Features of the Baiu Front Simulated in an AGCM (T42L52)

Journal of the Meteorological Society of Japan, Vol. 80, No. 4, pp. 697--716, 2002 697 Features of the Baiu Front Simulated in an AGCM (T42L52) Kozo NINOMIYA, Teruyuki NISHIMURA, Wataru OHFUCHI 1, Tuneaki SUZUKI and Shinji MATSUMURA Frontier Research System for Global Change, Yokohama, Japan (Manuscript received 17 October 2001, in revised form 7 May 2002) Abstract The simulation studies of the Meiyu-Baiu front, by global climate models, have not been performed in depth. A few experimental studies show insufficient precipitation in the Meiyu-Baiu front, without the detailed analysis on circulation systems in and around the Meiyu-Baiu front. In the present report, we study features of the Meiyu-Baiu front and associated circulation systems simulated in the climatological SST run by the T42L52 model (the maximum zonal wave-number is 42 in triangular wave transformation, and the number of the vertical level is 52) in comparison with the features described in observational studies. The Baiu front is not properly reproduced in the monthly averaged field for June and July. This is due to the alternation of relatively short Baiu phase and the longer non Baiu phase. In the Baiu phase, the large-scale circulation systems, such as the cut-off cyclones and blocking ridge in the northern latitudes, westward extending Pacific Subtropical anticyclone, monsoon westerly, and subtropical jet stream, are properly maintained. Only under this large-scale condition, a realistic Baiu frontal zone is formed in the model. The structure of the Baiu front, the features of precipitation, and the thermal stratification around the frontal zone are reasonably reproduced. In contrast, in the non Baiu phase, features in the large-scale field are significantly different from the observed features in the break period, while the continent-ocean thermal contrast is reasonably maintained. Judging from the results, the formation of the Baiu front does not only depend on the direct local effects of physical processes, and continent-ocean thermal contrast, but depends on the maintenance of large-scale environmental circulation systems favorable to sustain the Meiyu-Baiu front. 1. Introduction The summer monsoon rainfalls over East Asia are identified as Meiyu in China, and Baiu in Japan. The Meiyu-Baiu rainfalls are characterized by the west to east oriented narrow rain band forms with a stationary front, which is called the Meiyu-Baiu front (e.g., Ninomiya and Murakami 1987; Tao and Chen 1987; Ding 1991; Ninomiya and Akiyama 1992). Corresponding author: Kozo Ninomiya, Frontier Research System for Global Change, Yokohama 236-0001 Japan. E-mail: nmiya@jamstec.go.jp 1 Present affiliation: Earth Simulator Center ( 2002, Meteorological Society of Japan Recent progress of the global climate models enables us to make simulation studies of various climate systems. However, the simulation study of the Meiyu-Baiu front has not been performed in depth. A few reports (e.g., Sugi et al. 1995) indicate that representation of the Meiyu-Baiu front is difficult in an AGCM with a low resolution (T42L21; the maximum zonal wave-number is 42 in triangular wave transformation, and the number of the vertical level is 21). Some experiments with higher resolution (e.g., Kar et al. 1996 by T106L21) also show insufficient precipitation in the Meiyu- Baiu front. Kawatani and Takahashi (2000) show that a T106L20 model can simulate properly the Meiyu-Baiu frontal precipitation for

698 Journal of the Meteorological Society of Japan Vol. 80, No. 4 certain periods, though the precipitation in July is insufficient. Yoshikane et al. (2001) studied the effect of the continent-ocean thermal contrast on the Meiyu-Baiu front in a regional model. They conclude that the Meiyu-Baiu front is formed and sustained by the thermal contrast between the continent and ocean, and that the effect of large-scale topography is subordinate. Emori et al. (2001) report that the insufficient Meiyu- Baiu precipitation in a regional model is due to the insufficient moisture transport from the subtropical zone to the frontal zone, since the moisture in the subtropical zone is overmuch consumed by strong precipitation. They demonstrate that the frontal precipitation increases if the precipitation in the subtropical zone is reasonably suppressed through the tuning of the convective parameterization scheme. However, in these previous studies, the distribution of the simulated precipitation has been mainly discussed without the detailed analysis on circulation systems in, and around, the Meiyu-Baiu front. We need detailed examination of the simulated Meiyu-Baiu front in comparison with observational studies to evaluate the simulation, and also to further the knowledge about the Meiyu-Baiu front in the real atmosphere. In the present report, we will study features of the Meiyu-Baiu front and its associated circulation systems simulated in the T42L52 model in comparison with the features described in several observational studies. Interannual variations of Meiyu-Baiu, or sensibility experiments, are not the subject of the present report. 2. Model and simulation The model used in the present study, T42L52, is the high vertical resolution version of CCSR/NIES T42L20, which is developed by the Center for Climate System Research of the University of Tokyo, and the National Institute of Environmental Sciences ( Numaguti et al. 1997). Although physical process schemes used in T42L52 are almost the same as these used in T42L20, some tuning is needed when the vertical resolution is increased. The simplified Arakawa-Schubert convective scheme is modified so as not to increase the cloud base mass flux when the vertical resolution is increased. For this, the cloud base mass flux is assumed to be proportional to the thickness of the detrainment layers. The detail of this modification will be discussed in the separate report. The thickness of the model layers is @40 m in the lower troposphere, but increased gradually to @600 m in the lower stratosphere, with the top at @1 hpa. We expect that the important features in the planetary boundary layer will be simulated reasonably by this vertical resolution. The horizontal resolution of T42, which corresponds to @2:8 Gaussian grid, will be able to simulate circulation systems of synoptic-scale. Of course, this horizontal resolution is insufficient to reproduce meso-a-scale convective systems, which play an important role to form intense precipitation in the frontal zone (Akiyama 1990; Ninomiya and Akiyama 1992; Ninomiya 2000). The integration is performed with climatological SST (the sea surface temperature). The results obtained at 6-hour interval, in the fourth year integration after the quasi-equilibrium state, are used for the present analysis. 3. Observed features of the Meiyu-Baiu front and associated circulation systems Basic features of the Meiyu-Baiu front, and its associated circulation systems, which are to be simulated in the model, are first summarized. Although observational studies for the Meiyu-Baiu season of 1968 (Akiyama 1973), 1979 (Ninomiya and Muraki 1986), 1982 (Akiyama 1989) and 1991 (Ninomiya 2000) indicate significant intraseasonal and interannual variations of the Meiyu-Baiu, the following common features in the typical Meiyu- Baiu period are pointed out from articles listed in the references: (1) Quasi-periodic alternations of the active and inactive phase of precipitation appear during a Meiyu-Baiu season; (2) During the active phase of Meiyu-Baiu, the intense precipitation zone of 300 500 km width extends from the eastern foot of the Tibetan Plateau to the western North Pacific, along 30 35 N latitude circle. The 10-day averaged precipitation reaches 20 40 mm d 1, during the active phase. Precipitation is very small to the north and the

