Meiyu-Baiu Front Research Articles

The role of the Kuroshio Current on the diurnal cycle of Meiyu-Baiu rainband in mid-June

This study investigated the mechanism of the strong diurnal cycle of precipitation over the Kuroshio Current (KC) in mid-June when the climatological location of the Meiyu-Baiu (MB) front is aligned with the KC. The diurnal cycle of precipitation exhibits double peaks, with the primary peak in the morning (03–12 LST) and the secondary peak in the afternoon (12–15 LST). It was revealed that the morning peak is induced by convective instability associated with the large temperature difference between the sea surface and the surface air (TDiff), whereas the cross-frontal circulation is responsible for the afternoon peak. In the morning, heating from the KC intensifies as the TDiff maximizes. Intensified convective instability due to the enhanced heating from the KC increases the mean amount of convective precipitation (CP) as well as the probability of stronger CP. The surface wind convergence and the confluence sustaining the convection intensify the frontogenesis, which leads to the maximization of the frontal intensity in the afternoon. The afternoon peak of precipitation is in accordance with the intensified MB front and the associated cross-frontal circulation. However, direct contribution of diabatic heating to the frontogenesis is relatively weak. This study provides a physical understanding of the intensified diurnal cycle of MB rainfall in terms of the interaction between oceanic and atmospheric fronts, the KC and MB, respectively.

Impacts of moisture supply from the subtropical western Pacific on the subtropical high and the atmospheric river during the heavy rain of 2020 in Japan

Our recent study suggested that moisture from the subtropical Western Pacific (WP) contributed the most to an atmospheric river (AR) event and the related heavy rainfall during the heavy rain of 2020 in Japan based on a Lagrangian approach. However, the actual role of moisture from the subtropical WP region in the AR and heavy rain formations remains unclear. To evaluate that, we conducted a set of numerical sensitivity experiments by adjusting the surface moisture supply over the subtropical WP region with factors of 0%, 50%, and 200%. The sensitivity experiments suggest that the reduced surface evaporation over the subtropical WP suppressed the local convective activity and decreased moisture content in the whole troposphere, leading to shallow and weak positive geopotential height anomalies. Although the slightly strengthened WP subtropical High (WPSH) and related anomalous anticyclonic circulation enhanced the southwesterly wind, convective activities along the Meiyu-Baiu front were still weakened due to the largely reduced moisture supply, resulting in another anomalous anticyclonic circulation over Japan but had much stronger and deeper structures. These two anomalous circulations and the reduced moisture modulated the AR over Japan, which eventually caused the weakened rainfalls and the northward migration of the rainband. By contrast, larger surface evaporation enhanced the local convective activities and weakened the dominant WPSH, resulting in the weakening and the southward migration of the AR. Overall, this study confirmed the large contributions of moisture supply from the subtropical WP region to the AR and related precipitation over Japan during the record-breaking Meiyu-Baiu season in 2020 via both dynamic and thermodynamic influences. In addition, it reveals that, although larger evaporation over the WP region would increase the total rainfalls but would not have led to more several rainfalls over certain regions, such as the relatively small Kyushu Island.

Raindrop Size Distribution Characteristics for Tropical Cyclones and Meiyu-Baiu Fronts Impacting Tokyo, Japan

