.jp

1. Introduction

Initial study of explosively developing cyclones by Sanders and Gyakum (1980) was followed by many numerical studies conducted to enhance our knowledge of the systems that cause the strong winds, heavy precipitation, and floods accompanying cyclones. However, forecasting of such cyclones has been difficult (Harr et al. 1992; Sanders et al. 2000) because of a poor understanding of the cyclone processes, including the upper-level short-wave trough, lower-level baroclinicity, and their interactions in the presence of diabatic heating (Shapiro et al. 1999; Kuo et al. 1991). Scientific interest has also focused on the energy and vapor transport of explosive cyclones, providing the mechanisms for their rapid development and their contribution to local and global climate changes.

Statistical analyses (Sanders and Gyakum 1980; Roebber 1984) have shown that the northwestern Pacific Ocean region is one of the most active areas for explosive cyclone development. Chen et al. (1992) and Yoshida and Asuma (2004) identified two active areas in the northwestern Pacific region: one over the Sea of Japan and the Sea of Okhotsk and another over the northwestern Pacific Ocean. Several case studies and numerical studies for cyclone development in the northwestern Pacific region have been conducted (Liou and Elsberry 1987; Mullen and Baumhefner 1988; Takano 2002). Chen and Dell’Osso (1987) simulated cyclone development over the Sea of Japan and suggested the importance of latent heat release and the presence of a low-level jet for the cyclone development. Nuss and Kamikawa (1990) compared the explosive cyclone with a nonexplosive cyclone that developed over the northwestern Pacific Ocean and reported that the maintenance of strong surface fluxes in the updraft region is attributed to the interaction of warm frontal dynamics with a downstream upper-level jet streak, leading to the explosive cyclogenesis. These studies suggested that explosive cyclogenesis requires not only baroclinicity but also diabatic heating. Kuo and Low-Nam (1990) simulated nine explosive cyclones over the western Atlantic Ocean and concluded that the influence of diabatic heating on explosive development varied substantially for each case. However, their study did not provide a discussion of the reasons for the variation.

Yoshida and Asuma (2004) classified explosively developing extratropical cyclones over the northwestern Pacific region into three types, based on the locations of formation and most rapid deepening: Okhotsk–Japan Sea (OJ) cyclones form over the Asian continent and develop over the Sea of Japan or the Sea of Okhotsk, Pacific Ocean–land (PO–L) cyclones also form over the Asian continent and develop over the northwestern Pacific Ocean, and Pacific Ocean–ocean (PO–O) cyclones form and develop over the northwestern Pacific Ocean.

The authors concluded that each type of cyclone reflects the characteristic environmental and mesoscale structures at the location of most rapid development and the contributing development factors systematically reflect the larger-scale atmospheric environment. Frequently, OJ cyclones occur in late autumn, have the smallest deepening rates, and upper-level vorticity advection with a short-wave trough and lower-level thermal advection are essential for their development. PO–L cyclones have moderate deepening rates, frequently occur in early and late winter, and develop under a zonally extended jet stream. PO–O cyclones mainly occur in midwinter, have the largest deepening rates, and develop with a large amount of latent heat release near the cyclone center. The main objectives of the present paper are to investigate the development of extreme cases of each cyclone type and to clarify the role of latent heat release and other processes in the explosive cyclogenesis using numerical simulations and analyses.

2. Data and model description

The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5), version 3.6.1, which is a nonhydrostatic, primitive equation model (Grell et al. 1995), was used in the present study. Simulations were started 24 h before the maximum observed deepening rate and integrated for 48 h. Two nested grid domains with two-way nesting were used with the Lambert conformal projection map. The horizontal grid spacing of the outer domain was 45 km having 200 × 160 grid points and that of the inner domain was 15 km having 301 × 301 grid points. The center of each domain was set to be the surface cyclone center at the location of the maximum deepening rate. Twenty-three vertical layers, with sigma coordinates from the surface to 100 hPa, were calculated. The explicit simple ice moisture scheme, which predicts water vapor, cloud water, and rainwater composition for air temperatures above 0°C, and cloud ice and snow composition below 0°C, was used as the outer domain (Dudhia 1989). A mixed-phase explicit moisture scheme, which also predicted supercooled water along with the simple ice scheme, was used for the inner domain (Reisner et al. 1998). Grell’s cumulus parameterization (Grell 1993) was used to represent subgrid-scale convective precipitation for each domain.

The global objectively analyzed dataset (GANAL), provided by the Japan Meteorological Agency (JMA), was used for initial and lateral boundary conditions. The GANAL dataset contains sea level pressures, geopotential heights, temperatures, horizontal winds, and dewpoint depressions with horizontal resolutions of 1.25° in latitude and longitude and 18 vertical levels between the surface and 10 hPa. A Reynolds sea surface temperature (SST) dataset, provided by the National Oceanic and Atmospheric Administration–Cooperative Institute for Research in Environmental Sciences Climate Diagnostics Center (NOAA–CIRES CDC), was used as the SST condition. This dataset provides a weekly mean SST for every 1° grid point in latitude and longitude.

3. Methodology and results

a. Selection of extreme cases

The “Eady index” (σBI), derived by Lindzen and Farrell (1980), indicates the maximum growth rate of an adiabatic perturbation in quasigeostrophic baroclinic flow on a β plane with a constant vertical wind shear and static stability, is used to diagnose the midlatitude cyclone growth rate, and is defined as follows:
i1520-0493-136-2-712-e1
where v is the horizontal wind, f is the Coriolis parameter, N is the Brunt–Väisälä frequency, and z is the height. The explosive cyclone’s deepening rates (in Bergeron) have been evaluated based on the sea level pressures at the cyclone center following Sanders and Gyakum (1980). Yoshida and Asuma (2004) calculated the deepening rate using the following definition:
i1520-0493-136-2-712-e2
where t is analyzed time in hours, p is the sea level pressure at the cyclone center, and ϕ is the latitude at the cyclone center. An explosively developing cyclone was defined as having a deepening rate of at least 1 Bergeron. Although the definition of Bergeron in Sanders and Gyakum (1980) used a 24-h pressure change, a 12-h pressure change is used by Yoshida and Asuma (2004) and in the present paper to find the instance of most rapid deepening in a cyclone’s life.