August 2002 K. NINOMIYA, T. NISHIMURA, W. OHFUCHI, T. SUZUKI and S. MATSUMURA 699 south of the Meiyu-Baiu front. The large precipitation in the active phase results in the formation of the Meiyu-Baiu precipitation zone even in the monthly mean maps; (3) In general, convections are more active in the western part of the precipitation zone; (4) The westward extending North Pacific subtropical anticyclone is an important feature seen in the active Meiyu-Baiu period. The confluence/convergence of the Indian monsoon westerly and the Pacific trade wind easterly results in the strong southerly moisture transport along the western rim of the subtropical anticyclone; (5) The Meiyu-Baiu precipitation zone is situated to the south of the subtropical jet stream, and to the north of the Meiyu-Baiu low-level jet stream; (6) The ascent motion, strong apparent moisture sink and strong apparent heat source are found in the precipitation zone. Nearly moist neutral stratification is maintained in the Baiu precipitation zone, while moist unstable stratification is sustained in the Meiyu precipitation zone; (7) The Meiyu front forms in the boundary between the moist maritime tropical airmass and dry hot continental airmass. Airs of higher temperature cover wide area to the north of the frontal zone, and therefore the reverse thermal gradient is seen in the north side of the front, since the frontal zone is cooled by precipitation. There is a baroclinic zone in @50 N, between the hot continental airmass and the cold polar airmass; (8) The Baiu front forms between the tropical maritime airmass and the polar maritime airmass. Southwestward intrusion of the polar airs to the north side of the Baiu front is an important feature, and, (9) In the northern latitude zone (45 60 N), a blocking ridge tends to develop around 140 E during the active phase of Meiyu- Baiu, although its longitudinal location varies from case to case. Cut-off lows developed to west (over Siberia) and east (over the Aleutian Islands or the western North Pacific) of the blocking ridge. Figure 1 summarizes the aforementioned large-scale features associated with the active Fig. 1. Features of large-scale circulation systems observed in the active Meiyu- Baiu period. The shaded area indicates the highland area higher than 3000 m. Fig. 2. Latitude-time section of TBB (cloud top equivalent blackbody temperature) along 129.5 E for the period between 25 June and 15 July 1991. The isopleths of TBB are given at 10 C interval. The minus sign is omitted from the labeled numeral. The shading indicates cloud areas where TBB is colder than 30 C. Meiyu-Baiu front. The latitude-time section of TBB (cloud top equivalent blackbody temperature) obtained from GMS (Geostationary Meteorological Satellite) IR observation, along 129.5 E meridian for Baiu period in 1991 is presented in Fig. 2 as a typical example of the Baiu front. The Baiu frontal cloud zone, which is clearly separated from the ITCZ cloud zone, and middle-high latitude cloud systems, is sustained as a quasi-stationary cloud zone.

700 Journal of the Meteorological Society of Japan Vol. 80, No. 4 Fig. 3. The latitude-time section of the simulated precipitation rate (unit in mm d 1 ) along 135 E in June and July. Isopleths are given for 5, 10, 20, 40, 60 and 80 mm d 1. Fig. 4. The monthly averaged simulated precipitation for June and July. Isopleths are given for 2, 4, 6, 8, 12, 16 and 20 mm d 1. 4. Overall features of precipitation in the model Overall features of the simulated precipitation are examined by showing the latitude-time section of precipitation along 135 E meridian for June and July (Fig. 3). Peaks of the precipitation in 40 70 N appear quasi-periodically with an interval of about 5 days. The precipitation maps, and surface maps (not presented), indicate that these precipitation peaks are associated with the synoptic scale depressions passing over the northern latitudes. Precipitation in 10 20 N is sustained almost continuously during June and July. A few imminent peaks of precipitation exceeding 20 40 mm d 1, are accompanied by depressions in the tropical latitudes. In 30 40 N, the Baiu precipitation zone is not sustained as a separated quasi-stationary precipitation zone, but forms sporadically during the studied period. At 135 E, the Baiu precipitation zone is clearly seen only in the limited periods, which are 2 6 June, 11 19 June, 13 18 July, and 20 27 July of the simulated year. Consequently, the Meiyu-Baiu precipitation zone is not well reproduced in the monthly averaged field for June and July in our model (Fig. 4). This is due to alternation of relatively short Baiu phase and the longer non- Baiu phase. Therefore, the evaluation for the monthly or seasonal mean field would not have significant meaning. Evaluation, and examination, of the simulation for both Baiu phase and the longer non-baiu phase is needed. 5. Features in model Baiu phase in late July By inspecting maps of simulated precipitation at 24-hour interval, a few active Meiyu- Baiu phases are identified. The 8-day period