Tropical cyclones and meiyu-baiu fronts, as the two main synoptic systems over East Asia, bring heavy rain during summers, but their long-term and vertical raindrop size distribution (RSD) features over the midlatitude Japan Islands are limited. Radar-based quantitative precipitation estimation (QPE) techniques require RSD observations. In this study, five-year observations from Tokyo with a ground-based impact Joss-Waldvogel disdrometer (JWD) and a vertically pointing micro rain radar (MRR) with a vertical range of 0.2–6.0 km were used to study the vertical structures of RSD and QPE parameters. The results showed that the convective rain associated with tropical cyclones had a maritime nature, while the rain associated with the meiyu-baiu front had a continental nature. The rain associated with tropical cyclones had a relatively higher concentration of raindrops and a larger average raindrop diameter than the rain associated with the meiyu-baiu front. The Z–R (radar reflectivity-rain rate) relationships (Z = ARb) based on the JWD data for tropical cyclones, the meiyu-baiu front and total summer rainfall in Tokyo were Z = 189 R1.38, Z = 214 R1.35 and Z = 212 R1.33, respectively. When the Z–R relationships obtained in this study were used to replace the operational relationship of Z = 300 R1.4, the standard deviation of the rain rate was reduced from 5.50 mm/h (2.34 mm/h) to 2.34 mm/h (1.32 mm/h) for typhoon (meiyu-baiu front) rainfall, although the change for total summer rainfall was small. In addition, with increasing height below 4 km, the value of A and b decreased.

Influence of the Kuroshio Interannual Variability on the Summertime Precipitation over the East China Sea and Adjacent Area

AbstractMuch attention has been paid to the climatic impacts of changes in the Kuroshio Extension, instead of the Kuroshio in the East China Sea (ECS). This study, however, reveals the prominent influences of the lateral shift of the Kuroshio at interannual time scale in late spring [April–June (AMJ)] on the sea surface temperature (SST) and precipitation in summer around the ECS, based on high-resolution satellite observations and ERA-Interim. A persistent offshore displacement of the Kuroshio during AMJ can result in cold SST anomalies in the northern ECS and the Japan/East Sea until late summer, which correspondingly causes anomalous cooling of the lower troposphere. Consequently, the anomalous cold SST in the northern ECS acts as a key driver to robustly enhance the precipitation from the Yangtze River delta to Kyushu in early summer (May–August) and over the central ECS in late summer (July–September). In view of the moisture budget analysis, two different physical processes modulated by the lateral shift of the Kuroshio are identified to account for the distinct responses of precipitation in early and late summer, respectively. First, the anomalous cold SST in the northern ECS induced by the Kuroshio offshore shift is likely conducive to the earlier arrival of the mei-yu–baiu front at 30°–32°N and its subsequent slower northward movement, which may prolong the local rainy season, leading to the increased rain belt in early summer. Second, the persistent cold SST anomalies in late summer strengthen the near-surface baroclinicity and the associated strong atmospheric fronts embedded in the extratropical cyclones over the central ECS, which in turn enhances the local rainfall.

Multi-scale temporospatial variability of the East Asian Meiyu-Baiu fronts: characterization with a suite of new objective indices

The Meiyu-Baiu front (MBF) is the primary weather system modulating the monsoonal rainfall in East Asia, and it exhibits pronounced variability across multiple temporal and spatial scales. A suite of new objective indices based upon daily reanalysis data are developed to characterize such variability, which include an “intensity” index effectively measuring the meridional gradient of equivalent potential temperature across the frontal zone, a “location” index documenting the central latitude and longitude of the front, and a “stationarity” index corresponding to the standard deviation of the daily central latitude. All these indices are characterized by strong interannual to interdecadal variations. Specifically, the MBF has shown a tendency of southwestward shift since the mid-1990s and become more stationary over southern China with reduced intraseasonal meandering. The western North Pacific subtropical high (WNPSH) is the main circulation feature affecting the location of the MBF on interannual timescales. The MBF moves northward (southward) when the WNPSH is anomalously strong (weak) and the surface high over the region of 35°–50°N/100°–120°E is anomalously weak (strong). When the WNPSH is strong, the MBF is often situated northwest to its climatological position and becomes more stationary. On interdecadal timescales, enhanced activity of high latitude blocking over East Asia is often accompanied with a stronger but less stationary MBF while a negative phase of the Arctic Oscillation favors a southward-shifted and more stationary MBF.

