Baiu Season Research Articles

Mesoscale Variations in the Number of Thunder Days Over the Japanese Archipelago During the Past Fifty‐Six Years

ABSTRACTThe spatial and temporal variations of cumulonimbus clouds (Cbs) that produce lightning have mesoscale characteristics due to the nature of Cbs and the influence of topography; however, previous studies have lacked a mesoscale perspective because historical lightning observations relied on human observation. In this study, mesoscale variations in the number of thunder days (TDs) over the Japanese Archipelago over a 56‐year period were investigated using manned observation data (Period 1: 1954–1963) and radio‐wave observation data (Period 2: 2010–2019). A comparison of the two periods of Period 1 and Period 2 revealed that the locations of mesoscale areas with high TDs did not change significantly; however, the number of TDs increased in most mesoscale areas and it was not possible to identify any mesoscale areas with a significant decreasing trend. The characteristics of mesoscale variation varied with the season as follows: (1) Winter (November–January): the number of TDs along the coastal lands of the Sea of Japan increased significantly. The variations in the synoptic‐scale winter pressure pattern with low stability were one of the factors contributing to the increasing trend of TDs. (2) In the first half of the Baiu season (June), the number of TDs increased in two regions: one region was the south of Japan associated with the Baiu front, while the second region was the northern area of Japan, where the influence of the Baiu front is limited. (3) Summer (August): the number of TDs in mountainous areas tended to increase significantly, and the variation in the water vapour content was significantly correlated with the variation in the number of TDs. The number of TDs in the foothills did not increase significantly. (4) Akisame season (September): the number of TDs along the Pacific Ocean coast significantly increased.

Statistical Analysis of “Senjo-Kousuitai” in East Asia and Characteristics of Associated Large-Scale Circulations in the Baiu Season

Statistical Analysis of “Senjo-Kousuitai” in East Asia and Characteristics of Associated Large-Scale Circulations in the Baiu Season

Spatiotemporal patterns of synchronous heavy rainfall events in East Asia during the Baiu season

Abstract. Investigating the synchrony and interdependency of heavy rainfall occurrences is crucial to understand the underlying physical mechanisms and reduce physical and economic damages by improved forecasting strategies. In this context, studies utilizing functional network representations have recently contributed to significant advances in the understanding and prediction of extreme weather events. To thoroughly expand on previous works employing the latter framework to the East Asian summer monsoon (EASM) system, we focus here on changes in the spatial organization of synchronous heavy precipitation events across the monsoon season (April to August) by studying the temporal evolution of corresponding network characteristics in terms of a sliding window approach. Specifically, we utilize functional climate networks together with event coincidence analysis for identifying and characterizing synchronous activity from daily rainfall estimates between 1998 and 2018. Our results demonstrate that the formation of the Baiu front as a main feature of the EASM is reflected by a double-band structure of synchronous heavy rainfall with two centers north and south of the front. Although the two separated bands are strongly related to either low- or high-level winds, which are commonly assumed to be independent, we provide evidence that it is rather their mutual interconnectivity that changes during the different phases of the EASM season in a characteristic way. Our findings shed some new light on the interplay between tropical and extratropical factors controlling the EASM intraseasonal evolution, which could potentially help to improve future forecasts of the Baiu onset in different regions of East Asia.

Sensitivity to Horizontal Resolution of Regional Climate Model in Simulated Precipitation over Kyushu in Baiu Season

Sensitivity to Horizontal Resolution of Regional Climate Model in Simulated Precipitation over Kyushu in Baiu Season

Objective Classification of Controlling Factors for the Occurrence of the Wide-spread Extreme Precipitation Events during the Baiu Season over Western Japan

Objective Classification of Controlling Factors for the Occurrence of the Wide-spread Extreme Precipitation Events during the Baiu Season over Western Japan

Heavy Precipitation over Southwestern Japan during the Baiu Season due to Abundant Moisture Transport from Synoptic-Scale Atmospheric Conditions

Heavy Precipitation over Southwestern Japan during the Baiu Season due to Abundant Moisture Transport from Synoptic-Scale Atmospheric Conditions