August 2002 K. NINOMIYA, T. NISHIMURA, W. OHFUCHI, T. SUZUKI and S. MATSUMURA 701 of 20 27 July is selected as one of the Baiu phases. In the present section, the features in this phase will be studied in detail. The simulated precipitation, sea-level pressure, and 500-hPa heights averaged for this phase are presented in Fig. 5. A precipitation zone of @500 km width elongates from the eastern foot of the Tibetan Plateau to the western North Pacific along @35 N latitude circle. The areas with small precipitation extend to both north and south of the precipitation zone. It is noted that the large precipitation also appears over the northern latitudes in this simulation. Sometimes, relatively large precipitation occurs in the real atmosphere in the northern latitudes in association with the southerly flow, when the mid-latitude depression developed to the north of the Meiyu-Baiu front (Ninomiya 2001). However, the model precipitation in the northern latitude is too large as compared with the observed precipitation in the Meiyu-Baiu season. In the sea-level pressure map (Fig. 5B), we find vast low-pressure area over the Continent, the westward protruding Pacific subtropical anticyclone, and quasi-stationary low over the northwestern Pacific, the Okhotsk anticyclone and low-pressure zone along the precipitation zone. The height field at 500 hpa ( Fig. 5C) indicates the westward extending subtropical anticyclone, a blocking ridge over 140 E, and cut-off lows over the Siberia and the Bering Sea. These features are commonly seen in the active Baiu period in the real atmosphere. This indicates that proper reproduction of largescale circulations is important for the simulation of the Meiyu-Baiu precipitation zone. The simulated wind velocity at 850 hpa, air temperature at 850 hpa and the specific humidity at 700 hpa are shown in Panel A, B and C in Fig. 6, respectively. The wind field in this phase is characterized by the strong southwesterly/westerly along the rim of the Pacific subtropical anticyclone. This low-level strong wind zone is signified as the Meiyu-Baiu low-level jet stream, in many observational studies. The circulation at 850 hpa has strong influence on the thermal and moisture distribution in the lower troposphere through the advection. The strong southwesterly/westerly along the rim of the Pacific subtropical anticyclone maintain large moisture gradient in the Fig. 5. A: Averaged simulated precipitation in the Baiu phase in 20 27 July. Isopleths are given for 2, 4, 6, 8, 12, 16 and 20 mm d 1. B: Averaged surface pressure. Isobars are given at 2-hPa intervals. Row of the blacked circles indicates axis of precipitation zone. C: Averaged height at 500 hpa. Contours are given at 30 m interval. Row of the arrowheads shows axis of 200 hpa jet stream with wind speed more than 25 m s 1. The shaded area indicates the highland area higher than 3000 m.

702 Journal of the Meteorological Society of Japan Vol. 80, No. 4 Fig. 6. A: Averaged wind velocity at 850 hpa (unit: m s 1 ) in the Baiu phase in 20 27 July. B: Averaged temperature at 850 hpa (unit: C). The areas warmer than 20 C are indicated by hatch. C: Averaged specific humidity at 700 hpa (unit: g kg 1 ). Meiyu-Baiu frontal zone through the moisture advection. The southerly flow, associated with the cyclonic circulation centered over @55 N/ 110 E, transports the warm and moist airs from @45 N/120 E to the northern latitudes. The anticyclonic circulation centered over @45 N/160 E sustains the large thermal gradient along the rim of the maritime polar airmass. At 850 hpa, the Meiyu frontal zone (@35 N) is relatively cool (18 20 C). This is due to the cooling by precipitation and decrease of the insolation by large cloud cover. It is noted that the weak thermal gradient between the southern China and the Meiyu frontal zone (@35 N, 110 120 E) is not due to the continent-ocean thermal contrast, but due to the relatively cool precipitation zone. The area to the north (40 45 N) of the Meiyu front is significantly hot (@24 C), due to the adiabatic warming associated with the subsiding motion. Therefore, the reversed temperature gradient (north is warmer) appears to the north of the Meiyu front. This reversed temperature gradient is usually seen for the real Meiyu front. The simulated specific humidity at 700 hpa ( Fig. 6C) is consistent with the precipitation (Fig. 5A), and vertical velocity (Fig. 7A). The zone of ascent motion elongates over the precipitation zone, while areas of significant subsiding motion appears over the area of smaller specific humidity to both north and south of the precipitation zone. The zone of large total heat source Q 1 (Q 1 is not the apparent heat source, but the total heat source calculated in the model) at 500 hpa (Fig. 7B), which is mainly due to the release of latent heat, is seen over the areas of large precipitation. It is evident that the strong anticyclonic flow at 200 hpa (Fig. 7C) is sustained just over the large heat source region. The Tibetan anticyclone, and the anticyclonic zone elongating over the Meiyu- Baiu precipitation zone, indicate strong and direct link between the heating in the midtroposphere and the anticyclonic circulation in the upper troposphere. The relation between the precipitation and the vertical stability shall be examined next. The simulated convective precipitation, which is calculated by the cumulus parameterization scheme, and large-scale condensation are shown in Fig. 8A and 8B. The vertical stability ðqye/qpþ in the 850 500 hpa layer are presented in Fig. 8C. It is the interesting result that the large portion of precipitation in the Meiyu front, and the western part of the Baiu

August 2002 K. NINOMIYA, T. NISHIMURA, W. OHFUCHI, T. SUZUKI and S. MATSUMURA 703 Fig. 7. A: Averaged vertical velocity o (unit: hpa h 1 ) in the Baiu phase in 20 27 July. B: Averaged heat source Q 1 at 500 hpa (unit: K d 1 ). C: Averaged wind velocity at 200 hpa (unit: m s 1 ). front, are caused by the convective precipitation, while the contribution by the large-scale precipitation is relatively large over the eastern part of the Baiu front, and over the northern latitudes. The natures of the simulated precipitation are consistent with the observed features of precipitation. Fig. 8. A: Averaged convective precipitation (unit: mm d 1 ) in the Baiu phase in 20 27 July. B: Averaged large-scale precipitation (unit: mm d 1 ). C: Averaged vertical stability ðqye/qpþ in 850 500 hpa layer. Unit is in K (100 hpa) 1. The airs over the Asian Continent indicate strong convective instability (negative value of ðqye/qpþ), while maritime airs show stable stratification. Almost moist neutral stratification is sustained in the convective area of the Baiu front, while the strong convective insta-