Sensitivity of Asian Summer Monsoon precipitation to tropical sea surface temperature anomalies

Sensitivity of Asian Summer Monsoon (ASM) precipitation to tropical sea surface temperature (SST) anomalies was estimated from ensemble simulations of two atmospheric general circulation models (GCMs) with an array of idealized SST anomaly patch prescriptions. Consistent sensitivity patterns were obtained in both models. Sensitivity of Indian Summer Monsoon (ISM) precipitation to cooling in the East Pacific was much weaker than to that of the same magnitude in the local Indian–western Pacific, over which a meridional pattern of warm north and cold south was most instrumental in increasing ISM precipitation. This indicates that the strength of the ENSO–ISM relationship is due to the large-amplitude East Pacific SST anomaly rather than its sensitivity value. Sensitivity of the East Asian Summer Monsoon (EASM), represented by the Yangtze–Huai River Valley (YHRV, also known as the meiyu–baiu front) precipitation, is non-uniform across the Indian Ocean basin. YHRV precipitation was most sensitive to warm SST anomalies over the northern Indian Ocean and the South China Sea, whereas the southern Indian Ocean had the opposite effect. This implies that the strengthened EASM in the post-Nino year is attributable mainly to warming of the northern Indian Ocean. The corresponding physical links between these SST anomaly patterns and ASM precipitation were also discussed. The relevance of sensitivity maps was justified by the high correlation between sensitivity-map-based reconstructed time series using observed SST anomaly patterns and actual precipitation series derived from ensemble-mean atmospheric GCM runs with time-varying global SST prescriptions during the same period. The correlation results indicated that sensitivity maps derived from patch experiments were far superior to those based on regression methods.

Different sub-monsoon signals in stable oxygen isotope in daily precipitation to the northeast of the Tibetan Plateau

This study presents a stable oxygen isotope (δ18O) record in daily precipitation from two sites located to the northeast of the Tibetan Plateau (TP): Yushu on the eastern TP and Xi'an on the eastern Chinese Loess Plateau. It attempts to reveal the unique features associated with variations in atmospheric circulation patterns over inland China. For δ18O in daily precipitation at both stations, temperature effect is significant (p < 0.01) only during non-monsoon, while amount effect is significant only during monsoon. This suggests the coexistence of local recycling with large-scale atmospheric circulation on regional precipitation, which is further verified by the significant correlation of relative humidity with δ18O at both stations during monsoon season. The similarity of δ18O in regions under the supposedly same atmospheric circulation streams is tested for Yushu with that at Lhasa, Lulang and Delingha, demonstrating the lag days of δ18O depletion at Yushu with that at Lulang as varying from 15 to 25 d. This confirms the Bay of Bengal monsoon dominance over Yushu. Daily δ18O at Xi'an is compared with contemporary data at Changsha and Guangzhou, featuring a close correlation with the East Asian summer monsoon evolution processes over eastern China, and reflecting the Meiyu-Baiu front influence during July. Back-trajectory analysis in October–November at Xi'an identified the combined effect of cooling of the atmospheric column by the colder air from the west and the lifting of the warmer air from the east, which coexists with local water vapour source. Interactions of the three result in condensation at lower temperatures that is coupled with the long-distance transport of 2/3 of the available water vapour, thus leading to extremely low δ18O values in the post-monsoon precipitation.

Orographic Effects of the Tibetan Plateau on the East Asian Summer Monsoon: An Energetic Perspective