Carbon and nitrogen exports from forested headwater catchment in southwestern Japan

Accurate estimation of carbon and nitrogen exports from forest ecosystems via streams is important because of the critical role these losses play in the carbon and nitrogen budgets in forest ecosystems. However, little is known about the export of carbon and nitrogen from small hilly catchment under an East Asian monsoon climate In this study, carbon and nitrogen exports in both particulate and dissolved forms were measured under baseflow and stormflow conditions of a forested catchment in southwestern Japan. Coarse particulate matter (CPM) was collected continuously by using mesh frames deployed across a channel. Exports of fine particulate matter (FPM) and dissolved organic and inorganic matter were calculated by using load–discharge relationships. Total estimated yields for carbon and nitrogen were 369.6 kg ha−1 year−1 and 18.3 kg ha−1 year−1, respectively. FPM accounted for more than 60% of the total stream yield of both carbon and nitrogen, and was on the upper end of the range found in worldwide reviews. The contribution of CPM to the total yield was limited to 2% and 1% for carbon and nitrogen, respectively. Most of the carbon (81%) and nitrogen (93%) was exported by stormflows which occurred only 8% of the time. Large rainstorms occurred during the bai-u season and typhoons, which is a typical rainfall pattern in the monsoon climate. Therefore, to estimate the total yield of carbon and nitrogen from hilly catchments experiencing large frequent storms, adequate sampling and computation particularly of FPM export by stormflows is critical.

Precipitation Characteristics over East Asia in Early Summer: Effects of the Subtropical Jet and Lower-Tropospheric Convective Instability

Abstract A quasi-stationary front, called the baiu front, often appears during the early-summer rainy season in East Asia (baiu in Japan). The present study examines how precipitation characteristics during the baiu season are determined by the large-scale environment, using satellite observation three-dimensional precipitation data. Emphasis is placed on the effect of subtropical jet (STJ) and lower-tropospheric convective instability (LCI). A rainband appears together with a deep moisture convergence to the south of the STJ. Two types of mesoscale rainfall events (REs; contiguous rainfall areas), which are grouped by the stratiform precipitation ratio (SPR; stratiform precipitation over total precipitation), are identified: moderately stratiform REs (SPR of 0%–80%) representing tropical organized precipitation systems and highly stratiform REs (SPR of 80%–100%) representing midlatitude precipitation systems associated with extratropical cyclones. As the STJ becomes strong, rainfall from both types of mesoscale precipitation systems increases, with a distinct eastward extension of a midtropospheric moist region. In contrast, small systems appear regardless of the STJ, with high dependency on the LCI. The results indicate that the STJ plays a role in moistening the midtroposphere owing to ascent associated with secondary circulation to the south of the STJ, producing environments favorable for organized precipitation systems in the southern part of the rainband. The horizontal moisture flux convergence may also contribute to precipitation just along the STJ. On the other hand, the LCI plays a role in generating shallow convection. In high-LCI conditions, deep convection can occur without the aid of mesoscale organization.

Classification and forecast of heavy rainfall in northern Kyushu during Baiu season using weather pattern recognition

AbstractIn this study, the Self‐Organizing Maps in combination with K‐means clustering technique are used for classification of synoptic weather patterns inducing heavy rainfall exceeding 100 mm day−1 during the Baiu season (June–July) of 1979–2010 over northern Kyushu, southwestern Japan. It suggests that these local extreme rainfall events are attributed to four clustered patterns, which are primarily related to the Baiu front and the extratropical/tropical cyclone/depression activities and represented by the intrusion of warm and moist air accompanied by the low‐level jet or cyclonic circulation. The classification results are then implemented with the analogue method to predict the occurrence (yes/no) of local heavy rainfall days in June–July of 2011–2016 by using the prognostic synoptic fields from the operational Japan Meteorological Agency (JMA) Global Spectral Model (GSM). In general, the predictability of our approach evaluated by the Equitable Threat Score up to 7‐day lead times is significantly improved than that from the conventional method using only the predicted rainfall intensity from GSM. Although the false alarm ratio is still high, it is expected that the new method will provide a useful guidance, particularly for ranges longer than 2 days, for decision‐making and preparation by weather forecasters or end‐users engaging in disaster‐proofing and water management activities.

