Baiu Frontal Zone Research Articles

Multi-scale Interaction Processes Associated with Development of a Sub-Synoptic-Scale Depression on the Meiyu-Baiu Frontal Zone

A sub-synoptic-scale depression, with a wavelengtJh of 2000-3000 km, propagated along the Meiyu-Baiu frontal zone from China to Northwestern Pacific during 03-05 July 1991. We investigated the development and change in the structure of the frontal depression in detail, by using the Geostationary Meteorological Satellite (GMS) Infrared (IR) and objective global analysis (GANAL) data, of the Japan Meteorological Agency.This sub-synoptic-scale depression develops in association with low/middle-level vorticity maxima and meso-x-scale cloud systems. However, the simple one-to-one correspondence among them is not lasting for the whole life time of the depression, but the multiple coupling among the depression, vorticity cores and cloud systems occurs during the evolution process. The vertical structure of the depression varies in accordance with the low-level and middle-level vorticity cores, indicating the differentmovement and evolution in the coupling process.The present study thus demonstrates an example of multi-scale interactions related to the complicated evolution process of the sub-synoptic-scale depression on the Baiu frontal zone.

Formation Factors of 2004 Niigata-Fukushima and Fukui Heavy Rainfalls and Problems in the Predictions using a Cloud-Resolving Model

Localized, band-shaped heavy rainfall was observed over the Niigata-Fukushima area on July 13th 2004 and the over Fukui area on July 18th 2004. Both areas are located on the Japan-Sea side of the Japan Islands. These heavy rainfall events were a result of an intensification of convective instability over the Baiu frontal zone, induced by the inflows of low-level humid air and middle-level dry air. The middle-level air was considerably warm, not colder than the surrounding air. This indicates that the formation factors of heavy rainfall should be examined using convective instability, rather than potential instability.Numerical simulations using a cloud-resolving model with a horizontal grid of 1.5 km were carried out in an attempt to reproduce these heavy rainfall events. The Niigata-Fukushima heavy rainfall was reproduced well, while the Fukui heavy rainfall was not. This failure of the Fukui case could be the result of the uncertain analysis of the wind field over the Sea of Japan that determined the movement of low-level humid air. edevelopment of new observation systems over the sea, where upper-air sounding is seldom operated, are required to improve heavy rainfall predictions and to prevent such failures.

Recent Climate Trends in the East Asia During the Baiu Season of 1979-2003

Recent climate trends in the East Asia during the Baiu season (June) of 1979-2003 are investigated. The precipitation is significantly decreasing around Philippines and the northeastern part of China, while increasing along the Baiu frontal zone (the southeastern part of China and Japan) and the western part of Siberia. The pressure trend that corresponds to the precipitation has barotropic structure in the midlatitudes and baroclinic structure in the subtropics. For the barotropic structure to be maintained, the Rossby wave propagation from the western part of Siberia and Tibetan Plateau seems to play an important role. On the other hand, the trend of diabatic heating around Philippines and Japan is important for the baroclinic structure.The pressure trend is investigated by a linearized primitive model. The results support the above idea that the effect of the Rossby wave propatation is important for the barotropic pressure trend in the midlatitudes and that the trend of apparent diabatic heating is an important factor for the baroclinic pressure trend in the subtropics. Interannual variability is also investigated. It is suggested that the trend of surface temperature around Mongolia and Philippines is related to the pressure trend.

Characteristics of the Averaged Vertical Profiles of Hydrometeors in the Baiu Season Simulated with a Non-hydrostatic Regional Climate Model

The Baiu (Mei-yu) front over East Asia in the global warming climate as well as that in the present climate is simulated using a non-hydrostatic regional climate model (NHM) with a horizontal resolution of 5 km, which explicitly calculates the microphysical processes without a cumulus parameterization. The simulation results of the present and global warming climates were compared to clarify the change in the characteristics of the distributions of cloud and precipitation particles. The vertical profile of the mixing ratio of each hydrometeor category slightly shifts about 1 km higher in the global warming climate. The mixing ratios are generally larger in the global warming climate by a factor of about 1.3 at the middle of the Baiu frontal zone. No significant difference is found in the mass ratios of solid precipitating water to total water and non-precipitating water to total water between the climates. This is a reflection of that the shape of each hydrometeor profile is similar between the climates. The approximate formulas of the mass ratios among water substances were obtained.