To compare the growth rates of actual cyclones with those determined using adiabatic linear baroclinic theory, the Eady index was calculated at the maximum deepening rate for all extratropical cyclones that continued for at least 2 days during three cold seasons from 1 October 1996 to 31 March 1999. Figure 1 shows a scatter diagram correlating the maximum deepening rates and the Eady indices at 650 hPa, which were calculated for 300- and 1000-hPa winds and potential temperatures 6 h before the occurrence of the maximum deepening rate, and the values were averaged over the 1000-km area surrounding the cyclone center.

The slope of the regression line measures the extent to which the adiabatic (dry) baroclinic theory explains the cyclone-deepening rate. Although each panel in Fig. 1 shows a positive correlation between the maximum deepening rates and the Eady indices, above a 99% significance level, OJ cyclones show the smallest regression line slope [about 1.2 Bergeron (day−1)−1], among the three types, while PO–L and PO–O cyclones have larger slopes [about 1.6 and 1.7 Bergeron (day−1)−1, respectively]. Several extreme cases, which had larger deepening rates, fall distinctly large distances from their regression lines. It is interesting that the Eady index does not exceed 1.5 day−1, and the maximum deepening rate seems not to have a maximum limitation, especially for the region in which the Eady index is larger than 1 day−1. These results suggest that the explosive cyclogenesis cannot be explained simply on the basis of the Eady index by assuming adiabatic (dry) baroclinic instability, especially for larger deepening rates. To explain these results, the most rapidly developing cyclones in each of the three types, were selected and simulated using the MM5.

b. Synoptic overview of extreme three cases

A synoptic overview of the extreme cases was developed using the GANAL dataset. Figure 2 shows the cyclone track and a time series of the central sea level pressures for each extreme case from formation to disappearance. Figures 3 –5 show the distributions of sea level pressures, potential temperatures, and their horizontal gradients at 850 hPa, and geopotential heights, potential vorticities, and wind speeds at 300 hPa—at the time of maximum deepening rate, as well as 24 h before and after the occurrence of the maximum deepening rate for the extreme OJ case, PO–L, and, PO–O, respectively.

The extreme OJ cyclone formed over the Asian continent at 1800 UTC 24 February 1999 (Fig. 2a), moved eastward, and arrived over the northern Sea of Japan, with a 1004-hPa central sea level pressure, recorded 24 h before the maximum deepening rate (Figs. 3a and 3b). The cyclone’s cold front extended along the east coast of the Asian continent, bending westward at 40°N. Another weak frontal system existed over the southwestern part of the Japanese mainland (Honshu), the East China Sea, and southern China. A strong anticyclone formed over Mongolia, with a central sea level pressure of 1044 hPa, and extended over the Asian continent. A weak jet streak appeared at the leading edge of an upper-level trough extending from Siberia, accompanied by large potential vorticity (PV), which existed over the east coast of the Asian continent at 300 hPa. When the cyclone moved over the Sea of Okhotsk at 1200 UTC 27 February 1999 (Figs. 3c and 3d), it experienced its maximum deepening rate of 1.84 Bergeron (Table 1) and its central sea level pressure dropped to 972 hPa. The two frontal zones merged and became a strong cold frontal zone elongated from the cyclone center to the southwest over the northwestern Pacific Ocean. A PV maximum, associated with the upper-level trough, existed over the Sea of Japan, just to the west of the surface cyclone center. The cyclone then moved northeastward at 1200 UTC 28 February 1999 (Figs. 3e and 3f), and its central sea level pressure dropped further to 960 hPa, which was the minimum recorded throughout the cyclone’s lifetime. The cold front elongated southeastward from the cyclone center, bending southwestward over the northwestern Pacific Ocean. The upper PV maximum overlapped the surface cyclone and the enlarged upper-level trough meandered. After deepening, the cyclone stagnated, achieving its minimum central sea level pressure of 960 hPa at 0600 UTC 28 February 1999, and disappeared at 1200 UTC 3 March 1999.

The cyclone track and the central sea level pressures for the extreme PO–L cyclone are shown in Figs. 2c and 2d. The cyclone formed over the Asian continent at 0600 UTC 8 February 1998, moved rapidly eastward and was located over the southern coast of the Sea of Japan at 1800 UTC 9 February 1998 (Figs. 4a and 4b; 24 h before the maximum deepening rate appeared), and its central sea level pressure at formation was 1010 hPa. A weak frontal zone extended just north of the surface cyclone. We can also see that a large cyclone existed over the central North Pacific Ocean and a relatively weak anticyclone (1024 hPa) formed over China. At 300 hPa, a strong jet stream zonally extended from southern China to the northeastern Pacific Ocean and a northern jet stream associated with a weak upper-level trough entered the stronger jet east of the surface cyclone. The cyclone continued to rapidly move eastward, explosively developing, to a level of 2.54 Bergeron (Table 1), offshore east of Japan at 1800 UTC 10 February 1998 and its central sea level pressure at the time of maximum deepening dropped to 992 hPa (Figs. 4c and 4d). There was no distinct frontal structure in the lower level and the upper-level jet stream maintained its strength. Its central sea level pressure dropped to a first minimum at 971 hPa at 1800 UTC 11 February 1998 (Fig. 4e). After a small increase in central pressure, it redeveloped and reached the 962-hPa minimum pressure of the cyclone life cycle at 1200 UTC 13 February 1998, disappearing over the Gulf of Alaska at 0000 UTC 16 February 1998.

