Baiu Frontal Zone Research Articles

Intensive Radiosonde Observations of Environmental Conditions on the Development of a Mesoscale Convective System in the Baiu Frontal Zone

AbstractMesoscale convective systems (MCSs) that occur in the Baiu frontal zone (BFZ) can cause devastating flash floods during early summer in Japan; however, the environmental conditions necessary for their development require further investigation. High‐frequency atmospheric soundings, conducted using multiple marine vessels in the East China Sea on 19 June 2022, captured the detailed environmental conditions pertaining to the development of an MCS within the BFZ. The MCS, which developed rapidly without any remarkable preceding synoptic or mesoscale disturbance in the mid‐ or upper troposphere, caused intense precipitation exceeding 80 mm/hr. The MCS persisted for approximately 6 hr, and it intensified when the influx of nearly saturated air near the sea surface toward a weak surface front overlapped with the influx of free‐tropospheric moist air. The influx of nearly saturated air near the sea surface ensured conditional instability within the lower troposphere. The influx of moist air in the free troposphere contributed to the near‐saturation conditions above the boundary layer, a feature inherent to the BFZ, and played an important role in minimizing the reduction in the buoyancy of air parcels. The results of this study indicate that a better forecast of the horizontal distribution of free tropospheric moist air is beneficial for limiting the potential area of genesis of MCS in the BFZ, and a more comprehensive understanding of the vertical variations in moisture transport contributes to an improved forecast skill for MCS in the BFZ.

Multi-scale Uncertainty of Mesoscale Convective Systems in the Baiu Frontal Zone: A Case Study from June 2022

Multi-scale Uncertainty of Mesoscale Convective Systems in the Baiu Frontal Zone: A Case Study from June 2022

Remote dynamic and thermodynamic effects of typhoons on Meiyu–Baiu precipitation in Japan assessed with bogus typhoon experiments

To advance comprehensive knowledge of typhoon-induced remote precipitation and anomalous atmospheric circulations during the early summer rainy season (Meiyu-Baiu), we performed numerical simulations in which different bogus typhoons are imbedded in the July climatological mean field as ideal initial and lateral boundary conditions using a regional atmospheric model, and we compared experiments with and without the bogus typhoon. The dominance of an anomalous local high (ALH) remotely induced by a typhoon leads to a poleward shift of the primary Meiyu–Baiu frontal zone in the form of the enhancement of the North Pacific subtropical high (NPSH). Typhoon-induced negative potential vorticity advection plays a dominant role in forming the ALH around Japan. Despite the shift of the frontal zone, heavy precipitation in excess of 100 mm per day occurs along the Pacific coast of western Japan, which is triggered by two primary moisture inflows. One is the low-level moisture inflow along the moisture conveyor belt (MCB), which is an appreciable low-level moisture flux zone stretching from the Indian Ocean to a typhoon over the western North Pacific, due to the intensified horizontal pressure gradient between the typhoon and the NPSH. The MCB is more robust in the experiment in which the typhoon is close to the South China Sea where the monsoon westerlies prevail. The parcels along the MCB bring moisture into western Japan even though moisture losses due to condensation occur on the way, to some degree. The other is the moisture inflow through the atmospheric boundary layer along the southwestern periphery of the NPSH. The parcels along the NPSH periphery easily receive moisture from the underlying ocean as they approach western Japan. Both inflows are enhanced by the ALH formation over Japan. The combined effect of the typhoon and its induced ALH is responsible for the prominence of the two major moisture inflows into western Japan, leading to local heavy precipitation in that region.

Large- and Meso-α-scale Features of Baiu Frontal Zone observed in July 1982

Large- and Meso-α-scale Features of Baiu Frontal Zone observed in July 1982

Contribution of sea-surface wind curl to the maintenance of the SST gradient along the upstream Kuroshio Extension in early summer

The seasonal cycle of the meridional sea-surface temperature (SST) gradient in the upstream Kuroshio Extension (KE) region was examined using satellite observation data and model simulations. In general, the meridional SST gradient is small in summer. However, in early summer (June and July), the SST front is sustained or intensified on the northern side of the KE near the coast of eastern Japan. This observed seasonal cycle was successfully simulated in the North Pacific Ocean model for the Earth Simulator (NP-OFES). Analysis of the forecast data revealed that the vertical profiles of temperature and salinity are shifted upward along the KE in early summer. As a result, the permanent thermocline depth is shallowest during summer, causing a relatively small SST tendency. In addition, significant cyclonic vorticity in the lower atmosphere related to the southwesterly sea-surface wind was found to the south of the KE, associated with the Baiu frontal zone (BFZ). It was inferred that the positive vorticity causes Ekman upwelling over the KE region, resulting in suppressed SST warming on the northern side of the KE. These results suggest that the BFZ contributes to maintaining or strengthening the SST front.