Reason for the Failure of the Simulation of Heavy Rainfall during X-BAIU-01—Importance of a Vertical Profile of Water Vapor for Numerical Simulations—

A heavy rainfall band was observed over southern Kyushu on 23 June 2001. This heavy rainfall was not predicted by a nonhydrostatic model (NHM) that was operated twice a day to support the field experiment of X-BAIU-01. The reason for this failure was examined in comparison with X-BAIU-01 special observation and satellite data. The maritime wind convergence zone was retrieved over the sea southwest of Kyushu Island from NASA QuickSCAT data, before the heavy rainfall system formed. This wind convergence was also found in the initial field of NHM. The lower atmosphere observed by aerosondes was very humid, while that simulated by NHM was considerably drier. The existence of this low-level humid air was also supported by total precipitable water (TPW) data derived from Tropical Rainfall Measuring Mission Microwave Imager (TMI) and Defense Meteorological Satellite Program Special Sensor Microwave/Imager (SSM/I). These comparisons indicate that the fact that the lower atmosphere was drier than it is in reality, was the reason that the NHM failed to simulate the heavy rainfall. This indication was ascertained by humidifying the lower atmosphere of NHM to correspond to the observation (MOD), and applying the four-dimensional variational data-assimilation technique to TMIderived TPW data (4DV). For MOD, the band-shaped rainfall was reproduced successfully. For 4DV, the simulated precipitation intensity was not strong enough to forecast a heavy rainfall, because 4DV could not retrieve the vertical profile of water vapor by using only the TMI-derived TPW data. Humid air concentrated in the lower atmosphere is necessary to induce heavy rainfall over the Baiu frontal zone.

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

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.

Formation and Maintenance Processes of a Stationary Band-shaped Heavy Rainfall Observed in Niigata on 4 August 1998.

On 4 August 1998, a torrential rainfall was observed in the Niigata area, in central Japan. The rainfall was induced by a quasi-stationary rainband aligned in the east-southeast direction from the west of Sado Island. The rainband lasted for several hours with the characteristic features of a back-building formation, in which new convective cells were successively generated in an upstream direction. The rainfall area and intensity simulated by a nonhydrostatic mesoscale model with a 5-km horizontal grid agreed well with the observation. The rainband with a back-building nature was reproduced successfully by a nonhydrostatic model with a 2-km grid. The simulation results showed that the heavy rainfall formed as a result of strong convective instability enhanced by low-level humid and middle-level dry inflows from the southwest into the weak convergence region associated with the Baiu frontal zone. Sensitivity experiments indicated that neither the terrain of Sado Island, nor the evaporative cooling of raindrops affected the formation of the rainband. Although moved to the downstream when the low-level wind convergence was not enhanced enough, the simulated rainband stagnated after the convergence became so strong that the whole low-level air could be lifted upward while crossing the convergence zone. Therefore, the quasi-stationary rainband was maintained by the convection-enhanced low-level wind convergence existing upstream of the rainband. Over the convergence zone associated with the quasi-stationary rainband, new convective cells successively formed and separated from the zone after having developed higher. This successive formation of new cells could maintain a back-building formation. Such a back-building formation was found at several points over the wind convergence zone. These meso-β-scale convective systems with a back-building nature produced the precipitation region with a length over 200 km.

Long-term Variations of the Baiu Frontal Zone and Midsummer Weather in Japan.

A comparison of midsummer (August 1 to 10) daily sunshine duration averaged over 10 years for the periods 1986∼1995 and 1959∼1968 indicates that it dropped by as much as 2 hr day-1 in some observational sites in central Japan. The diurnal temperature range and water vapor pressure in midsummer also decreased. A long-term examination of the Baiu frontal zone location suggests that its slower northward movement in recent years affected the midsummer weather. Furthermore, the sea level pressure increased in northern Japan, while the equivalent potential temperature in the lower atmosphere dropped in central Japan. The change in the northward movement of the Baiu frontal zone is likely to be associated with the strengthening of the polar airmass around Japan.