Anomalous Weather Patterns in Relation to Heavy Precipitation Events in Japan during the Baiu Season

Abstract Anomalous weather patterns (WPs) in relation to heavy precipitation events during the baiu season in Japan are investigated using a nonlinear classification technique known as the self-organizing map (SOM). The analysis is performed on daily time scales using the Japanese 55-year Reanalysis Project (JRA-55) to determine the role of circulation and atmospheric moisture on extreme events and to investigate interannual and interdecadal variations for possible linkages with global-scale climate variability. SOM is simultaneously employed on four atmospheric variables over East Asia that are related to baiu front variability, whereby anomalous WPs that dominated during the 1958–2011 period are obtained. Our analysis extracts seven typical WPs, which are linked to frequent occurrences of heavy precipitation events. Each WP is associated with regional variations in the probability of extreme precipitation events. On interannual time scales, El Niño–Southern Oscillation (ENSO) affects the frequency of the WPs in relation to the heavy rainfall events. The warm phase of ENSO results in an increased frequency of a WP that provides a southwesterly intrusion of high equivalent potential temperature at low levels, while the cold phase provides southeastern intrusion. In addition, the results of this analysis suggest that interdecadal variability of frequency for heavy rainfall events corresponds to changes in frequency distributions of WPs and are not due to one particular WP.

都市気象LESモデルの開発による豪雨の「種」を捉えるための基礎研究

Localized torrential rainfall disasters in Summer and Baiu season of Japan is called as “Guerrilla-heavy-rainfall” in Japanese media. This rainfall is produced by an isolated cumulonimbus that grew rapidly. To prevent this disasters, the approach to detection baby-cell of rainfall earlier by X-band radar has been performed. However, in order to enhance prediction of Guerrilla-heavy-rainfall, we have to analyze convection genesis of which is affected to a large degree by urban area, because it is concerned a large component of that generate baby-cell. We aim to clarify this convection genesis by development of urban meteorological model based on large-eddy simulation.

Indirect Effects of Tropical Cyclones on Heavy Rainfall Events in Kyushu, Japan, during the Baiu Season

Indirect Effects of Tropical Cyclones on Heavy Rainfall Events in Kyushu, Japan, during the Baiu Season

Characteristics of Intense Rainfalls over Southwestern Japan in the Baiu Season in the CMIP3 20th Century Simulation and 21st Century Projection

Characteristics of Intense Rainfalls over Southwestern Japan in the Baiu Season in the CMIP3 20th Century Simulation and 21st Century Projection

Projections of Future Changes in Precipitation and the Vertical Structure of the Frontal Zone during the Baiu Season in the Vicinity of Japan Using a 5-km-mesh Regional Climate Model

Projections of Future Changes in Precipitation and the Vertical Structure of the Frontal Zone during the Baiu Season in the Vicinity of Japan Using a 5-km-mesh Regional Climate Model

Delay of the Baiu Withdrawal in Japan under Global Warming Condition with Relevance to Warming Patterns of SST

OLPDWRORJLFDOO ��DQDEUXSWHQKD DO� circulation model (MJ98 AGCM) utilizing these salient warm SST anomalies also show the southeastward shift of CJ in comparison with a control simulation. These results indicate that the change in the location of subtropical convections associated with the spatial distribution of warm SST anomalies could be an important factor for the late termination of the Baiu season over central Japan under global warming conditions.