An extreme PO–O cyclone formed off the east coast of China at 0000 UTC 29 December 1997. Moving northeastward, the cyclone was located over the south coast of Japan at 0000 UTC 30 December 1997 with a central sea level pressure of 1009 hPa (Figs. 2e, 2f and 5a). The cyclone had a very weak baroclinic zone extending from south of China to south of the coast of Japan. An upper-level jet stream core was located over the lower-level baroclinic zone and a second strong wind existed along the northeast of the cyclone (Fig. 5b). The surface cyclone formed under the southern entrance of the northeastern jet streak and the northern exit of the southwestern one. This is favorable for cyclogenesis because the jet streaks forced ageostrophic upward winds at that location (Keyser and Shapiro 1986). The cyclone experienced explosive development (2.96 Bergeron) over the northwestern Pacific Ocean at 0000 UTC 31 December 1997 (Table 1). The cyclone developed to 974 hPa and its cold front elongated southward. Southwest and northeast of the surface cyclone, 300-hPa jets intensified. The cyclone continued moving northeastward, and the central sea level pressure reached a minimum of 957 hPa by 0000 UTC 1 January 1998 (Fig. 5e). The southern upper-level jet streak remained along the southern coast of Japan, separating from the surface cyclone center, while the northern jet streak continued to exist near the cyclone (Fig. 5f). The cyclone then turned northwestward, filling rapidly, and disappearing north of the Sea of Okhotsk at 0600 UTC 2 January 1998.

c. Control experiments

Simulation results for the extreme OJ cyclone (OJ CNTL run) are shown in Fig. 6. Simulated maximum deepening rates and deepening rates at T = 24 h are listed in Table 1. A cyclone associated with an upper-level short-wave trough was simulated over the northern Sea of Japan at T = 12 h (0000 UTC 27 February 1999; Figs. 6a and 6b). An upper-level positive PV anomaly from the 2-day average was located at the southern edge of the trough and a relatively weak jet streak appeared over the southern Sea of Japan. The precipitable water, over 30 mm, existed along the southern coast of Japan and precipitation occurred in the humid region. The northern frontal zone, within which no precipitation occurred, can be identified and is the same as the results of the GANAL analysis in Fig. 3a. As the cyclone developed, the upper-level positive PV anomaly moved over the south of the surface cyclone center and the upper-level jet streak elongated northeastward (Figs. 6d and 6f). The negative PV anomaly enhanced to the east of the jet streak. Vertically integrated rainwater increased along the southern cold front, but was not remarkable around the cyclone center (Figs. 6c and 6e). The cyclone deepening rate at T = 24 h (1200 UTC 27 February 1999) was 1.55 Bergeron, smaller than that determined through the GANAL analysis (1.84 Bergeron). The maximum deepening rate of the control run was 1.70 Bergeron at T = 30 h (1800 UTC 27 February 1999). Although there was a 6-h lag in appearance time, it was almost comparable in magnitude to that determined using the GANAL analysis.

The results for the PO–L CNTL run are shown in Fig. 7. At T = 12 h (0600 UTC 10 February 1998; Figs. 7a and 7b), the surface cyclone center was located just east of Japan. In the upper level, a jet stream zonally extended along the southern coast of Japan and a small trough associated with a high-PV anomaly existed over the western part of the surface cyclone (Figs. 7b and 7d). As the upper-level trough moved over the surface cyclone center at T = 36 h (Fig. 7f), the surface cyclone developed and its central sea level pressure reached its minimum. Precipitation appeared near the cyclone center, with a narrow larger region of precipitable water extending southwestward from the cyclone center. The upper-level negative anomaly, located to the east of the surface cyclone, and a short ridge developed in the same region. Although the GANAL analysis showed a cyclone deepening rate of 2.54 Bergeron, the control run showed 1.62 Bergeron at T = 24 h. The maximum deepening rate was recorded at T = 18 h, although the deepening rate, 2.01 Bergeron, was also smaller than that determined in the GANAL analysis (Table 1), while the cyclone position and surface central sea level pressure variation were simulated well, as shown in Figs. 2c and 2d.

Figure 8 shows the results for the PO–O CNTL run. At T = 12 h (1200 UTC 30 December 1997; Figs. 8a and 8b), the surface cyclone was located over the eastern coast of Japan and the moist region was spread around the cyclone center and eastward. The precipitation volume was the largest among the three cases, over 2 mm (Fig. 8c). In the upper level, a jet streak was located to the southwest of the surface cyclone center and another weaker jet streak appeared to the northeast. At T = 24 h (0000 UTC 31 December 1997; Figs. 8c and 8d), the positive PV anomaly overlapped the surface cyclone center and the northeastern jet streak strengthened, as did the northeastern negative PV anomaly. A larger amount of precipitable water was concentrated just to the east of the surface cyclone center, and the precipitation amount was also larger. At T = 36 h (1200 UTC; Figs. 8e and 8f), the larger precipitation and moisture area separated from the cyclone center and weakened. At the same moment, the northeastern jet streak weakened and spread; and the southwestern jet streak maintained its strength. Both of the deepening rates, 2.26 Bergeron at T = 24 h and 2.64 Bergeron at T = 18 h, which was the maximum deepening rate, were comparable to the results of the GANAL analysis (2.96 Bergeron).

d. Sensitivity experiments

Yoshida and Asuma (2004) suggested that the contribution of latent heat release to cyclone deepening depended on the cyclone type. Numerical simulations also suggested different features for the relative configuration of the precipitation area and the surface cyclone center, as well as the precipitation amount. To clarify how latent heat release affects cyclone development, sensitivity experiments were conducted and are described in this section. The simulations were conducted in the same manner described in the previous section, except that no latent heat release occurred during the water vapor condensation. The cyclone tracks and central sea level pressures of simulations with (CNTL) and without (DRY) the latent heat release are summarized in Fig. 2 (dashed line and dotted line, respectively, in each panel) and the deepening rates at T = 24 h and the maximum deepening rate of each simulation are listed in Table 1.