Roles of the Atmospheric Heat Sources in Maintaining the Subtropical Convergence Zones: An Aqua-Planet GCM Study

The subtropical convergence zones (STCZs), that is, the Baiu frontal zone and subtropical portions of the South Pacific and South Atlantic convergence zones, are unique subtropical rainfall systems connected to the tropical monsoons to the west. An aqua-planet GCM without any land was used to study diabatic heating contributions in tropical monsoons and the STCZs that maintain the STCZs. An STCZ was simulated in the GCM without any influence of land when a monsoonlike, namely, strong zonally localized off-equatorial heat source, was put in the Tropics. The monsoonlike heat source has two roles:1) making a strong subtropical jet through a local Hadley circulation in the summer hemisphere and 2) making a low-level poleward moisture flow, which supplies considerable moisture to the STCZ. Upward motion maintained by the diabatic heating in the STCZ induces a low-level trough along the west and poleward side of the STCZ, keeping the vorticity balance between stretching and advection of low absolute vorticity air from the Tropics. This trough intensifies the low-level wind along the STCZ. This wind maintains strong convergence in the STCZ, together with the poleward moisture flow from the Tropics. The upward motion also induces an upper-level trough and ridge with the STCZ between, as a result of vorticity balance between shrinking and eastward vorticity advection by the mean westerlies. The radiation of a stationary Rossby wave from the STCZ was also suggested.

1993年8月1日に南九州で発生したライン状豪雨の数値シミュレーション

The stationary band-shaped torrential rain, observed over southern Kyushu on 1 August 1993, was successfully simulated by a three-dimensional anelastic non-hydrostatic model. A warm rain scheme that explicitly predicts cloud water and rainwater is employed in the model. The Japan Spectral Model (JSM) had previously failed to reproduce the stationary feature of the rainband and had predicted the precipitation area to move eastward.The failure of JSM was examined by using a 10km-resolution model. The failure was traced to the use of the moist convective adjustment scheme. Neither coarse-resolution nor absence of hydrostatic water loading led to the failure. By the moist convective adjustment scheme stabilizing unstable layers, the air was cooled and dried in the lower adjusted levels. This drying reduced the near-surface moisture and cut off the supply of highly humid air required for the maintenance of the rainband.The causes of the organization and maintenance of the simulated stationary rainband were then investigated using 2km- and 5km-resolution models. Highly humid air in the lower layer was advected by southwesterlies, condensing over the region with a large temperature contrast in the north-south direction corresponding to the Baiu frontal zone. At first, shallow convective rolls formed parallel to the low-level westerlies. As they developed, individual cells grouped into severe convective systems, resulting in a marked depression of pressure in the lower layer by more than 1.5hPa over the region of (100km)2 for two hours after the condensation. This low pressure induced the convergence of winds. An inflow of intensified cold northwesterlies increased meridional temperature contrast, forming a strong convergence line and an organized rainband. The maintenance system of this rainband had the characteristic features of the back-building type, with significant vertical wind shear in the repeated generation of convective cells upstream of the rainband. For this torrential rain event, evaporation was not very important in the enhancement and maintenance of the rainband, while the relatively large meridional temperature contrast over the Baiu frontal zone played a significant role.The maintenance and enhancement processes of the low-level jet (LLJ) in the torrential rain event were also investigated by calculating the mean horizontal momentum budget and examining the trajectories of air parcels. The pressure gradient force produced by the convection-induced low pressure accelerated the mean horizontal velocity in the LLJ core, while horizontal advection acted to decelerate the jet. It was found that the acceleration and deceleration of the jet canceled each other to maintain the jet over southern Kyushu. The inflow of southwesterly winds into the lower layer below 1km was also accelerated, by 6ms-1 over 150min after the start of integration, by the pressure gradient force. These accelerated horizontal winds were transported upward by convection, a portion of which intensified the LLJ to the north of the rainband.

1991年アジア夏季モンスーンの降水と水蒸気収支

The large-scale features of the moisture transport and the water balance of the Asian summer monsoon in 1991 were studied by utilizing the 24-hour prediction by a T106 spectral global model. In part I of the present study, we will describe the seasonal change of the precipitation in relation to the change of the major circulation systems over the Asian monsoon region. The most remarkable change in the onset period of the Asian summer rainy season over the Indian Ocean and the Southeastern Asia is the rapid displacement of the major precipitation areas from the equatorial zone to the Indochina Peninsula and Indian subcontinent. The notable change in the onset phase of the rainy season over East Asia is the formation of a precipitation belt, which corresponds to the Meiyu and Baiu frontal zone. In order to realize the relation between the variations of the precipitation with the those of the low-level circulation, several major circulation systems (CSs) in the lower troposphere are defined. Among them, the variations of the clockwise circulation centered over the Indian Ocean (CS-3) and a cyclonic circulation over the northern part of the Indian subcontinent (CS-6) are closely related with variations of the Indian monsoon rainfalls, while the cross-equatorial flow from the Australia anticyclone (CS-4) and the circulation around the North Pacific subtropical anticyclone (CS-5) are the major systems related with the East Asia rainfalls. The variation of the mixing ratio of water vapor, equivalent potential temperature, vertical stability and the vertical motion are also described in relation with the variations of the precipitation and CSs. During the Asian summer monsoon season, the gradual decrease of the mixing ratio and the increase of the vertical stability, in association with the subsidence, are significant features over the western tropical Indian Ocean, while the increase of the specific humidity and the vertical instability are the notable features over the Indian sub continent, Indochina Peninsula and the southern part of China.