Influence of the Kuroshio on Mesoscale Convective Systems in the Baiu Frontal Zone over the East China Sea

Abstract Two mesoscale convective events in the baiu frontal zone (BFZ) were documented, based on intensive atmospheric soundings and oceanic castings in the East China Sea during May 2011, in addition to continuous surface meteorological observations, satellite products, and objective analyses. These events occurred while the BFZ was nearly stagnant and a mesolow was deepening in the zone. Near-surface southerlies associated with the low-level jet transported a warm, humid air mass from south of the BFZ. Enhanced evaporation, which was mainly attributable to the high sea surface temperature of the Kuroshio, augmented the moisture content of the air mass and helped maintain a convectively unstable stratification in the lower troposphere around the BFZ.

Two regimes of cloud water over the Okhotsk Sea and the adjacent regions around Japan in summer

AbstractThis study derived two regimes of cloud water with a dipole structure between over the Okhotsk Sea and over the adjacent regions around Japan in summer by using a climate index for cool summer. When the Okhotsk high develops, clouds are confined to a thin low‐level layer owing to the enhanced stability in the lower atmosphere induced by the downward motion associated with the Okhotsk high. The resulting optically thin clouds allow more downward shortwave radiation to reach the surface of the Okhotsk Sea. In contrast, the low‐level easterly winds blowing toward the Japanese Islands and the Eurasian continent enhance cloud formation. This is due to the convergence of the water vapor flux induced by the easterly winds associated with the Okhotsk high and the southerly winds associated with the Baiu frontal zone and the Pacific high and due to the orographic uplift of air mass. When a cyclonic circulation occurs over the Okhotsk Sea, a thick layer of low‐level clouds extending close to the sea surface is formed. The convergence of the water vapor flux over the subarctic sea surface temperature (SST) frontal zone and the cool SST promote fog formation, and upward motion associated with the cyclonic circulation supports the high cloud water content from the lower to the upper troposphere. The resulting optically thick clouds reduce the downward shortwave radiation at the surface of the Okhotsk Sea. Over the regions around Japan, water vapor flux diverges owing to dry air originating from land and cloud water decreases.

Oceanic influence on the Baiu frontal zone in the East China Sea

AbstractA high‐resolution transect of atmospheric soundings across the Kuroshio Current in the East China Sea was conducted onboard a ship in June 2012 with the objective of analyzing the influence of the complex sea surface temperature (SST) distribution on the Baiu frontal zone (BFZ). Expendable bathythermograph castings and continuous surface meteorological observations were also examined. Two distinct mesoscale atmospheric fronts, characterized by changes of wind direction in the lower troposphere and surface air temperature, were found in the BFZ. One (northern) atmospheric front was observed around the SST front in relation to a warm water tongue extending from the Kuroshio. A high SST region around the northern atmospheric front enhances unstable near surface stratification and intensifies turbulent heat flux. They help modify the marine atmospheric boundary layer in the BFZ. The other (southern) atmospheric front was at the southern end of the BFZ. Intense evaporation over the Kuroshio and moisture transport by southerly winds were important in forming the conditionally unstable air masses in the lower troposphere of the BFZ.

Possible Impact of a Tropical Cyclone on the Northward Migration of the Baiu Frontal Zone

The impact of a tropical cyclone on the northward migration of the Baiu frontal zone (BFZ) is investigated in the case of the tropical cyclone MAWAR (2012) using a global cloud-system resolving model, called NICAM. From 4 to 6 June in 2012, the BFZ rapidly shifts northward with MAWAR. A simulation with the initial data of 29 May reproduces the northward migration of the BFZ and the tropical cyclone. Strong southerlies on the eastern side of the tropical cyclone transport moist and high-temperature air into the BFZ. This horizontal advection affects the northward migration of the BFZ. In contrast, the BFZ stagnates to the south of Japan in another simulation with the initial data of 30 May because the tropical cyclone track is diverted eastward. Thus, realistic reproducibility of a tropical cyclone is needed for better simulations and prediction of the BFZ migration.