The results of the no-latent heat release simulation for the extreme OJ cyclone (hereinafter, OJ DRY run) are shown in Figs. 9a and 9b. When compared with the OJ CNTL (Figs. 6a and 6b), the cyclone mesoscale structures are not very different from each other and the cyclone tracks were similar (Figs. 2a and 2b), although the deepening rate (0.88 Bergeron at T = 24 h) was weaker than the OJ CNTL. The amounts of vertically integrated rainwater and precipitable water slightly decreased for the DRY run (Figs. 6c and 9c), and the northward extension of the upper-level jet streak was reduced at T = 24 h (Figs. 6d and 9d) and T = 36 h (Figs. 6f and 9f). The simultaneous weakness of the upper-level negative PV anomaly and upper-level jet reveals that the latent heat release enhanced updraft around the cold front and strengthened divergence in the upper levels. The divergence created an upper-level negative PV anomaly, which induced anticyclonic circulation. As a result, the upper-level jet streak extended northward. The minimum central sea level pressure of the DRY run was about 10 hPa higher than that of the GANAL analysis and 17 hPa higher than that of the CNTL run (Fig. 2d).

To examine the three-dimensional structural differences between the CNTL and DRY runs, a trajectory analysis of air parcels was conducted. In total, 343 air parcels around the cyclone center were traced. The parcels initiated from 7 × 7 points [each point horizontally separated by 225 km (15 grid points) and each centered at the cyclone surface center, extending vertically for seven levels: 200, 300, 400, 500, 600, 700, and 850 hPa], as shown in Fig. 10. Trajectories were obtained for 24 h backward and forward from T = 24 h. Winds were interpolated every 15 min using 2-h interval model outputs. Figure 11 shows the perspective views of the trajectories categorized by their mean distances into four groups using Ward’s clustering method (Ward 1963), which clusters to minimize the sum of the squared distances to the central mean of each cluster. The green lines correspond to the trajectories moving almost horizontally in the upper levels, the purple lines are upward-moving air parcels in the midlevel, the orange lines are downward-moving air parcels from the cyclone’s upstream from 200–400 hPa to the surface cyclone, and the blue lines are upward-moving air parcels from the lower level.

Comparing the OJ CNTL run (Fig. 11a) with the OJ DRY run (Fig. 11b), the number of downward-moving air parcels in the DRY run is lower than in the CNTL run and the number of midlevel upward-moving parcels in the DRY run is higher than in the CNTL run. These are reflected in the decreased altitude attained by the upward-moving parcels. Figures 12a and 12b show the vertical cross sections along line A–A′ in Fig. 6c and line B–B′ in Fig. 9c, the projected upward-moving air parcels from the lower levels (blue lines). The upper-level positive PV anomaly is located just upstream of the upward trajectories in the OJ CNTL run; however, the upper-level positive PV anomaly is weaker in the OJ DRY run. In the CNTL run, air parcels reached the cyclone center from the southward direction in the lower level, suddenly rising to a height of 7 km, which corresponds to the tropopause, and the isentropic gradient below 6 km was steeper downstream of the cyclone than it was for the DRY run. This indicates that a larger amount of warm advection was associated with cyclone development in the CNTL run. However, trajectories in the DRY run were different from those in the CNTL run. Air parcels in the DRY run rose gradually along the isentropic surfaces and were lifted only up to a height of 5 km, which is below the tropopause.

The deepening rate in the PO–L DRY run decreased 0.73 Bergeron from 1.62 Bergeron in the CNTL run at T = 24 h, as shown in Table 1. The results of the DRY run for the extreme PO–L cyclone are shown in Fig. 13. There are distinct differences from the CNTL run in the precipitation and moisture distributions (Fig. 7). Although most of the vertically integrated rainwater was concentrated along the cold frontal zone in the CNTL run, it was spread over the warm sector during the DRY run. The amount of precipitable water in the DRY run was less than that in the CNTL run due to a smaller lower-level convergence in the warm sector. Although the upper-level short-wave trough became deeper and the upper-level jet streak was divided into two regions at T = 24 h (Fig. 7d) and T = 36 h (Fig. 7f) during the CNTL run, the upper-level trough was shallower and the upper-level jet was straighter during the DRY run (Figs. 13d and 13f). Although the amplitude and pattern of the upper-level positive PV anomaly were almost the same during the CNTL and DRY runs, the downstream negative PV anomaly disappeared only in the DRY run. Structures that are more detailed can be seen in the trajectory analysis.

Air parcels, which moved upward in the middle level, disappeared in the DRY run (Fig. 11d) and the number of upward-moving air parcels from the lower levels decreased, as compared with the CNTL run (Fig. 11c). Many air parcels rising near the cyclone center can be identified during the CNTL run (Fig. 12c), while the lower-level positive PV anomalies appeared over the surface cyclone center. Air parcels rapidly rose to a height of 7 km, moving eastward, and the isentropic surface at 304 K was lifted up to the altitude where the upper-level negative PV anomaly was located. However, during the DRY run, the lower-level positive PV anomaly disappeared, upward trajectories decreased, and the eastern tropopause was not lifted up. These differences are similar to the results in Davis et al. (1993), which suggested, using PV inversion analysis, that upward motion by latent heat release caused upward and northward advection of the tropopause aloft and enhanced the downstream upper-level ridge.

The CNTL and DRY runs of the PO–O cyclone exhibit the largest differences among the three cyclone types. First, surface cyclone development in the DRY run was much weaker than in the CNTL run. Although the deepening rate at T = 24 h and the minimum sea level pressure were 2.26 Bergeron and 955 hPa, respectively, during the CNTL run (Table 1; Fig. 2f), they were 0.71 Bergeron and 990 hPa during the DRY run, respectively. Vertically integrated rainwater spread over the warm sector as in the PO–L DRY run, and its amount and precipitable water remarkably decreased (Figs. 8 and 14). Although the northeastern upper-level jet streak appeared in the CNTL run, it did not appear in the DRY run. The upper positive and negative PV anomalies distinctly decreased in the DRY run.