梅雨前線に伴うメソスケールの降水に対する対流のパラメタリゼーションのインパクト

Four cumulus parameterizations, including the moist convective adjustment (MA), Kuo-Anthes (KA), Kain-Fritsch (KF) and Arakawa-Schubert (AS) schemes, were coupled into the Japan Spectral Model (JSM) and tested in the case of successively developed cloud clusters along the Baiu front during the period from 16 to 17 July 1993 in western Japan. All four experiments with these four parameterizations simulated synoptic features well: locations of lows and the front. The mesoscale precipitation associated with the Baiu front was, in contrast, sensitive to the convective parameterization. Intense precipitation in the Kyushu district was caused by the successive development of cloud clusters. This characteristic feature was simulated by the experiment with the AS scheme. The other three schemes were, however, not so effective in describing the mesoscale precipitation system in the Baiu front. Three sensitivity tests in which the number of types of clouds was reduced to two from six in the AS scheme were also performed to examine the importance of cloud interaction. None of the three simulated the characteristics of the Baiu frontal precipitation. The results suggest that a sophisticated parameterization of convection is necessary to simulate mesoscale precipitation associated with the Baiu front, which is characterized by weakly baroclinic forcing and significant convective instability. An additional experiment for the latent heating in the AS scheme was also performed. The result showed that precipitation in the Baiu frontal zone was greatly affected by the diabatic heating.

中国東部における積乱雲群出現の日変化と総観場の解析(1979年6～8月)

The appearance frequency of Cb-clusters and its diurnal variation were examined in relation to large-scale conditions based on the observational data from June to August 1979. The analysis was applied to the three stages in the seasonal march in that year, i.e., Pre-Meiyu (1-17 June), Meiyu (20 June-22 July) and Mid-summer (23 July-17 August). The main results are as follows. (1) The Cb-clusters appeared frequently not only in the Baiu frontal zone but also in North to Northeast China (Area N1) in Meiyu, and in Central China (Area C2) in Mid-summer. A remarkable diurnal variation of appearance frequency of the Cb-clusters with an evening maximum was observed in Area N1 in Meiyu and Area C2 in Mid-summer. The daily maximum of ground surface temperature in these two cases was very high. (2) In Area N1 in Meiyu, the Cb-clusters with diurnal variation were mainly embedded in large-scale cloud systems corresponding to slow-moving upper-level lows or troughs. The specific humidity in the lower layer increased from Pre-Meiyu to Meiyu. These two factors, as well as the strong surface heating, would provide a favorable condition for the frequent appearance of Cb-clusters due to the increase of convective instability. The large-scale horizontal convergence in the lower layer was stronger at 12 UTC (20 BST) than at 00 UTC (08 BST), which would lead to the development of Cb-clusters in the evening. (3) Area C2 was covered by the subtropical high, and the low-level divergence was generally stronger at 12 UTC than at 00 UTC. In spite of these conditions, the isolated Cb-clusters developed very frequently in the evening. The very strong surface heating in the daytime could contribute to release of the strong convective instability by generating meso-scale upward motion as a trigger.