Seasonal Migration of the Baiu Frontal Zone over the East China Sea: Sea Surface Temperature Effect

Seasonal Migration of the Baiu Frontal Zone over the East China Sea: Sea Surface Temperature Effect

Characteristics of the Meiyu and Baiu Frontal Precipitation Zone in the CMIP3 20th Century Simulation and 21st Century Projection

Features of the Meiyu frontal rain zone (MFZ, in 110–125°E) and Baiu frontal rain zone (BFZ, in 125–140°E) in the 20th Century simulation (20C3M) and 21st Century projection (Special Report on Emission Scenarios (SRES) A1B), obtained by 22 climate models contributed to the World Climate Research Programme's Coupled Model Intercomparison Project phase 3 (CMIP3) are studied. Characteristics of the MFZ and BFZ in the 20th century climate simulation are compared with two sets of observed precipitation data. The latitude of the zone and the precipitation in the zone (“precipitation in FZ”) reproduced by the models are examined, using the 20-year (1980–1999) averaged values for May, June and July. The “precipitation in FZ” in each month obtained from the multi-model ensemble average (MEA) coincides approximately with the observation data. However, MEA reproduces MFZ and BFZ to the north of their observed latitude in May. MEA reasonably reproduces the latitude BFZ for June and July, and the latitude of MFZ only for June. The standard deviation (STD) of the “precipitation in FZ” does not change widely from zone to zone, and from month to month. However, STD of the latitude of zones varies widely. The STD of latitude of MFZ and BFZ became larger in July. The STD of the latitude of MFZ is larger than that of BFZ. The low and medium horizontal resolution models with moist convective adjustment scheme tend to reproduce MFZ and BFZ in the northern latitude, while the models with mass flux scheme and models with Arakawa-Schubert scheme tend to reproduce MFZ and BFZ in the southern latitude. Features of the MFZ and BFZ in the 21st century climate projection by the models are examined using the 20-year (2080–2099) averaged values for May, June and July. The characteristics of the models in regard to MFZ and BFZ in 20th century simulation are commonly found in the 21st century projection. The MFZ and BFZ shift slightly northward, and the “precipitation in FZ” decreased slightly from the 20th century to 21st century. However, these changes are very small, as compared with the respective STD. The change of MFZ and BFZ under the climate changes in the 21st century can not be definitely concluded by models of CMIP3.

Climatology of Warm Rain and Associated Latent Heating Derived from TRMM PR Observations

Abstract The large-scale distribution of precipitation and latent heating (LH) profiles in the tropics, subtropics, and part of the midlatitudes was studied using a 9-yr dataset derived from Tropical Rainfall Measuring Mission precipitation radar observations, with emphasis on the contribution of warm rain. The distribution of warm rain showed features unique from those of rain in other categories and those of outgoing longwave radiation. Warm rain was weak over land but widely distributed over oceans, especially along the intertropical convergence zone (ITCZ) and the western part of the subtropical oceans. The observed amount of warm rain depended on the rainfall intensity rather than on the frequency of warm rain events. The amount of warm rain over ocean was positively correlated with sea surface temperature (SST); this dependency was found in the tropics, subtropics, and part of the midlatitudes, whereas dependency of SST on total rain was confined to the tropics. Both total rain and warm rain were concentrated in the ITCZ, which elongated along the local SST maximum. Small amounts of warm rain were found along subtropical convergence zones (the baiu frontal zone and subtropical portions of the South Pacific convergence zone and the South Atlantic convergence zone) with ample total rainfall. However, larger amounts of warm rain were observed at the lower-latitude sides of these zones in the upstream portions of low-level moisture flow toward the zones. Warm rain may cultivate the subtropical convergence zones by deepening the moist boundary layer and increasing moisture flux toward the zones. The statistical relationship between warm rain and low-level cloudiness showed that the warm rain amount was large when low-level cloudiness was 20%–30% and small when low-level cloudiness was greater than 40%. This indicates that intense warm rain is provided by convective clouds, not by stratiform clouds, in conditions of substantial cloudiness. Despite the small contribution to total rain, warm rain maintained positive LH values over most of the tropical and subtropical oceans. The LH by warm rain masked low-level cooling observed in stratiform rain and maintained positive LH in the lower atmosphere below the melting layer. Because warm rain was confined to oceans, a strong LH contrast was maintained along the coast; this contrast reached values of 1–2 K day−1 in certain places and may affect local and monsoonal circulation across continental coasts.