704 Journal of the Meteorological Society of Japan Vol. 80, No. 4 bility is maintained in the Meiyu front against the release of the instability due to the cumulus convection. We also note convectively unstable airs to the north (@40 N, 110 E), and the south (@23 N, 107 E), of the Meiyu front. In these areas, the convective instability is not released since the subsidence suppresses the development of convections. The simulated surface air temperature, sensible heat flux, and the latent heat flux at the surface, are presented in Fig. 9. The large longitudinal thermal contrast between the continent and the ocean is seen in the regions north to @35 N, while largest meridional thermal gradient in 30 40 N is found over the western North Pacific, between the tropical maritime airmass and polar maritime airmass ( Fig. 9A). Time averaged sensible heat flux is generally small over the oceanic areas, and the wet land areas (rainfall areas), while large heat flux (80 100 W m 2 ) is seen over the arid land areas (40 50 N, 90 110 E). The latent heat flux in the polar marine area is very small, whereas the latent heat flux over the subtropical and tropical oceanic areas is large, with maximum of @180 W m 2 over the ITCZ. Significantly larger latent heat flux (80 160 W m 2 ) is evaluated over the continent, except for the arid region. A simulation study related to GEWEX Global Soil Wetness Project (Chen and Mitchell 1999) obtain latent heat flux of 90 120 W m 2 over southern China in July. However, the latent heat flux obtained over the continent in the middle and higher latitudes, is 30 60 W m 2. The latent heat flux over the continent, obtained in the present study, is considerably larger as compared with their simulation. The large precipitation over the continent in the northern latitudes will be related to the large latent heat flux. Comparison with the observed latent heat flux is one of the remaining problems. Meridional vertical section along 132.2 Eof zonal wind velocity u, meridional wind velocity v, vertical velocity o, and equivalent potential temperature ye, are presented in Fig. 10. While the subtropical jet stream is located at 200 hpa/42 N, the low-level zonal wind maximum in 700 800 hpa is seen at @34 N (Fig. 10A). The maximum easterly wind at 70 hpa is located at @20 N. Although the features of simulated zonal wind consist with the observed Fig. 9. A: Averaged surface air temperature (unit: C) in the Baiu phase in 20 27 July. B: Averaged sensible heat flux (unit: Wm 2 ). C: Averaged latent heat flux (unit: Wm 2 ). features, the low-level westerly, and the upperlevel easterly, are weaker than those observed in a typical Meiyu-Baiu period ( Ninomiya 2000). The low-level southerly wind to the south of the Meiyu-Baiu precipitation zone, upper-level (@100 hpa) northerly wind over the

August 2002 K. NINOMIYA, T. NISHIMURA, W. OHFUCHI, T. SUZUKI and S. MATSUMURA 705 Fig. 10. Meridional vertical section along 132.2 E. A: Averaged zonal wind velocity u (unit: m s 1 ) in the Baiu phase in 20 27 July. B: Averaged meridional wind velocity v (unit: m s 1 ). C: Averaged vertical velocity o (unit: hpa h 1 ). D: Averaged equivalent potential temperature ye (unit: K). precipitation zone, and the ascent motion in the precipitation zone are significantly weaker than the observations in the typical Meiyu-Baiu period. This indicates that the simulated vertical circulation in and around the precipitation zone is considerably weaker than the actual circulation. The simulated equivalent potential temperature indicates nearly moist-neutral stratification in the 600 800 hpa layer over @35 N. However, the depth of the simulated neutral layer is considerably shallow, as compared with that in the typical Meiyu-Baiu period. 6. Vertical structure of the lower troposphere In this section, the vertical stratification in the lower troposphere in, and around, the simulated precipitation zone is examined by using the data at s-level, which is defined by s ¼ðp/p sfc Þ. Vertical distribution of time averaged ðs/c p Þ, ðh/c p Þ and ðh /c p Þ at selected gridpoints (see Fig. 11 for location of the gridpoints) are presented in Fig. 12, where s, h, h, and c p indicate static energy, moist static en-

706 Journal of the Meteorological Society of Japan Vol. 80, No. 4 Fig. 11. The selected grid points on which the vertical distributions of simulated ðs/c p Þ, ðh/c p Þ, and ðh /c p Þ averaged for Baiu phase in 20 27 July are presented. ergy, saturated moist static energy, and specific heat at constant pressure, respectively. It is noted that we have the following relation ðt/yþqy/qz ¼ð1/c p Þqs/qz; from the definition of s and y. We also have following approximation ( Holton 1992) ðt/yeþqye/qzað1/c p Þqh/qz; from the definition of h and ye. Figure 12A shows data at 43.3 N/95.6 E (continental arid region to the north of Meiyufront). Although there is large diurnal variation at this point (data is not presented), the time averaged profiles indicate very weak sta- Fig. 12. Vertical distribution (vertical axis: s ¼ðp/p sfc Þ) of simulated ðs/c p Þ, ðh/c p Þ, and ðh /c p Þ,in unit of K, averaged for Baiu phase in 20 27 July at 43.3 N/95.6 E (A), 34.9 N/135.0 E (B), 26.5 N/112.5 E (C), and 26.5 N/146.3 E (D).