亜熱帯収束帯(梅雨前線帯、SPCZ、SACZ)にみられる大規模場の共通性第2部 : 亜熱帯収束帯が生成するための循環場の条件

亜熱帯収束帯(梅雨前線帯、SPCZ、SACZ)にみられる大規模場の共通性第2部 : 亜熱帯収束帯が生成するための循環場の条件

西南日本における降水の継続性からみた東アジア梅雨季の総観場

In this study, the rainfall distribution and synoptic conditions concerning behavior of the frontal zones over East Asia during the Baiu season have been investigated in terms of the length of consecutive heavy rainfall duration in the southwestern part (between 130°E and 140°E) of the Japanese Islands. In the cases of consecutive rainfall which lasts several days in the southwestern part of Japan, the Subtropical Frontal Zone (STFZ), which corresponds to a convective cloud zone extending from the inland region of China, causes rainfall in Japan as the Baiu Frontal Zone (BFZ). The Eurasian Polar Frontal Zone (EPFZ) is located along the 50°N latitude line as a maximum zone of meridional temperature gradient. On the other hand, when rainfall lasts a day or two, the EPFZ is located between 35°N and 40°N to the east of 120°E, and rainfall in the southwestern part of Japan is caused by a sub-synoptic-scale cyclone moving along the EPFZ. In this case, the STFZ is inactive and it comes very close to or merges with the EPFZ. The structure of the BFZ of this case is considered to be different from that of the STFZ but comparable with that of the EPFZ. Moisture content and moisture flux around Japan during the heavy rainfall period are larger in the former case than in the latter case. Moreover, preceding the start of the rainfall of longer duration in the southwestern part of Japan, an area of active convective clouds appears in the inland region of China.

亜熱帯域の降水帯(梅雨前線帯、SPCZ、SACZ)にみられる大規模場の共通性第1部:亜熱帯前線帯としての特徴

In East Asia, a quasi-stationary frontal zone called the Baiu frontal zone (BFZ) forms during the early summer and provides nearly as much precipitation as the Intertropical Convergence Zone (ITCZ). Since the BFZ has several characteristics different from both the ITCZ and polar frontal zones, Ninomiya (1984) proposed the BFZ should be classified as a subtropical frontal zone.Using mainly ten-day mean data, we compare the BFZ around Japan with the subtropical portions of the South Pacific Convergence Zone (SPCZ) and the South Atlantic Convergence Zone (SACZ), which are significant precipitation zones in the summer Southern Hemisphere. These three precipitation zones are shown to have several common features as the following.The BFZ, the SPCZ, and the SACZ (hereafter referred to as the SPZs, the Subtropical Precipitation Zones) commonly form along the subtropical jet on the eastern side of a quasi-anchored trough, which lies to the northeast (in the Northern Hemisphere) or southeast (in the Southern Hemisphere) of the localized active convection of the tropical monsoon. The rainfall amount in the SPZs attains -400 mm/month when they are active. All of the SPZs are characterized by convergence zones with an interior thick moist layer and baroclinic zones with an upper subtropical jet. They are also characterized as poleward boundaries of the moist tropical or monsoon airmass associated with a low-level large gradient of moisture mixing ratio.Since the evaporation rate is much smaller than the precipitation rate along the SPZs, high rainfall in the SPZs is maintained by the convergence of two types of moisture currents in the SPZs. One is eastward along the SPZ and the other is along the northwestern (in the N.H.) or the southwestern (in the S.H.) periphery of the subtropical high. The latter transports moisture evaporated under the western part of the subtropical high. Generation of convective instability by the differential advection process is found along the SPZs and maintains active convection along the SPZs.Since the SPCZ and the SACZ have several unique characteristics different from the ITCZ and polar frontal zones but similar to the BFZ, it is concluded that all of the SPZs can be classified as subtropical frontal zones.

梅雨前線帯の meso-α-scale connective systemの発達と微細構造

The evolution process and fine structure of a long-lived MαCS(meso-α-scale convective system) in the subtropical East Asia was studied for 14-15 July 1979 using Geostationary Meteorological Satellite IR observation and radar observation data.This MαCS was generated in the Baiu frontal zone under the influence of a weak short-wave trough and developed into a weak Baiu frontal depression. The MαCS consisted of a few MβCSs (meso-β-scale convective systems). The evolution and propagation of the MαCS were related to both development and displacement of MβCSs.The successive generation of MβCSs over the western coast of Kyushu (western part of Japan) indicated the orographic influence on the formation of convection. The successive development of MβCSs to the west of the preexisting one over Kyushu resulted in the slow moving of the MαCS over Kyushu.The environmental conditions for development of MβCS and the strong response in the upper-level wind field to MβCSs are also discussed.