AbstractThis paper investigates the dynamical processes through which the Tibetan Plateau (TP) influences the East Asian summer monsoon (EASM) within the framework of the moist static energy (MSE) budget, using both observations and atmospheric general circulation model (AGCM) simulations. The focus is on the most prominent feature of the EASM, the so-called meiyu–baiu (MB), which is characterized by a well-defined, southwest–northeast elongated quasi-stationary rainfall band, spanning from eastern China to Japan and into the northwestern Pacific Ocean between mid-June and mid-July.Observational analyses of the MSE budget of the MB front indicate that horizontal advection of moist enthalpy, and primarily of dry enthalpy, sustains the front in a region of otherwise negative net energy input into the atmospheric column. A decomposition of the horizontal dry enthalpy advection into mean, transient, and stationary eddy fluxes identifies the longitudinal thermal gradient due to zonal asymmetries and the meridional stationary eddy velocity as the most influential factors determining the pattern of horizontal moist enthalpy advection. Numerical simulations in which the TP is either retained or removed show that the TP influences the stationary enthalpy flux, and hence the MB front, primarily by changing the meridional stationary eddy velocity, with reinforced southerly wind over the MB region and northerly wind to its north. Changes in the longitudinal thermal gradient are mainly confined to the near downstream of the TP, with the resulting changes in zonal warm air advection having a lesser impact on the rainfall in the extended MB region.

Study on the Dynamic Characteristics of an Eastward-offshore Mesoscale Vortex along the Meiyu-Baiu Front

The eastward-moving Meiyu-Baiu frontal mesoscale vortices (MBFMVs) appear frequently and often cause heavy rainfall events along their tracks. A move-off-shore MBFMV was selected to enhance our und...

A Lagrangian Climatology of Tropical Moisture Exports to the Northern Hemispheric Extratropics

Abstract Case studies have shown that heavy precipitation events and rapid cyclogenesis in the extratropics can be fueled by moist and warm tropical air masses. Often the tropical moisture export (TME) occurs through a longitudinally confined region in the subtropics. Here a comprehensive climatological analysis of TME is constructed on the basis of seven-day forward trajectories started daily from the tropical lower troposphere using 6-hourly 40-yr ECMWF Re-Analysis (ERA-40) data from the 23-year period 1979–2001. The objective TME identification procedure retains only those trajectories that reach a water vapor flux of at least 100 g kg−1 m s−1 somewhere north of 35°N. The results show four distinct activity maxima with different seasonal behavior: (i) The “pineapple express,” which connects tropical moisture sources near Hawaii with precipitation near the North American west coast, has a marked activity maximum in boreal winter. (ii) TME over the west Pacific is largest in summer, partly related to the East Asian monsoon and the mei-yu–baiu front. This region alone is responsible for a large portion of TME across 35°N. (iii) The narrow activity maximum over the Great Plains of North America is rooted over the Gulf of Mexico and the Caribbean Sea and has a clear maximum in summer and spring. (iv) TME over the western North Atlantic shows the smallest annual cycle with a maximum in winter and autumn. The interannual variability of (i) and (iv) is significantly modulated by El Niño. Over the African–European–Asian region, high orographic barriers impede TME. A typical TME trajectory evolution is poleward and quasi-horizontal in the subtropics and then more eastward and upward in the southern midlatitudes, where TME contributes up to 60% to climatological precipitation. The TME dataset presented here can serve as a basis for future studies on extreme events.

Simulation of Synoptic- and Subsynoptic-Scale Phenomena Associated with the East Asian Summer Monsoon Using a High-Resolution GCM

Abstract A 20-yr simulation using a global atmospheric general circulation model with a resolution of 0.5° latitude × 0.625° longitude is compared with observational findings. The primary goal of this survey is to assess the model performance in reproducing various summertime phenomena related to the continental-scale Asian monsoon in general, and the regional-scale East Asian monsoon in particular. In both model and observed atmospheres, the seasonal march of the precipitation centers associated with the Asian summer monsoon is characterized by onsets occurring earliest over the southeastern Bay of Bengal, followed by rapid northeastward advances over Indochina, the South China Sea–Philippine Sea and the western Pacific, northward evolution in the East Asian sector, as well as northwestward development over the Bay of Bengal, the Indian subcontinent, and the Arabian Sea. This onset sequence is accompanied by southwesterly low-level flows over the rainy regions, as well as northwestward migration of the 200-mb Tibetan anticyclone. Analysis of the heat sources and sinks in various regions illustrates the prominent role of condensational heating in the local energy budget during the mature phases of monsoon development. In accord with observations, the simulated monsoon rains in the East Asian sector are organized about zonally elongated “mei-yu–baiu” (plum rain) systems. These precipitation features advance to higher latitudes during the June–July period, in conjunction with displacements of the axis of the low-level anticyclone over the subtropical western Pacific. A detailed case study is performed on a prominent rainy episode in the simulation. The model is capable of reproducing the observed intense gradients in temperature, humidity, and moist static stability in the vicinity of the mei-yu–baiu front, as well as the spatial relationships between the rainband and the three-dimensional flow field. The axis of the mei-yu–baiu rainband in this event is aligned with the trajectory of a succession of mesoscale cyclonic vortices, which originate from southwestern China and travel northeastward over the Yangtze River basin.