August 2002 K. NINOMIYA, T. NISHIMURA, W. OHFUCHI, T. SUZUKI and S. MATSUMURA 707 ble stratification, in the very dry lower troposphere. Figure 12B shows data at 34.9 N/135 E (within the Baiu precipitation zone). The data at this point indicates the weak convectively unstable stratification. The value of ðh/c p Þ in the lowermost layer slightly exceeds the value of ðh /c p Þ in the layer above s-level of @0.9. The features seen in Fig. 12B are consistent with the observed features in the typical Baiu precipitation zone. Data at 26.5 N/112.5 E (continental moist region to the south of the Meiyu precipitation zone; Fig. 12C), and data at 26.5 N/146.3 E (the subtropical Pacific; Fig. 12D) indicate features of the mixed layer, caped by a stable layer. The convective instability over these regions is not released, since the convective motion is suppressed by the strong subsidence, and therefore, these regions play the role of the moisture source region to the precipitation zone. The resulted profiles of averaged ðs/c p Þ, ðh/c p Þ and ðh /c p Þ seem to be reasonable from our observational knowledge. However, the thickness of the mixed layer at point D is significantly thin as compared with observations. The detailed observation about the mixing layer in, and around, Meiyu-Baiu precipitation zone have not been made, and therefore, more detailed comparison with observations is need in future. 7. Features of the model Baiu phase in early June The Meiyu-Baiu season continues, climatologically speaking, about two months, and the latitudinal location of the precipitation shifts northward with the seasonal march. The stability in the lower troposphere and therefore, the nature of the precipitation also indicate seasonal change, from the stratiform rain to the convective precipitation. Therefore, it will be worth to examine the features of the model Meiyu-Baiu phase in early June (1 6 June), in comparison with the model Meiyu-Baiu phase in late July. Figure 13 presents simulated precipitation, sea-level pressure and 500-hPa height averaged in the Meiyu-Baiu phase in 1 6 June. In this phase, the precipitation zone extends from the eastern foot of the Tibetan Plateau to the western North Pacific along 25 30 N (Fig. Fig. 13. A: Averaged simulated precipitation in the Baiu phase in 1 6 June. Isopleths are given for 2, 4, 6, 8, 12, 16 and 20 mm d 1. B: Averaged surface pressure. Isobars are given at 2-hPa intervals. Row of the blacked circles indicates axis of precipitation zone. C: Averaged height at 500 hpa. Contours are given at 30 m interval. Row of the arrowheads shows axis of 200 hpa jet stream with wind speed more than 25 m s 1. The shaded area indicates the highland area higher than 3000 m.

708 Journal of the Meteorological Society of Japan Vol. 80, No. 4 13A). The maximum precipitation appears over the southeastern part of China (@29 N, 115 E). In this phase too, the zones with small precipitation extends to the south and the north to the precipitation zone. Another precipitation zone forms over the Asian Continent along @50 N latitude circle. This northern precipitation zone is seen along the boundary between the hot continental airmass, and the cool polar airmass (map of the surface temperature is not presented). The sea-level pressure field shows features seen around the real Meiyu-Baiu front with intense precipitation. That is, the westward extending Pacific subtropical anticyclone, a westeast elongating surface trough along the precipitation zone, quasi-stationary Aleutian low, the Okhotsk anticyclone (over @44 N, 155 E), and the vast low-pressure area over the continent. The 500-hPa heights also indicate features associated with active Meiyu-Baiu front. We see the westward extending Pacific subtropical anticyclone, a blocking ridge centered over @60 N/140 E, and two cut-off lows to the west and east of the blocking ridge. In this phase too, the reproduction of proper large-scale circulation systems is necessary for simulation of realistic Meiyu-Baiu precipitation zone. In this phase, convective precipitation (Fig. 14A) is significantly smaller than the largescale precipitation ( Fig. 14B). This is consistent with the observational fact that the Meiyu-Baiu precipitation indicates more convective natures in the later period of the rainy season. The map of vertical stability ðqye/qpþ in the 850 500 hpa layer ( Fig. 14C) shows the convectively unstable zone, only in the southern part of the precipitation zone over China. Figure 15 presents wind velocity at 850 hpa, total heat source Q 1 at 500 hpa (unit: K d 1 ), and wind velocity at 200 hpa. The typical Meiyu-Baiu low-level jet stream extends along the southern side of the precipitation zone, while the subtropical jet stream extends to the north of the precipitation zone. Significant anticyclonic circulation at 200 hpa appears just over the area of maximum heating over southern China, and the northern part of the Indochina Peninsula. In this phase, the subtropical jet stream is located over @35 N, while it located over @41 N in the Baiu phase in late July. Fig. 14. A: Averaged convective precipitation (unit: mm d 1 ) in the Baiu phase in 1 6 June. B: Averaged large-scale precipitation (unit: mm d 1 ). C: Averaged vertical stability ðqye/qpþ in 850 500 hpa layer. Unit is in K (100 hpa) 1. Meridional vertical section along 115.3 Eof zonal wind velocity u, meridional wind velocity v, vertical velocity o, and equivalent potential temperature ye are shown in Fig. 16. While the subtropical jet stream is located at @200 hpa/

August 2002 K. NINOMIYA, T. NISHIMURA, W. OHFUCHI, T. SUZUKI and S. MATSUMURA 709 Fig. 15. A: Averaged wind velocity at 850 hpa (unit: m s 1 ) in the Baiu phase in 1 6 June. B: Averaged heat source Q 1 at 500 hpa (unit: K d 1 ). C: Averaged wind velocity at 200 hpa (unit: m s 1 ). 35 N, the low-level zonal wind maximum in 700 800 hpa is seen at @26 N (Fig. 16A). The low-level southerly wind to the south of the precipitation zone, upper-level (@100 hpa) northerly wind over the precipitation zone, and the ascending motion in the precipitation zone are reasonably reproduced as compared with the observations in the typical Meiyu-Baiu front. The simulated equivalent potential temperature indicates quasi-moist-neutral stratification in 600 800 hpa layer over @27 N, not in the midst of the precipitation zone, but its southern part. The continent-ocean thermal contrast in this phase is briefly noted. The surface temperature (not shown) indicates strong thermal gradient over the border of the continent. The sensible heat flux is small over the oceanic areas, while large sensible heat flux of 20 40 W m 2 is seen in the wide areas over the continent (map is not presented). Large sensible heat flux of @100 W m 2 appears in the dry zone in 40 45 N. The latent heat flux is large (60 160 Wm 2 ) over the tropical and subtropical oceanic areas (map is not presented). The latent heat flux of 20 100 W m 2 is seen over the continent. Especially large flux of @120 W m 2 is seen over @35 N/105 120 E, immediately north of the precipitation zone. 8. Features in non-baiu phase in July As already pointed out, the Meiyu-Baiu precipitation zone is properly sustained only in the limited periods in the model. Consequently, the Meiyu-Baiu precipitation zone is poorly reproduced in the monthly averaged field for June and July (Fig. 4). This defect is due to alternation of the relatively short Baiu phase, and the longer non-baiu phase. Therefore, the evaluation and examination of the simulation for the non-baiu phase is also needed. It is seen in Fig. 3 that the precipitation in 10 25 N increases during the non-baiu phase. Although the features in the non-baiu phase varies according to cases, the non-baiu phase in 1 13 July is selected for the discussion in this section, since this non-baiu phase in the model continues during the peak period of the real Meiyu-Baiu period. Figure 17 presents simulated precipitation, surface pressure, and height at 500-hPa, averaged in the non-baiu phase in 1 13 July. The simulated wind velocity at 850 hpa and 200 hpa, and the simulated specific humidity at 700 hpa are shown in Fig. 18. The simulated large-scale circulations are different from the circulations simulated for the Baiu phase in the model. Further, the simu-