東アジアにおける晩夏季から秋季にかけての前線帯に伴う大規模場の特徴について

Large-scale situations over East Asia associated with the frontal zone extending from China to Japan are examined from late August to the beginning of November on the basis of 5-day averaged fields during 1981 to 1985, mainly in 1983.As a result, the investigated period from late summer to autumn is divided into four stages according to the frontal positions, stagnating feature of fronts and characteristics of large-scale thermal, moisture and height fields. The relationship with the retreat of the Indian summer monsoon is also examined.In Stage 1, the frontal zone was situated in the northern part of Japan. The center of a subtropical anticyclone was located to the south of Japan and an associated ridge extended over South Japan. This Stage is regarded as a part of mid-summer season. The beginning of this stage roughly coincided with the beginning of the retreat of the Indian summer monsoon from Northwest India. In Stage 2, the frontal zone shifted to the southern coast of Japan and stagnated there, although the stagnating nature was less pronounced than the Baiu frontal zone. Over the Eurasian Continent, the mid-latitude westerlies were meandering and a quasi-stationary trough was formed between 160°E and 160°W. The horizontal temperature gradient around the frontal zone over China in Stages 1 and 2 was small (-4°C/1000km), while the moisture gradient was large (-5g/kg/1000km). The temperature gradient was relatively large (6-8°C/1000km) over Japan during these stages. These characteristics are similar to those of the Baiu frontal zone. The frontal zone and this stage are regarded as the Shurin (Akisame) frontal zone and the Shurin season, respectively. The period of this stage was from September 6 to 26 on a five-year average.In Stage 3, the position of the frontal zone was not changed from Stage 2. However, the stagnating nature of fronts was not pronounced compared with Stage 2. The height field of the westerlies over the Eurasia Continent abruptly changed to the zonal pattern at the beginning of this Stage and a quasi-stationary trough was formed between 100°E and 150°E. The temperature gradient along the frontal zone over China strengthened (-10°C/1000km). This stage is the early period of the autumn season. The period was on a five-year average from September 27 to October 21.At the beginning of Stage 4, the frontal zone abruptly shifted southward to the southern part of Continental China and was located away from the Japanese Islands. At the same time, the subtropical jet stream suddenly shifted to the south of the Tibetan Plateau in its eastern part, and the northeast monsoon began in South India. This stage occurs in the late period of the autumn season.

梅雨前線帯中間規模対流系の予報実験

Forecast experiments on a long-lived meso-α-scale convective system (MaCS) in the Baiu frontal zone were made by using a 13-level 64km-mesh primitive equation model. This MaCS passed over the East China Sea and Kyushu (western part of Japan) on 14 July 1979 and developed into a Baiu frontal depression when it moved over the Pacific coast of Japan on 15 July 1979. Three 24-hour forecast experiments (Exp. CNT: control experiment, Exp. SMM: experiment from the initial condition of small specific humidity in the lower troposphere, and Exp. HET: experiment with the prescribed condensation heating in the first 1-hour of the time integration) were made from the initial state at 09LST 14 July 1979.The result of Exp. CNT showed slow spin-up of the MαCS in the early few hours. The spin-up of the MαCS in Exp. SMM was significantly slower than that in Exp. CNT. Exp. HET indicated improvement of the MαCS's spin-up. The forced condensation heating in the first 1-hour had strong influence throughout the whole integration period. The timely spin-up of the MαCS caused by the forced heating in the model resulted in the timely change of circulations around the MαCS, and this change contributed to the further development of the MαCS and the associated Baiu frontal depression.

夏期北半球の顕著な亜熱帯前線としての梅雨前線の特徴

The nature of the Baiu frontal zone (a stationary precipitation zone appearing in June ∼July around the Japan Islands) is examined in comparison with polar frontal zone in the northern hemisphere for 19∼26 June 1975.The polar frontal zone defined as the maximum zone of time variation of potential temperature and vorticity and the maximum zone of potential temperature gradient is located in 50∼60°N. Such characteristics of the polar frontal zone are not found for the Baiu frontal zone located along ∼30°N in 110∼150°E. The Baiu frontal zone is characterized by a narrow steady precipitation zone, strong gradient of equivalent potential temperature, thick moist neutral layer, steady generation of convective instability. It is concluded that the Baiu frontal zone is not a polar frontal zone but a significant subtropical front in the East Asia.It is infered that the southwestward extension of the Pacific anticyclone into the region of very moist tropical maritime air-mass is the primal condition for the formation of the Baiu front, because the frontogenesis in equivalent potential temperature field, generation of convective instability and the strong moisture convergence in the Baiu frontal zone are owing to the SW wind in the tropical maritime airmass in the western periphery zone of the westward protruding Pacific anticyclone.