Trajectory analysis also shows significant structural differences (Figs. 11e and 11f). During the CNTL run, air parcels with strong downdrafts (orange lines) intruded from the western upper levels of the cyclone and air parcels with strong updrafts (blue lines) suddenly rose up in the narrow area near the cyclone center from the western lower levels. The maximum updraft recorded speed was over 1.0 m s−1 at T = 24 h at 600 hPa in the CNTL run. In contrast, low-level air parcels with updrafts gradually rose up in the wide area during the DRY run (the updrafts had a maximum speed of 0.3 m s−1). In addition, there were fewer air parcels with downdrafts (orange lines) during the DRY run. The differences in upward trajectories can be seen in the vertical cross sections in Fig. 12. In the CNTL run, the upward air parcels suddenly rose up just above the surface cyclone center where the lower-level positive PV anomalies appeared and reached up to 11 km in height (Fig. 12e). The upward motion may have been forced by the upper-level positive PV anomaly because the anomaly was located just upstream of the surface cyclone.

The upwardly moving air parcels, heated by latent heat release, were lifted up and diverged at the tropopause, creating the upper-level negative PV anomaly and the strong westerly jet along the northern side. In contrast to the PO–L DRY run, the positive PV anomaly, as well as the upper-level negative PV anomaly, weakened in the PO–O DRY run. The upward trajectories rose up to only 8 km and baroclinicity in whole troposphere weakened. The divergence did not occur at the upper levels and the northeastern upper-level jet streak did not appear at 300 hPa, as can be seen in Figs. 12f and 14. These results reveal that the latent heat release near the cyclone not only created surface cyclonic circulation by positive PV anomaly formation, but also may have influenced the amplification of the upper-level trough and ridge through the upper negative PV anomaly formation. Although extremely rapid development was experience in the PO–O case, without latent heat release by the cyclone, interaction between the lower- and upper-level PV anomalies could not occur, and cyclogenesis was drastically slower and precipitation and water vapor convergence were reduced.

4. Discussion

The preceding analysis demonstrated the role of latent heat release in the three extreme cases: however, an explanation of the reason for their “extremely” rapid development is still needed. To help in finding the answer, three other cases were analyzed. These cyclones, referred to hereinafter as standard cases, were selected based upon the similarity of their track and their Eady index value relative to the extreme cases, previously discussed, but the maximum deepening rate was close to the regression line in Fig. 1 and larger than 1 Bergeron (triangles in Fig. 1). Figure 15 shows an upper-level PV anomaly that averaged between 200 and 500 hPa and vertically integrated rainwater at T = 24 h for the CNTL runs of the extreme and standard cases. At first, one can see that the approaching upper-level positive PV anomalies in the extreme cases were much stronger than in the standard cases. Although the amount of vertically integrated rainwater was larger in every extreme case, their patterns show similar tendencies among the types; that is, the OJ cases were accompanied by smaller amounts of rainwater, the PO–L cases also had smaller amounts of rainwater extending eastward, and the PO–O cases had larger amounts and a more systematic distribution of rainwater near the cyclone center.

The mesoscale characteristic structures were also sensitive to latent heat release. Figures 16 and 17 show the results of the CNTL and DRY runs at T = 24 h for the standard cases. For the standard OJ runs, the pressure, temperature and water vapor distributions in the lower level (Figs. 16a and 16b) and the positive PV anomaly in the upper level (Figs. 17a and 17b) were almost the same during the DRY and CNTL runs, with the exception of a smaller amount of vertically integrated rainwater and a somewhat weaker upper-level negative PV anomaly in the DRY run. For the standard PO–L runs, the amount of vertically integrated rainwater decreased, especially to the east of the cyclone center in the DRY run, and the pressure pattern became more rounded because of the absence of a depression from the latent heat release around the warm eastern frontal region (Figs. 16c and 16d). The influences of latent heat release appeared in the weaker upper-level negative PV anomaly, as was seen in the extreme PO–L cases (Figs. 17c and 17d). For the standard PO–O run, the influence of the latent heat release was the largest among the three standard runs, as was the case in the extreme cases. Although vertically integrated rainwater amount maxima occurred near the surface cyclone center in the CNTL run, these rainwater maxima almost disappeared, water vapor concentration became weaker, and the cyclone center itself moved slightly northward during the DRY run (Figs. 16e and 16f). In the upper level, a strong negative PV anomaly and northwestern jet streak at the surface cyclone center disappeared and a positive PV anomaly moved northwestward in the DRY run (Figs. 17e and 17f). These lower- and upper-level differences between the CNTL and DRY runs were consistent for each cyclone case. In particular, the movement of the surface cyclone center revealed that latent heat release plays an important role in the cyclone development, even in the standard case of the PO–O cyclone.

A series of comparisons between the extreme and standard cases suggest that although positive PV advection in the upper level basically determined the extreme deepening, it is possible to say that mesoscale cyclonic features, in particular, the distribution and strength of the latent heat release (determined by the amounts of cloud and precipitation water), affect cyclone development through the nonlinear interaction among the upper-level short-wave trough, the jet (PV anomaly), and the latent heat release.

In the previous section, water vapor was determined to be an energy source for rapid cyclone development. However, after development, cyclones transport moisture for a considerable distance and affect the regional climate and/or the next-generation cyclone development (Lackmann et al. 1998; Smirnov and Moore 1999). To investigate moisture transport in the three extreme cases described in section 3, further analyses of backward air parcel trajectories and the water budget were conducted using the GANAL dataset, not simulated results, because of the long-term integration of the simulation-enhanced numerical errors, especially those appearing in the determination of cyclone position and the precipitation distribution.

The size of the moisture budget was calculated by the amount of moisture in the air column and its local change. The size of the water vapor budget corresponds to the precipitation amount minus the amount of the surface evaporation (PE, where P is the amount of precipitation and E is the amount of surface evaporation), written as
i1520-0493-136-2-712-e3
where g is the gravitational acceleration, q is the specific humidity, p is the pressure, pt is the upper boundary of the air column (=300 hPa), pb is the sea level pressure, and v is the horizontal wind vector.

To calculate the trajectories of the air parcels, vertical velocities were estimated using the kinematic method (O’Brien 1970). Backward trajectories start from the maximum point of vertically integrated vapor flux convergence, along the surrounding four grids at the 500-, 600-, 700-, 850-, and 925-hPa levels, at the time of maximum deepening and 48 h before and after its occurrence. Figure 18 shows backward trajectories of air parcels and precipitable water content averaged for 96 h. Precipitation amounts, using 1 degree daily (1DD) data of the Global Precipitation Climatology Project (GPCP; Huffman et al. 2001) accumulated during the cyclone’s life cycle in the cyclone influence area—defined as the area within next sea level pressure ridges—are plotted in the figure. Figure 19 shows similar composites of estimated PE values and vertically integrated water vapor flux using the GANAL dataset.