Characteristics of Precipitation in the Meiyu-Baiu Season in the CMIP3 20th Century Climate Simulations Kozo NINOMIYA

Characteristics of precipitation in the Meiyu-Baiu season in twenty-two 20th century climate simulations contributed to the World Climate Research Programme’s Coupled Model Intercomparison Project phase 3 (CMIP3) are studied comparing with two observed precipitation data. The distribution of precipitation in the Meiyu-Baiu season is characterized by the narrow Meiyu and Baiu frontal precipitation zone (MFZ and BFZ) extending along the northern rim of the North Pacific subtropical anticyclone (NPSA), the maximum precipitation zone (S. Max zone) along the southern rim of the NPSA, and the minimum precipitation zone (S. Min zone) to the south of MFZ and BFZ. The latitude of these zones and the precipitation within these zones produced by the models are examined, using the 20-year (1980-1999) averaged values for May, June and July.The precipitation in these zones in each month obtained from the respective many-model ensemble average (MEA) coincides approximately with the observation data, except the western “S. Max zone” in May. However, the MEA simulates MFZ, BFZ and “S. Min zone” to the north of their observed latitude in May. MEA reasonably reproduce the latitude of both MFZ and BFZ for June, and the latitude of BFZ and “S. Min zone” for July. The latitude of the “S. Max zone” is approximately reproduced for MJJ.The standard deviation (STD) of the precipitation from the MEA does not change widely from zone to zone and from month to month. However, STD of the latitude of zones varies widely. The STD of the latitude of MFZ and BFZ became larger in June and July. The STD of the latitude of MFZ is larger than that of BFZ. The STD of the latitude for “S. Max zone” is especially large. The models show large deviation from the MEA around the western and southern rim of the NPSA. Low horizontal resolution models tend to yield the larger deviation, in regard to both the latitude and precipitation, as compared with models of medium resolution. However, a few high resolution models still produce large deviation from the MEA. It is hard to state conclusively the influence of the cumulus parameterization scheme on the intense precipitation zones.

Detailed Structure of Wind and Moisture Fields around the Baiu Frontal Zone over the East China Sea

An aircraft observation was conducted over the East China Sea to reveal detailed distributions of wind and moisture around the Baiu frontal zone. Using a numerical simulation, the structure of the Baiu frontal zone was examined and four airstreams around the zone were identified: northeasterly, west-southwesterly, and southwesterly along the Baiu frontal cloud zone, as well as southwesterly along the oceanic zone. Each airstream had different characteristics of stratification. These differences and the convergence of the airstreams resulted in two different rainfall areas: a weak and wide rainfall area in the northern part of the cloud zone, and an intense and narrow rainfall area in the southern part.

Similarities and Differences among the South Indian Ocean Convergence Zone, North American Convergence Zone, and Other Subtropical Convergence Zones Simulated Using an AGCM