710 Journal of the Meteorological Society of Japan Vol. 80, No. 4 Fig. 16. Meridional vertical section along 129.4 E. A: Averaged zonal wind velocity u (unit: m s 1 ) in the Baiu phase in 1 6 June. B: Averaged meridional wind velocity v (unit: m s 1 ). C: Averaged vertical velocity o (unit: hpa h 1 ). D: Averaged equivalent potential temperature ye (unit: K). lated large-scale circulations for this period are significantly different from the observed circulations in the real break period of Meiyu-Baiu. It is therefore difficult to interpret the features of the non-baiu phase simulated in the model in comparison with the observed features in the break period. Large-scale circulation in this phase is characterized by the eastward retreat of the Pacific subtropical anticyclone, and disappearances of the blocking ridge and cut-off lows from the northwestern Pacific (@50 N, 160 E), and the continent (@65 N, 90 E). On the surface map, a predominate ridge extends from the western North Pacific (@45 N, 165 E) to the Japan Sea, and further to the Yellow Sea. The lowpressure center, which is located over @55 N/ 120 E in the Baiu phase, is centered over @45 N/100 E in this phase. The strong pressure gradient between the low-pressure area over the continent and the ridge, results in very strong southerly wind around 120 E. In the lower latitude zone (10 20 N), the Indian monsoon westerly extends to 145 E. Consequently, the confluence and convergence zone between the Indian monsoon westerly, and

August 2002 K. NINOMIYA, T. NISHIMURA, W. OHFUCHI, T. SUZUKI and S. MATSUMURA 711 Fig. 17. A: Averaged simulated precipitation in the non-baiu phase in 1 13 July. Isopleths are given for 2, 4, 6, 8, 12, 16 and 20 mm d 1. B: Averaged surface pressure. Isobars are given at 2-hPa intervals. Row of the blacked circles indicates axis of precipitation zone. C: Averaged height at 500 hpa. Contours are given at 30 m interval. Row of the arrowheads shows axis of 200 hpa jet stream with wind speed more than 25 m s 1. The shaded area indicates the highland area higher than 3000 m. Fig. 18. A: Averaged wind velocity at 850 hpa (unit: m s 1 ) in the non-baiu phase in 1 13 July. B: Averaged specific humidity at 700 hpa (unit: g kg 1 ). C: Averaged wind velocity at 200 hpa (unit: m s 1 ). the Pacific easterly trade wind, shifts eastward. In the upper troposphere, the subtropical jet stream axis is along @45 N, but its maximum wind speed is very weak (@25 m s 1 at 90 E, @15 m s 1 at 120 E, and @30 m s 1 at 150 E). The Baiu precipitation zone is not produced in the model. Instead, the model produces four

712 Journal of the Meteorological Society of Japan Vol. 80, No. 4 Fig. 19. Averaged temperature at 850 hpa (unit: C) in the non-baiu phase in 1 13 July. The areas warmer than 20 C are indicated by hatch. precipitation zones. The first one, in 15 20 N/ 90 140 E, is accompanied by the eastward protruding Indian monsoon westerly. The second one, seen around 30 N/150 E, may be regarded as the Baiu precipitation zone, since it is formed along the northwestern rim of the Pacific subtropical anticyclone. The third one, around 30 N/110 E, is along the northern border of the southerly winds. The forth one is seen in 60 65 N zone over eastern Siberia. Precipitation is very small over the Sea of Okhotsk, the Sea of Japan, the Yellow Sea and the East China Sea, which are covered by the surface ridge. Figure 18B (the simulated specific humidity at 700 hpa) also shows the moist zone over 20 N/90 145 E, while the moist zone, which is usually seen over the Japan Islands, disappears in this phase. The temperature distribution at 850 hpa simulated for this phase ( Fig. 19), is significantly different from that for the Baiu phase (Fig. 6B). The area over 45 60 N/140 E indicates significantly higher temperature associated with the westerly winds from the hot continent to the cool oceanic area, while that over 60 N/ 90 E is associated with southerly winds over the continent. Figure 20 presents simulated surface air temperature, sensible heat flux and latent heat flux averaged in this phase. The thermal contrast at the surface, between the continent and the oceanic areas (Fig. 20A) in the non Baiu Fig. 20. A: Averaged surface air temperature (unit: C) in the non-baiu phase in 1 13 July. B: Averaged sensible heat flux (unit: Wm 2 ). C: Averaged latent heat flux (unit: Wm 2 ). phase is almost the same as that found for the Baiu phase (Fig. 9A). The distribution of sensible heat flux in this phase (Fig. 20B) is also almost the same as that obtained for the Baiu phase (Fig. 9B). Significant difference between the non-baiu phase in 1 13 July and the