In the OJ case (Fig. 18a), the cyclone was located over the drier Asian continent at 48 h before the maximum deepening rate, and all air parcels came from the west during this time. There was no distinct precipitation around the cyclone center while the cyclone moved over the land. The precipitation amount maximized along the cold front over the northwestern Pacific Ocean during the occurrence of maximum deepening, as shown in Fig. 6. Air parcels in the lower level moved from the southward moisture area. For the backward trajectories 48 h after the most rapid deepening, a maximum point of vertically integrated vapor flux convergence was located northeast of the Sea of Okhotsk. Air parcels in the lower level came from the Bering Sea and the central North Pacific Ocean, which are relatively moist areas. From the results for PE in Fig. 19a and the GPCP precipitation amount (Fig. 18a), one can surmise that the evaporation was greater over the Sea of Japan and south of Japan, and precipitation was larger over the Sea of Okhotsk and on both sides of the Kamchatka Peninsula. Stronger eastward vertically integrated vapor fluxes extended from the eastern coast of China to offshore of eastern Japan, where evaporation was dominant and northward fluxes appeared offshore of eastern Hokkaido, where precipitation was dominant. The results demonstrate that the warm sector of the OJ cyclone transported precipitation northward, while the cold sector absorbed heat and moisture from the ocean south of Japan, the Kuroshio Current region.

In the PO–L case (Figs. 18b and 19b), the cyclone tracked southward of the OJ case, and there was no distinct net moisture transport north of the cyclone track. After passing the Japanese islands, stronger precipitation occurred on the northern side, associated with the rapid deepening. The cyclone moved directly eastward and a larger amount of precipitation occurred around the track. Most of the air parcels came from the west and moved directly eastward, and a northward component of the vertically integrated vapor flux can be identified in the lower levels. The moisture budget and vapor flux analyses in Fig. 19b show that there were strong eastward vapor fluxes in the southern part of the cyclone track, which generated a large amount of precipitation over the eastern North Pacific and the west coast of the North American continent. A large amount of evaporation (PE is about −45 mm day−1) was identified in the western part of the North Pacific Ocean. Similar vapor fluxes and transportation were identified by Bao et al. (2006) using the Special Sensor Microwave Imager (SSM/I) dataset.

For the PO–O case, the cyclone formed along the east coast of China at 48 h before maximum deepening. At that time, moist air parcels came from the vicinity of Taiwan, which is a very humid area (Fig. 18c), affecting the PO–O cyclone with moisture from the beginning. During the maximum deepening period, moist air parcels in the lower latitudes entered into the cyclone center, resulting in a large amount of precipitation (about 30 mm day−1). Forty-eight hours after the maximum deepening, the precipitation area moved to the western Bering Sea. During this period, moist air parcels came from the central North Pacific Ocean. Figure 19c shows that strong northward water vapor fluxes appeared along the eastern side of the cyclone track. A large amount of water vapor (PE is −30 mm day−1), evaporating over the ocean south of Japan, was transported to the Kamchatka Peninsula and the central North Pacific.

In all three cases, a large amount of moisture evaporation occurred over the southern Japanese islands. These results are consistent with the results of a study conducted by Chen et al. (1995), which analyzed global water transports and suggested that the southern Japanese islands supplied a large amount of moisture to the atmosphere in winter. However, positive PE and observed precipitation occurred in different areas during the three cyclones. Additionally, Yoshida and Asuma (2004) show that the direction and strength of the water vapor flux supplied to the cyclone center are different among the three types and the difference may affect the cyclone’s rapid genesis. These results suggest that explosive cyclones may play an important role in global water circulation, especially during winter. A better understanding of the global water cycle requires an understanding of the variations in cyclone tracks and their mechanisms.

5. Summary and conclusions

The present study of explosive cyclogenesis in the northwestern Pacific region simulated six cyclone cases using the PSU–NCAR MM5 and analyzed their evolutions. Extreme and standard cases of each cyclone type, that is, OJ, PO–L, and PO–O, were examined. Cyclone types were classified by Yoshida and Asuma (2004) using cyclone tracks and their most explosively developing positions. They followed characteristic evolutions, which could be successfully simulated using the mesoscale numerical model (PSU–NCAR MM5). Sensitivity experiments for latent heat release showed the importance of latent heat release in cyclogenesis, the differences among the three types, and the similarities between explosive and standard cases in each type. The analysis of backward air parcel trajectories showed that the distribution of latent heat release was closely connected with cyclone structure and cyclogenesis.

Neither of the OJ cases was sensitive to latent heat release since the cyclone’s precipitation area was far from the cyclone center, which resulted in weaker cyclogenesis than observed in the other types, caused mainly by the interaction between the upper vorticity advection and lower-level baroclinicity. The extreme PO–L case was somewhat sensitive to latent heat release, but the cyclone structure did not change much even during the DRY run because of a strong upper-level jet stream, the quick movement of the cyclone, and weaker precipitation. In contrast, the cyclone shape of the standard PO–L case varied under DRY runs because the contribution of the latent heat release was relatively larger. The PO–O cases showed the most drastic responses to latent heat release. The deepening rate significantly decreased and the structures of the surface cyclone, as well as the upper-level jet stream, differed greatly between the control and sensitivity runs. These results may be attributed to the evolutions and environments of the cyclones. The PO–O cyclones originally formed in the moist environment and developed with a large amount of precipitation. The environment of the PO–O cyclone formation may supply moist air easily into the lower-level cyclone center. Thus, PO–O cyclones demonstrate more sensitivity to latent heat release than to other cases. In addition, the moisture budget and water vapor transport analyses showed that, although the moisture source area was almost the same region (the ocean south of Japan), the vapor transport direction and strength differed among the cyclone types, resulting in precipitation in different areas.