I examined features of the South Indian Ocean convergence zone (SICZ) and the North American convergence zone (NACZ) simulated using an atmospheric general circulation model (AGCM; T106L56: a spectral primitive-equation model with 56 σ levels and triangular spectral truncation at wave-number 106). The 24-year model integration from 1979 to 2002 was constrained by observed sea-surface temperature and sea-ice distribution. I selected a typical case for each zone (SICZ and NACZ) from the 1985-1996 simulation. The AGCM properly simulates African and Indian Ocean monsoon circulation and precipitation. The precipitation zone of the SICZ extends southeastward from the southeastern part of Africa to the southwestern rim of the Mascarene high during Southern Hemisphere summer. North American summer monsoon circulation and precipitation were also correctly reproduced. The precipitation zone of the NACZ extends northeastward along the southeastern coast of North America to the northwestern rim of the Bermuda high during the North American summer monsoon season. I compared the features of the simulated SICZ and NACZ with features of the South Atlantic convergence zone (SACZ) and the Baiu frontal zone (BFZ) simulated using the same AGCM. The SACZ, SICZ, NACZ, and BFZ were characterized as subtropical convergence zones (STCZs) and are commonly sustained along their respective subtropical anticyclones that form over the ocean east of continents. However, their geographical environments differ significantly. Whereas the respective cool oceans at the poleward sides of the SACZ and SICZ provide significant baroclinicity for the SACZ and SICZ, the respective hot continents to the poleward sides of the BFZ and NACZ create weak baroclinicity for the BFZ and NACZ.

Similarity and Difference between the South Atlantic Convergence Zone and the Baiu Frontal Zone Simulated by an AGCM

Features of the South Atlantic convergence zone (SACZ) and the Baiu frontal zone (BFZ) simulated by an AGCM (T106L56: a primitive equation spectral model which has 56 s-levels and triangular spectral truncation at wave-number 106) are studied. The 24-year integration, from 1979 to 2002, by the model constrained by observed sea-surface temperature and sea-ice distribution, is used for this study. The detailed analysis is made for the typical case of SACZ and BFZ selected from data in 1985-1996.The South American monsoon circulation and the associated precipitation are reasonably reproduced. The precipitation zone of the SACZ, which extends southeastward from the southern part of Brazil to the South Atlantic, is sustained along the southwestern rim of the South Atlantic subtropical anticyclone during the southern summer. The Asian monsoon circulation and the associated precipitation are also reasonably reproduced. The precipitation zone of the BFZ, which extends northeastward from the southern part of China to Japan is formed along the northwestern rim of the North Pacific subtropical anticyclone in June.Many common features are found in the simulated SACZ and BFZ, in regard to the frontal structure and the associated synoptic- and meso-α-scale disturbances, and the relation to the respective subtropical anticyclone. However, an important difference is seen between their geographical environments. The cold south Atlantic in the poleward side of the SACZ provides the significant baroclinicity for SACZ, while the existence of warm land mass to the poleward side of the BFZ brings on weak baroclinicity of the BFZ. Another significant difference is seen in the large-scale environmental condition. The SACZ is influenced, in essence, by the trade winds and the subtropical anticyclone over the South Atlantic. The BFZ is strongly influenced by the moisture transport due to the Indian monsoon westerly and variation of the North Pacific subtropical anticyclone related to the convective activity over the western North Pacific.

Statistical Study on Cloud Top Heights of Cumulonimbi Thermodynamically Estimated from Objective Analysis Data during the Baiu Season

The level of neutral buoyancy (LNB) around the Japan Islands during the Baiu season in 2001-2005, estimated by lifting a low-level air with the maximum equivalent potential temperature in a vertical direction, is statistically examined from objective analysis data with the horizontal resolution of 20 km, in order to clarify the surrounding atmospheric conditions under which cumulonimbi can form and develop. Three and two peaks are found in the vertical profiles of the appearance rate of the LNB over the sea and on the land, respectively. The high-level (∼ 150 hPa) peak is very weak in June, and the low-level (∼ 900 hPa) peak does not appear on the land. The other peak is found at the middle level (N 700 hPa).The theoretical study shows that the LNB during the Baiu season strongly depends on the lapse rate between the low and middle levels, when the variation of the low-level temperature is within a few degrees. This is because the LNB during the Baiu season rapidly becomes higher as the lapse rate increases. The statistical results show that the lapse rate around the Japan Islands strongly depends on convective activities over upstream regions, especially the southern part of China. Therefore, strong convective activities over the Baiu frontal zone in June produce the middle-level LNB around the Japan Islands to suppress the development of cumulonimbi. In July, the northward shift of the area with strong convective activities considerably releases such suppression, associated with that of the Baiu frontal zone.The middle-level LNB also frequently appears over the other areas around the Baiu frontal zone, although it scarcely appears over the western part of the North Pacific Ocean. Therefore, the middle-level LNB that suggests the existence of cumulonimbi with a cloud top lower than several kilometers is one of the characteristic features around the Baiu frontal zone.