Multi-Scale Features of the Meiyu-Baiu Front and Associated Precipitation Systems

The multi-scale features of the Meiyu-Baiu front (MBF) and associated precipitation systems are described based on the recent observational studies for the intense rainfalls period in July of 1991.The MBF extends from the Yangtze River basin to the Japan Islands along the northwestern rim of the westward protruding North Pacific subtropical anticyclone. A blocking ridge develops over Primorskiy Kray of Russia, while a cold upper low develops over Mongolia during this period. The low-leveljet stream, nearly moist neutral stratification, and a strong gradient of the specific humidity characterize the MBF. The thermal gradient in the Meiyu front is weak, whereas the relatively large thermal gradient in the Baiu front is sustained between the tropical maritime airmass and the polar maritimeairmass.The strong meridional convergence of the moisture flux sustains the large precipitation in the MBF. The differential advection of the equivalent potential temperature generates convective instability against the stabilizing effect of the cumulus convection, and sustains moist neutral stratification and intense precipitation. The northward ageostrophic winds from the northwestern rim of the westward protruding subtropical anticyclone causes strong low-level convergence in the MBF.The short-wave trough that propagates along the southern rim of the upper cold low couples with the short-wave trough in the MBF. The coupling of the troughs causes the development of the subsynoptic-scale depression (SD) and the associated subsynoptic-scale cloud system. Successively with the development of the SD, a series of the meso-α-scale cloud systems are formed along the long trailing portions of the SD. Thus the ”cloud system family“ with a length of ∼2000 km, which consists of a subsynoptic-scale cloud system and a few meso-α-scale cloud systems aligned along the trailing portion of the SD, is formed along the MBF. Features and the development process of the subsynoptic- and meso-α-scale cloud systems are studied in detail.

Thermodynamic Impact of the Heated Landmass on the Nocturnal Evolution of a Cloud Cluster over a Meiyu-Baiu Front

Numerical simulations of a cloud cluster over a Meiyu-Baiu front were conducted using a cloud-resolving non-hydrostatic model. The purpose was to demonstrate thermodynamic impact of the diurnal heating of the mainland China upon the nocturnal evolution of the cluster. Simulations were conducted within a domain covering eastern and southern China to reproduce land-surface heating on the southern side of the front. The case simulated was a rainband associated with the cluster, which formed on the late afternoon of 22 June 2003. The environment is characterized by a synoptic-scale southerly inflow of warm and moist air in the lower troposphere. A control simulation reproduced the rainband successfu11y, and showed that the evolution resulted from the strong latent instability over the frontal convergence zone, due to the southerly inflow of the warm and moist air. The origin of this air was the surface heat flux over a cloud-free area to the south of the front. The importance of the heating was proved by a sensitivity simulation without insolation, which failed to reproduce deep convective updrafts. The water budget analysis demonstrated that the evaporation from the ground, rather than the moisture convergence, contributed to the increase in the precipitable water before the rainband evolution. This study, thus, indicates that the continental surface, heated by insolation, has a significant impact upon the nocturnal evolution of a cloud cluster over a Meiyu-Baiu front.