In conclusion, the evolution of explosively developing cyclones is closely related to the cyclone mesoscale structure, reflecting the larger-scale environment, in particular, the moisture supply and the upper PV anomaly, which leads to the distribution of the latent heat release. This suggests that detailed information concerning vapor distribution and winds over the ocean, and the positioning and depth of upper-level short-wave troughs, is needed to improve the forecasting of explosively developing cyclones. Harr et al. (1992) reported that the cyclone track affected the success of forecasting, for which the evaluation of the role of diabatic heating in cyclogenesis was difficult. The results in the present paper are consistent with their results.

Additionally, a conceptual model of an extratropical cyclone, such as the Norwegian model or the Shapiro–Keyser model, may reflect the larger-scale environments of the cyclones. The OJ cyclone tends to present itself with a distinct cold front and at a stage to occlude structure, as shown in the Norwegian model, while the PO–O cyclone produces heavy precipitation around the warm front and cyclone center, leading to the formation of a bent-back warm front with a warm core, as shown in the Shapiro–Keyser model. Our cyclone classification may be able to contribute to a resolution of these problems. Fantini (2004) also reported that moisture affected the frontal structures using numerical simulations of the ideal cyclone.

The evolutions and structures of extratropical cyclones should be treated as interactions between the larger-scale environment and smaller-scale phenomena. Statistical analysis of the cyclones may be useful in understanding these interactions. Observational research is also important. Recently, various satellites with microwave passive sensors and/or radar systems on board have been used to monitor water bodies. Dropsondes, radar systems on aircraft, aerosondes, and driftsondes are all effective tools for observing cyclone structure over the ocean. Since most of the rapid deepening of cyclones occurs over the ocean, where observational data are scarce, our observational knowledge of this phenomenon is small; observation by these tools should be increased for a better understanding of cyclones.

In contrast, the effects of explosively developing cyclones on climate and their relationships to climate change are still not understood. One of the reasons is that realistic explosively developing cyclones cannot be simulated in the current climate numerical models because of insufficient model resolution (Walthorn and Smith 1998). Recently developed computer systems may be able to research the interaction of climate change and mesoscale phenomena (Ohfuchi et al. 2004; Tomita et al. 2005; Shen et al. 2006). Clarifying the interaction between cyclone activity and climate change is an exciting topic for future research.

Acknowledgments

A part of this paper represents the first author’s Ph.D. dissertation at Hokkaido University. The authors express their thanks to Masaya Kato, Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, for his technical assistance in the analyses, especially for providing tools to operate MM5 and analyze its output. They also express their thanks to Yoshi-Yuki Hayashi, Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, and Koji Yamazaki, Graduate School of Environmental Earth Science, Hokkaido University, for their suggestions and encouragement. Part of the research was supported by a grant-in-aid for scientific research [(A)-(1)-No. 13373003] from the Ministry of Education, Culture, Sport, Science and Technology of Japan (Monbu-Kagaku-sho). GPCP data were provided by the Laboratory for Atmospheres, NASA Goddard Space Flight Center (information online at http://precip.gsfc.nasa.gov/).

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APPENDIX

Sensitivity to Cumulus Parameterizations

A sensitivity experiment using numerical models always includes parameterization problems. In the current study, the cumulus parameterization may have an important contribution to the rapid cyclone deepening because precipitation formation is a key factor. To estimate the sensitivity of cumulus parameterization, several comparisons were conducted using two experimental setups: one has an additional finer nested domain with a 5-km grid without cumulus parameterization in a 15-km grid domain within a 45-km grid outer domain; the other has a 15-km grid domain without cumulus parameterization within a 45-km grid outer domain. Figure A1 shows an example of the results of the extreme PO–O case at T = 24 h. Cyclone development in both cases, with the 5-km grid and the 15-km grid without cumulus parameterization, was almost the same as that observed during the extreme PO–O CNTL run. These results indicate that the arguments presented in this paper do not depend heavily on cumulus parameterization.

Fig. 1.
Fig. 1.

Scatter diagrams for the Eady index estimated for 300- and 1000-hPa horizontal winds and potential temperatures averaged over an area within a 1000-km radius from the cyclone center when observing the maximum deepening rate vs maximum deepening rate for (a) OJ, (b) PO–L, and (c) PO–O cyclones. Boldface lines are regression lines corresponding to equations for the correlation coefficient (r) given in the upper-left panels. Squares show the extreme cases, and triangles show the standard cases.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 2.
Fig. 2.

(left) Surface cyclone tracks (solid lines, crosses represent 6-h interval positions, closed circles show the position at every 0000 UTC), and (right) time series of central sea level pressure between the formation (triangle) and the disappearance (square) from the GANAL dataset; shown are the extreme (a),(b) OJ, (c),(d) PO–L, and (e),(f) PO–O cases. Stars show the minimum sea level pressure. Arrows show the maximum deepening rate. Broken lines show the results of the CNTL runs, and dotted lines show results of the DRY runs.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 3.
Fig. 3.

Synoptic weather chart for the extreme OJ case. (left) Sea level pressure (solid line; contour intervals are 4 hPa), potential temperature at 850 hPa (broken line; contour intervals are 2 K), and horizontal gradient of potential temperature at 850 hPa (shading). Here, L shows the surface cyclone center. (right) The geopotential height (solid line; contour intervals are 120 m), potential vorticity (broken line; contour intervals are 1 PVU = 10−6 m−2 s−1 K kg−1), and wind speed (shading) at 300 hPa at (a), (b) 1200 UTC 26 Feb, (c), (d) 1200 UTC 27 Feb, and (e), (f) 1200 UTC 28 Feb 1999.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 4.
Fig. 4.

As in Fig. 3, but for the extreme PO–L case: (a), (b) 1800 UTC 9 Feb, (c), (d) at 1800 UTC 10 Feb, and (e), (f) 1800 UTC 11 Feb 1998.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 5.
Fig. 5.