Multiple Frontal Structures in the Baiu Frontal Zone Observed by Aircraft on 27 June 2004

The structure of the Baiu frontal zone over the East China Sea on 27 June 2004 was investigated using data from aircraft observations. During the period of 09-12 JST (Japanese Standard Time: UTC + 9 hours), three cloud zones appeared over the East China Sea. The Baiu frontal zone is conveniently divided into three zones on the basis of the three cloud zones. The zones on the southern side, in the middle, and on the northern side are called FZ1, FZ2, and FZ3, respectively. The position of FZ2 corresponded with that of a stationary front on the surface weather map. Remarkable variations of winds, potential temperature, and moisture corresponding to the three zones were detected using in situ measurements at 500 m MSL (Mean Sea Level). In FZ1, a decreased southerly wind area associated with a remarkable moisture gradient was seen, although the potential temperature gradient was unclear. FZ2 had remarkable gradients of potential temperature and moisture associated with a drastic change from southwesterly to northwesterly winds. FZ3 also had remarkable gradients of potential temperature and moisture. This study shows observational evidence of the fact that multiple frontal structures exist inside the Baiu frontal zone.

Periodic Evolution of Multiscale Precipitation Systems Developed within a Baiu Frontal Cloud Cluster

A meso-α-scale cloud cluster (CC) was observed in the Baiu frontal zone, located southwest of Kyushu, Japan, on July 7, 1996. It was characterized by a lifespan of approximately 20 hours, stationary motion, and heavy precipitation of over 200 mm in a period of 12 hours. This paper contains a report of the characteristic features of internal multiscale precipitation systems observed within the CC, and their periodic evolutions as newly found facts.The CC consisted of a convective rainfall region, characterized by a meso-βL-scale (100-200 km) line-shaped convective system (convective line, MβLCL), and a weak stratiform rainfall region on the lee side of the MβLCL. The MβLCL consisted of several band-shaped meso-βS-scale (20-100 km) convective systems (MβSCSs), and each MβSCS consisted of meso-γ-scale cumulonimbus clouds. The environmental atmosphere was characterized by a warm and moist inflow in the shallow layer (below 500 m in height) associated with a cyclonic circulation of slow moving and shallow depression. Additionally, the CC was located within a large temperature gradient zone in the lower atmosphere (below 4000 m in height) along the Baiu front. These were favorable environments for the generation and maintenance of theMβLCL. The MβLCLs were periodically generated with an interval of 5-6 hours in almost the same region, and they showed a common evolution of structure and processes.The behavior of cold pools formed by the MβLCLs on the northern side of the MβLCL played a key role in the periodic evolution. The developed cold pool intensified the MβSCSs in the MβLCL and modified one of the MβSCSs into an arc-shaped MβSCS, that had similar characteristics to those ofsquall lines. Simultaneously, the arc-shaped MβSCS became the sole MβSCS that constituted MβLCL. Finally, the MβLCL decayed with the expansion of the cold pool. Another MβLCL was generated when the favorable environment was restored after the cold pool dissipated, and the evolution process of the MβLCL was repeated.

Changes in the Characteristic Features of Disturbances Appearing in the Baiu Frontal Zone over Western Japan Due to Global Warming

Numerical experiments are performed using a non-hydrostatic regional climate model with a horizontal resolution of 5 km to study changes in the characteristic features of disturbances appearing over the Baiu frontal zones due to global warming. In this study, disturbances are defined as those with precipitation greater than 20 mm/6 hr within a radius of 100 km. An increase in the number of disturbances is found in the Baiu frontal zone over western Japan in the warming climate. The increase is caused by the lengthening of the Baiu duration. In addition, the disturbances are likely to be much more detected by the intensification of precipitation. Among such disturbances, those with intense precipitation and eastward-tilting vertical structures in high-pass-filtered fields are more frequently seen in the warming climate. They are considered to be meso-α-scale baroclinic instability modified by diabatic heating due to large precipitation. Our results suggest that the changes in the numbers of disturbances are induced by an increase in the supply of water vapor to the Baiu frontal zone and convectively unstable stratifications in the lower atmosphere.