Strengthening of the Southern Side of the Jet Stream and Delayed Withdrawal of Baiu Season in Future Climate

Strengthening of the Southern Side of the Jet Stream and Delayed Withdrawal of Baiu Season in Future Climate

Long-Term Changes of Seasonal Progress in Baiu Rainfall Using 109 Years (1901-2009) Daily Station Data

This study investigates long-term changes in Baiu rainfall in Eastern and Western Japan using daily precipitation records at 37 stations for the years 1901 through 2009, focusing on its seasonal progress. This period is much longer than various data analyzed in previous observational studies. In the early Baiu season (early to mid June), significant long-term decreasing trends are observed in Eastern and Western Japan, accompanying large inter-decadal variation in the former half of the 20th century. In the late Baiu season (mid to late July), in contrast, significant long-term increasing trends are observed on the Japan Sea side of Eastern and Western Japan. No significant trends are recognized either in the mid Baiu season (late June to early July) or in the entire Baiu season (June to July) over all regions. It is interesting to note that the observed tendency of delayed Baiu withdrawal in the last 109 years, when global warming has been in progress, is similar to its future changes projected by climate models.

Climatology of Extratropical Transition of Tropical Cyclones in the Western North Pacific Defined by Using Cyclone Phase Space

Tropical cyclones (TCs) in the western North Pacific are examined to present climatology of their extratropical transition (ET) during the period of 1979–2004. Two parameters of the cyclone phase space (CPS) are calculated using the Japanese 25-year Reanalysis (JRA-25) dataset as indices of ET for the TCs in the best track data from the Japan Meteorological Agency. The onset and the completion of ET are defined as the beginning of an increase in thermal asymmetry and the evolution from the warm to the cold-core structure, respectively. Forty percent of all TCs are assumed to complete ET. The mean transition period from the ET onset until ET completion is estimated to be about 18 hrs, although 16.8% of all ET storms are transformed into the cold-core structure before the increase in thermal asymmetry. Meridional fluctuation in the location of ET completion by season attains its peak in August, while the peak of ET events occurs in September-October with regards to the ratio of ET frequency to all TCs, the transition period, and intensity at the time of ET completion. The background environment of ET events are also examined using the monthly mean of the JRA-25 and sea surface temperature of the centennial in situ observation-based estimates (COBE). Both air-sea thermal contrast and the tropospheric vertical shear are greater in autumn than in the Baiu season and summer. The results suggest that, whereas a TC can translate into higher latitudes with a warm-core structure in an environment of weak vertical shear and small air-sea contrast in summer, it tends to complete ET in lower latitudes in an environment of strong vertical shear and large air-sea contrast in autumn-winter. These characteristics of ET affect the seasonal variation in the structure of landfalling TCs on the main islands of Japan.

Interannual Variability of the Baiu Season near Japan Evaluated from the Equivalent Potential Temperature

Baiu is a Japanese name for rain and the rainy season that occurs in boreal early summer in the western North Pacific (WNP). This work proposes a methodology to determine the Baiu onset and closing dates using only an atmospheric parameter, the equivalent potential temperature (θe) in the lower troposphere, to reveal the interannual variability of Baiu season. First, the location of the Baiu front is determined by the maximum of the negative meridional gradient of θe, which identifies the zonal boundary of air masses. Then, the Baiu onset and closing dates are fixed on the basis of the northward movement of the Baiu front and development of the southern air mass in the WNP. The mean onset date was estimated to be June 2 near Japan, which is comparable to the date published by the Japan Meteorological Agency (JMA) for South Kyushu, although the correlation was not significant between the two interannual variations. The meridional shift of the monsoon southwesterlies was concurrent with early or late onset dates. The mean closing date was July 21, about 1 week earlier than the JMA date for North Tohoku, but the correlation was significant between the two interannual variations. The closing date varied with an anomalous meridional dipole in atmospheric circulation, largely extending from the eastern Eurasian continent to the central North Pacific. The correlation was insignificant between the interannual variations of the onset and closing dates, although both had dominant 2–4-year periodicity. There was also no significant correlation with typical large-scale interannual variations (i.e., the El Nino/Southern Oscillation, the tropospheric biennial oscillation of the Asian monsoon, and the North Atlantic Oscillation). Hence, these three interannual variations do not appear to be direct causes of the modulation of the onset and closing dates of the Baiu season near Japan.

He case analysis of the remarkable precipitation in the Ehime district in the Baiu season period in 2011

he case analysis of the remarkable precipitation in the Ehime district in the Baiu season period in 2011