As in Fig. 3, but for the extreme PO–O case: (a), (b) 0000 UTC 30 Dec 1997, (c), (d) 0000 UTC 31 Dec 1997, and (e), (f) 0000 UTC 1 Jan 1998.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 6.
Fig. 6.

(left) Sea level pressure (thin solid line; contour intervals are 4 hPa), potential temperature at 850 hPa (broken line; contour intervals are 4 K), vertically integrated rainwater (thick solid line; 0.2 and 2 mm are contoured), and precipitable water (shading). (right) Geopotential height (solid line; contour intervals are 120 m), potential vorticity anomaly from 2-day averages (shading), and horizontal wind speed (boldface line; contour intervals are 10 m s−1 and are drawn for speeds >50 m s−1) at 300 hPa at (a), (b) T = 12 h, (c), (d) T = 24 h, and (e), (f) T = 36 h of the extreme OJ CNTL run. Here, L is the position of the surface cyclone center, and the A–A′ line is the cross section in Fig. 12a.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 7.
Fig. 7.

As in Fig. 6, but for the extreme PO–L CNTL run.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 8.
Fig. 8.

As in Fig. 6, but for the extreme PO–O CNTL run.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 9.
Fig. 9.

As in Fig. 6, but for the extreme OJ DRY run.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 10.
Fig. 10.

Schematic illustration of the initial arrangement of air parcels for the backward trajectory analysis. The L represents the surface cyclone center at the maximum deepening rate.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 11.
Fig. 11.

Major trajectories for the extreme (a) OJ CNTL, (b) OJ DRY, (c) PO–L CNTL, (d) PO–L DRY, (e) PO–O CNTL, and (f) PO–O DRY runs. Sea level pressure (solid line; contour intervals are 4 hPa) and precipitable water (shading) at T = 24 h are shown at the bottom of each cube. The colors of the trajectories indicate four groups by Ward’s clustering method (see text for details).

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 12.
Fig. 12.

Vertical cross section of the projected trajectories rising near the surface cyclone center (thin line, corresponding to blue lines in Fig. 11), the potential vorticity anomaly from 2-day averages (shading), the potential temperature (medium line; contour intervals are 8 K), and the wind speed (thick line; contour intervals are 10 m s−1 and are drawn for speeds >50 m s−1) at T = 24 h for the extreme (a) OJ CNTL along A–A′ in Fig. 6, (b) OJ DRY along B–B′ in Fig. 9, (c) PO–L CNTL along C–C′ in Fig. 7, (d) PO–L DRY along D–D′ in Fig. 13, (e) PO–O CNTL along E–E′ in Fig. 8, and (e) PO–O DRY along F–F′ in Fig. 14.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 13.
Fig. 13.

As in Fig. 6, but for the extreme PO–L DRY run.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 14.
Fig. 14.

As in Fig. 6, but for the extreme PO–O DRY.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 15.
Fig. 15.

Upper potential vorticity anomaly averaged between 200 and 500 hPa from 2-day averages (shading) and vertically integrated rainwater (solid line; mm) at T = 24 h for (a) the extreme OJ CNTL, (b) the standard OJ CNTL, (c) the extreme PO–L CNTL, (d) the standard PO–L CNTL, (e) the extreme PO–O CNTL, and (f) the standard PO–O CNTL runs.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 16.
Fig. 16.

Sea level pressure (thin solid line; contour intervals are 4 hPa), potential temperature at 850 hPa (broken line; contour intervals are 4 K), vertically integrated rainwater (thick solid line; 0.2 and 2 mm are contoured), and precipitable water (shading) for (a) the standard OJ CNTL, (b) the standard OJ DRY, (c) the standard PO–L CNTL, (d) the standard PO–L DRY, (e) the standard PO–O CNTL, and (f) the standard PO–O DRY runs at T = 24 h.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 17.
Fig. 17.

Geopotential height (solid line; contour intervals are 120 m), potential vorticity anomaly from 2-day averages (shading), and horizontal wind speed (thick line; contour intervals are 10 m s−1 and are drawn for speeds >50 m s−1) at 300 hPa for (a) the standard OJ CNTL, (b) the standard OJ DRY, (c) the standard PO–L CNTL, (d) the standard PO–L DRY, (e) the standard PO–O CNTL, and (f) the standard PO–O DRY runs at T = 24 h.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 18.
Fig. 18.

The 48-h backward trajectories of air parcels from the vertically integrated vapor flux convergence maximum points at 500, 600, 700, 850, and 925 hPa [circles; colors shows altitudes of air parcels (hPa)] at the time of maximum deepening, and 48 h before and after that. Precipitable water amount averaged over 96 h (contours) and precipitation amount from GPCP composited near the cyclone during its lifetime (shading). White line and open circles show cyclone track (star, maximum deepening; triangle, 48 h before maximum deepening; square, 48 h after maximum deepening) for the extreme (a) OJ, (b) PO–L, and (c) PO–O cases.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Fig. 19.
Fig. 19.

Vertically integrated vapor flux (arrows) and PE (color shading) composited near the cyclone during its lifetime. Cyclone tracks are the same as in Fig. 18: the extreme (a) OJ, (b) PO–L, and (c) PO–O cases.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

i1520-0493-136-2-712-fa01

Fig. A1. (top) Sea level pressure (thin solid line; contour intervals are 4 hPa), potential temperature at 850 hPa (broken line; contour intervals are 4 K), vertically integrated rainwater (thick solid line; 0.2 and 2 mm are contoured), and precipitable water (shading). (bottom) Geopotential height (solid line; contour intervals are 120 m), potential vorticity anomaly from 2-day averages (shading), and horizontal wind speed (thick line; contour intervals are 10 m s−1 and are drawn for speeds >50 m s−1) at 300 hPa for (a), (d) CNTL, (b), (e) 15-km grid without cumulus parameterization, and (c), (f) 5-km grid without cumulus parameterization at T = 24 h. Here, L is the position of the surface cyclone center.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2111.1

Table 1.

Deepening rate (Bergerons) at T = 24 h, along with the maximum deepening rate for the GANAL analysis, CNTL run, and DRY run for the extreme cases.

Table 1.
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