August 2002 K. NINOMIYA, T. NISHIMURA, W. OHFUCHI, T. SUZUKI and S. MATSUMURA 713 Fig. 21. The difference between the non- Baiu phase (1 13 July) and the Baiu phase (20 27 July) in the model. A: The difference of sea-level pressure (unit: hpa). B: The difference of precipitation (unit: mm d 1 ). C: The difference of wind velocity at 850 hpa (unit: m s 1 ). Baiu phase in 20 27 July, is found only in the latent heat flux over the Pacific in15 20 N ( Fig. 20C, in comparison with Fig. 9C). A strong Indian monsoon westerly during 1 13 July, results in the large latent heat flux (maximum @280 W m 2 ) in this zone, which works to increase the precipitation, and condensation heat source in this zone. It is noted that almost the same continent-ocean thermal contrast is found for both the Baiu phase and the non- Baiu phase. 9. Comparison of no Baiu phase with Baiu phase The difference between the non-baiu phase (1 13 July) and the Baiu phase (20 27 July), will be studied in detail. Figure 21 presents the difference of the sea level pressure, the precipitation, and 850-hPa-wind velocity averaged for each phase. Figure 22 shows the difference of the 850-hPa temperature, and the 200-hPa wind velocity, respectively. Although, there are many differences between the non-baiu and the Baiu phases, the following features are noted as the most remarkable differences. The most significant differences are seen in 20 30 N in 90 150 E, where the decrease of the sea level pressure, with the increase of cyclonic circulation, is found in the non-baiu phase. This means the weakening, or eastward retreat, of the North Pacific subtropical anticyclone. At the same time, the westerly in 10 20 N increases, while the southwesterly in the 20 40 N zone decreases. Many observational studies (e.g., Akiyama 1973; Ninomiya 1984 and 2000), demonstrate that the confluence/convergence of the Indian westerly monsoon with the southerly winds along the western rim of the North Pacific anticyclone, is the moist important process to sustain the Meiyu-Baiu front. They also show that the moisture influx toward the Meiyu-Baiu frontal zone, which is due to the southwesterly wind along the western rim of the anticyclone, is an important process to sustain the Meiyu- Baiu precipitation. Suppression of the convection in the anticyclone is also important to inhabit the consumption of water vapor over the moisture source region. This means that the Meiyu-Baiu precipitation zone cannot be sustained if the Pacific subtropical anticyclone is not formed in the proper location. In this regard, the eastward retreat of the Pacific subtropical anticyclone is the key feature of the non-baiu phase in the model. The second significant feature is the warm area at 850 hpa over 45 60 N/140 E (Fig.

714 Journal of the Meteorological Society of Japan Vol. 80, No. 4 Fig. 22. The difference between the non- Baiu phase (1 13 July) and the Baiu phase (20 27 July) in the model. A: The difference of temperature at 850 hpa (unit: C). B: The difference of wind velocity at 200 hpa (unit: m s 1 ). 22A). In the actual Baiu phase, northeasterly winds in the polar maritime airs, to the east of the Japan Islands, advect the cold air and sustain the thermal gradient of the Baiu frontal zone in its northern side (Ninomiya and Muraki 1986; Ninomiya 2000). In the non-baiu phase, the westerly winds advect the warm continental air and thus, the thermal gradient over the Japan Sea is not sustained properly. Of course the aforementioned circulations are associated with disappearance of the blocking ridge, and quasi-stationary low, over the northwestern Pacific, and accompanied by formation of the surface ridge, which extend from the Pacific to the continent over @45 N. The Baiu front is not formed, if both the blocking ridge around 140 E, and the quasi-stationary low over the northwestern Pacific, are not properly maintained. The third significant difference is the weakening of the subtropical jet stream over 30 35 N in the non-baiu phase. It has been reported that the Meiyu-Baiu precipitation zone is maintained to the south of the subtropical jet stream (e.g., Akiyama 1973; Ninomiya 1984 and 2000; Yoshikane et al. 2000). In this, the weakening of the subtropical jet stream in the model may have some effect on the disappearance of the Meiyu-Baiu precipitation zone. The upper-level anticyclonic circulation over 25 40 N, which is seen in the Baiu phase, is weakened remarkably in the non-baiu phase. Instead, in the non-baiu phase, the 200-hPa anticyclonic circulation is remarkably intensified over the precipitation zone over @20 N. Ninomiya (1984 and 2000) shows that the subtropical jet stream is locally intensified to the north of the intense Baiu precipitation zone. He also notes that the zone of strong anticyclonic vorticity forms over strong condensation heating. It is not clear whether the subtropical jet stream has influence on the Meiyu-Baiu precipitation, or strong precipitation intensify the subtropical jet stream. 10. Conclusive remarks Global climate models have been used for simulation studies of various climate systems. However, the simulation study of the Meiyu- Baiu front has not been performed in detail. A few reports indicate that representation of the Meiyu-Baiu front is difficult in an AGCM with a low resolution. Some experiments with higher resolution also show insufficient precipitation in the Meiyu-Baiu front. However, these previous reports discuss the distribution of precipitation, without the analysis on features of circulation systems in and around the Meiyu-Baiu front. More detailed examination of the simulated Meiyu-Baiu front, in comparison with observational studies, is needed to evaluate the simulation, and also to further the knowledge about processes related to Meiyu-Baiu in the real atmosphere. In the present report, we therefore study features of the Meiyu-Baiu front and associated circulation systems simulated in the climatological SST run by the T42L52 model, which is