Interannual variation of the diurnal convection cycle in the western North Pacific

As revealed from the interannual variation of outgoing longwave radiation in the western Pacific, deep cumulus convection along the Meiyu-Baiu front and ITCZ is modulated by the anomalous summer circulation in the following manner: when the sea surface temperatures on the eastern tropical Pacific are anomalously warm (cold), cumulus convection is enhanced (suppressed) along the equator east of 150° E and along the Meiyu-Baiu front, but is suppressed (enhanced) along the equator west of 150° E and along a longitudinal zone (10° N–30° N) extending from the northern section of the South China Sea to the International Dateline. Since tropical deep cumulus convection exhibits a pronounced diurnal variation, the diurnal convection cycle in the western Pacific may undergo an interannual variation coherent with that of deep tropical cumulus convection. This inference is substantiated by our analysis of the diurnal convection cycle for 1980–1993 with 3-hour equivalent black-body temperature observed by the Japanese Geostationary Meteorological Satellite (GMS). As expected, the diurnal convection cycle in the western Pacific is subjected to an interannual variation in accordance with deep cumulus convection along the Meiyu-Baiu front and ITCZ. Except along the equator east of 150° E, the diurnal convection cycle does not exhibit a drastic interannual change in phase.

A study of formation and development of one kind of cyclone on the mei-yu (Baiu) front

The paper presents one diagnosis of baroclinity and the coupling of jets during the developing process of a cyclone that occurred on the mei-yu (Baiu) front around the end of the second stage of the mei-yu (Baiu) in 1998. Results have shown that: (1) The advantageous changes of upper-level large-scale circulation caused the appearance and maintenance of the coupling between the upper-level jet (ULJ) and lower-level jet (LLJ) over the cyclone’s area. The coupling of jets in this case possesses some different characteristics from previous cases. Moreover, the coupling between the ULJ and LLJ caused the intensification of both lower-level convergence and upper-level divergence, which was favorable for the development of this cyclone. (2) From the analysis of the voricity budget, the role of lower-level convergence in the development of the cyclone was emphasized. Divergent wind in the lower troposphere was a direct contributor to the development of the cyclone. (3) During the development of the cyclone, cold air and warm air were active over the cyclone’s domain. Although this cyclone occurred at the mei-yu (Baiu) front, its development assumed baroclinity to a certain extent, which was just the main difference between this kind of cyclone and the first kind of low which is usually barotropic (or quasi-barotropic). (4) In recent years, studies on mei-yu front lows have paid more attention to the lower troposphere. In this paper, the analysis of the energy budget further supports this point: the certain effect of baroclinity forcing in the upper troposphere on mei-yu front lows cannot be ignored.