August 2002 K. NINOMIYA, T. NISHIMURA, W. OHFUCHI, T. SUZUKI and S. MATSUMURA 715 the high vertical resolution version of CCSR/ NIES T42L20. The Baiu front is not properly reproduced in the monthly averaged field for June and July. This is due to alternation of the relatively short Baiu phase and the longer non-baiu phase. In the Baiu phase, the large-scale circulation systems such as the cut-off cyclones and blocking ridge in the higher latitudes, westward extending Pacific subtropical anticyclone, the subtropical jet stream, and monsoon westerly are simultaneously and reasonably maintained. Only under this large-scale condition, a realistic Meiyu-Baiu frontal zone is formed in the model. The structure of the Meiyu-Baiu front is consistent with that described in observational studies. Features of precipitation in the frontal zone, the thermal stratification in and around the frontal zone are also reasonably reproduced. The large-scale circulation systems in the non-baiu phase are significantly different from those in the Baiu phase, while the continent-ocean thermal contrast, in the largescale, is reasonably maintained. It is noted that the features of the non-baiu phase, in the model, are different from those in the real break period of Baiu. Judging from our analysis, the representation of the Baiu front is not dependent only on the direct local effect of the physical processes, and continent-ocean thermal contrast, but also depends on accurate simulation of the largescale circulation systems such as the North Pacific subtropical anticyclone, blocking ridge and the quasi-stationary low over the northwestern Pacific. In this regard, we note the difference between the present study and the experiment by Yoshikane et al. (2001), who conclude that the Meiyu-Baiu front is formed, and sustained, basically by the thermal contrast between the continent and ocean. They perform experiments for 20-day integration period by using a regional model, with the initial value and boundary condition given by ECMWF data, or zonal-averaged ECMWF data. Meanwhile, our AGCM simulation indicate important role of the large-scale circulation systems to sustain the Meiyu-Baiu precipitation zone, in addition to the continent-ocean thermal contrast. We will agree Emori et al. (2001) about the importance of the subtropical zone as a moisture source region for the Meiyu-Baiu front. They demonstrate that the Meiyu-Baiu frontal precipitation increases, when the precipitation in the subtropical zone is reasonably suppressed through the tuning of the convective parameterization scheme. However, the precipitation in the subtropical anticyclone is reasonably controlled by the large-scale subsidence in the Baiu phase in this simulation, without a certain tuning of the cumulus parameterization. Unfortunately, the key to ameliorate the model, in regard to the simulation of the Meiyu-Baiu precipitation zone, has not yet been obtained, since dynamical processes to maintain the blocking ridge and cut-off lows in the northern latitudes, and the subtropical anticyclone, in both the numerical model, and the real atmosphere, cannot be studied in the present report. Nevertheless, the present paper has significant meaning, in that it is the first attempt to study the Meiyu-Baiu precipitation zone in the AGCM in detailed comparison with features found in observational studies. Acknowledgement The present authors would like to express their thanks to the joint climate modeling group of the Center for Climate System Research, the National Institute of Environmental Sciences, and the Frontier Research System for Global Change, for permitting the use of T42L52 for the present study. We also thank two anonymous reviewers for their valuable comments and advise. References Akiyama, T., 1973: The large-scale aspects of the characteristic features of the Baiu front. Pap. Meteor. Geophy., 24, 151 188., 1989: Large, synoptic and meso scale variations of the Baiu front, during July 1982. Part 1: Cloud features. J. Meteor. Soc. Japan, 67, 57 81., 1990: Large, synoptic and meso scale variations of the Baiu front, during July 1982. Part 2: Frontal structure and disturbances. J. Meteor. Soc. Japan, 68, 557 574. Chen, F. and K. Mitchell, 1999: Using the GEWEX/ ISLSCP forcing data to simulate global soil

716 Journal of the Meteorological Society of Japan Vol. 80, No. 4 moisture fields and hydrological cycle for 1987 1988. J. Meteor. Soc. Japan, 77, 167 182. Ding, Y.-H., 1991: Monsoon over China, Kluwer Academic Pub. 419 pp. Emori, S., T. Nozawa, A. Numaguti and I. Uno, 2001: Importance of cumulus parameterization for precipitation simulation over East Asia in June. J. Meteor. Soc. Japan, 79, 939 947. Kar, S.C., M. Sugi and N. Sato, 1996: Simulation of the Indian summer monsoon and its variability using the JMA global model. Papers Meteor. Geophy, 47, 65 101. Holton, J.R., 1992: An introduction to Dynamic Meteorology, third edition. Academic Press, 511 pp. Kawatani, Y. and M. Takahashi, 2000: Baiu front appearing in a high-resolution GCM simulation. Proceeding of The Second International Symposium on Asian Monsoon System, Cheju, Korea. March 27 31, 2000, 239 245. Ninomiya, K., 1984: Characteristics of the Baiu front as a predominant subtropical front in the summer northern hemisphere. J. Meteor. Soc. Japan, 62, 880 894., 2000: Large- and meso-a-scale characteristics of Meiyu-Baiu front associated with intense rainfalls in 1 10 July 1991. J. Meteor. Soc. Japan, 78, 141 157., 2001: Large l-shaped cloud zone formed around July 6, 1991 with pole-ward moisture transport from intense rainfall area in Meiyu-Baiu front. J. Meteor. Soc. Japan, 79, 805 813. and T. Akiyama, 1992: Multi-scale features of Baiu, the summer monsoon over Japan and the East Asia. J. Meteor. Soc. Japan, 70, 467 495. and T. Murakami, 1987: The early summer rainy season (Baiu) over Japan. Monsoon Meteorology, Edit. P.-C. Chang and T.N. Krishnamurti, Oxford Univ. Press, 93 121. and H. Muraki, 1986: Large-scale circulation over the East Asia during Baiu period of 1979. J. Meteor. Soc. Japan, 64, 409 429. Numaguti, A., M. Takahashi, T. Nakajima and A. Sumi, 1997: Description of CCSR/NIES atmospheric general circulation model. CGER Super Computer Monograph Report, No. 3, 1 48, Center for Global Environmental Research, National Institute for Environmental Studies. Sugi, M., R. Kawamura and N. Sato, 1995: The climate simulated by the global model. Part 1: Global features. Report of the National Research Institute for Earth Sciences and Disaster Prevention, No. 54, 155 180. Tao, S. and L. Chen, 1987: A review of recent research on the East Asia summer monsoon over China. Monsoon Meteorology, Edit. P.-C. Chang and T.N. Krishnamurti, Oxford Univ. Press, 50 92. Yoshikane, T., F. Kimura and S. Emori, 2001: Numerical study on the Baiu front generated by heating contrast between land ocean. J. Meteor. Soc. Japan, 79, 671 686.