Cloud System Families in the Meiyu-Baiu Front Observed during 1-10 July 1991

This paper studies the cloud systems in the stationary Meiyu-Baiu front observed in 1-10 July 1991, using the Geostationary Meteorological Satellite IR data and ECMWF re-analysis data. The Meiyu-Baiu frontal cloud zone consists of a few trains of cloud systems with a length of ∼2000 km, which is recognized as the “cloud system family”. Each family consists of a preceding subsynoptic-scale cloud system, and a few meso-α-scale cloud systems aligned along the trailing portion of the sub-synoptic-scale cloud system. During the 10-day period, 4 sub-synoptic-scale cloud systems, accompanied by Meiyu-Baiu frontal depressions, are formed with an interval of 2-3 days, in association with the short-wave westerly troughs. The sub-synoptic-scale depressions indicate the peak value of vorticity more than 6× 10-5 s-1, at both 850 and 500 hPa. The sub-synoptic-scale cloud system consists of convective clouds in its southern portion, and wide spread clouds in the northeastern portion. A zone of potential vorticity appears in the trailing portion of the sub-synoptic-scale depression. The cold and dry advection occurring to the back of the depression sustain thermal and moisture gradient in the narrow frontal zone after the passage of the depression. A few meso-α-scale cloud systems develop along the trailing portion of the sub-synoptic-scale depression. The main part of the meso-α-scale cloud system consists of the active convective clouds. Some of the cloud systems show features of oval shaped cloud cluster, while others indicate features of leaf shaped cloud system with a cloud band in its trailing portion. These meso-a-scale cloud systems are categorized into three groups based on the horizontal extension of the cloud area, duration and the magnitude of the relative vorticity associated with the cloud system. Type-A meso-α-scale cloud systems are identified for∼2 days, and are accompanied by positive vorticity of ∼4 × 10-5 s-1 in the lower and middle troposphere. Type-B meso-a-scale cloud systems are identified only for∼1 day, and are accompanied by positive vorticity of ∼2 × 10-5 s-1 mainly in the lower troposphere. Type-C meso-α-scale cloud systems are generated during the daytime and decay during the nighttime. The wavelength estimated by Eddy’s baroclinic instability model for the unsaturated frontal zone coincides with the wavelength of the observed sub-synoptic-scale cloud system, while that for the moist frontal zone is consistent with the scale of Type-A meso-a-scale cloud systems. The cloud system families are most clearly observed in 110-140°E, where the Pacific subtropical anticyclone, and the cold low in the northern latitude, provide favorable conditions for the development of cloud systems.

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.

Large .LAMBDA.-shaped Cloud Zone Formed around July 6, 1991 with Pole-ward Moisture Transport from Intense Rainfall Area in Meiyu-Baiu Front.

Many previous papers stressed that the Meiyu-Baiu rain zone exhibits the quasi-stationary nature, except the abrupt transitional period. This is true only for the 10-day averaged large-scale field. The actual Meiyu-Baiu front indicates complicated variations even in “the quasi-stationary period”. The present paper will study a large λ-shaped cloud zone seen around July 6, 1991 as one of the cases of significant variations of the Meiyu-Baiu front. This λ-shaped cloud zone is formed when a north-south oriented cloud zone elongated from the intense rainfall area of Meiyu-Baiu front with pole-ward moisture transport. This process is quite different from “the abrupt transition of the Meiyu-Baiu front”, since the Meiyu-Baiu frontal zone itself remains in almost the same latitude belt. The λ-shaped cloud zone is associated with a north-south extending trough, which resulted from coupling of the short-wave troughs in northern (40-50°N) and southern (30-40°N) latitude. When the extending trough passed over the Tibetan Plateau and approached a quasi-stationary ridge over ∼120°E, the narrow north-south elongated cloud zone, which is associated with southerly wind and ascending motion, is formed in front of the trough. Convective precipitation occurs within the north-south elongated cloud zone. However, subsidence to the west of the trough and that to the east of the ridge block the northward shift of the Meiyu-Baiu front. The north-south elongating cloud zone disappears when the eastward propagating southern short-wave trough is de-coupled from the northern short-wave trough by the blocking ridge. The λ-shaped cloud zone occurs only under the specific phase relations among the short-wave troughs in northern and southern latitudes, quasi-stationary ridge and intense rainfall area in the Meiyu-Baiu front. Typical large λ-shaped cloud zone appears only a few times in a Meiyu-Baiu season.

Numerical experiments of Meiyu(baiu) rainfall by Quasi-Lagrangian limited area model with terrain

In this paper, a 10-level Quasi-Lagrangian Limited Area Model is used to simulate the process of Meiyu(Baiu) front of 1979. Some physical processes, such as large-scale condensation and cumulus convection, are included in the model. The simulation results are encouraging. 24-h numerical simulation shows that the invading of cold air from North China and rapidly northward moving of warm air from South China can be successfully reproduced. The terrain with a maximum of 4175 m is incorporated in the model. Three different kinds of terrain schemes are tested and the dynamic effect of the Plateau on the process of heavy rainfall is found to be very important.