Diagnostic study on the structural characteristics of a typical mei-yu front system and its maintenance mechanism

In this paper, a typical mei-yu front process with heavy rainfall from June 12 to 15 in 1998 is analyzed. The results show that the mei-yu front is a front system which consists of an iso-θe dense area with strong horizontal gradient, a deep-convective cloud tower band, a passageway transporting warm and moist air flow from the summer monsoon surge in the mid and low levels to the south of the mei-yu front,and a migrating synoptic scale trough to the north of the mei-yu front, which transports cold and dry air southward in the mid and upper levels. The maintenance of the mei-yu front is realized by: (1) is a positive feedback between the moist physical process enhancing frontogenesis and the development of the strong convective system in front of the mei-yu front; (2) the sustaining system to the north of the mei-yu front which is a migrating synoptic scale trough transporting cold and dry air to the mei-yu front and positive vorticity to the mesoscale system in front of the mei-yu front.     

The Relationship of Land-Ocean Thermal Anomaly Difference with Mei-yu and South China Sea Summer Monsoon

Based on the NCEP/NCAR reanalysis data for the period of 1948--2004 and the monthly rainfall data at 160 stations in China from 1951 to 2004, the relationships among the land-ocean temperature anomaly difference in the mid-lower troposphere in spring (April--May), the mei-yu rainfall in the Yangtze River-Huaihe River basin, and the activities of the South China Sea summer monsoon (SCSSM) are analyzed by using correlation and composite analyses. Results show that a significant positive correlation exists between mei-yu rainfall and air temperature in the middle latitudes above the western Pacific, while a significant negative correlation is located to the southwest of the Baikal Lake. When the land-ocean thermal anomaly difference is stronger in spring, the western Pacific subtropical high (WPSH) will be weaker and retreat eastward in summer (June--July), and the SCSSM will be stronger and advance further north, resulting in deficient moisture along the mei-yu front and below-normal precipitation in the mid and lower reaches of the Yangtze River, and vice versa for the weaker difference case. The effects and relative importance of the land and ocean anomalous heating on monsoon variability is also compared. It is found that the land and ocean thermal anomalies are both closely related to the summer circulation and mei-yu rainfall and SCSSM intensity, whereas the land heating anomaly is more important than ocean heating in changing the land-ocean thermal contrast and hence the summer monsoon intensity.     

Mesoscale predictability of mei-yu heavy rainfall

Recently reported results indicate that small amplitude and small scale initial errors grow rapidly and subsequently contaminate short-term deterministic mesoscale forecasts. This rapid error growth is dependent on not only moist convection but also the flow regime. In this study, the mesoscale predictability and error growth of mei-yu heavy rainfall is investigated by simulating a particular precipitation event along the mei-yu front on 4-6 July 2003 in eastern China. Due to the multi-scale character of th...     

The Thermodynamic and Dynamical Features of Double Front Structures During 21-31 July 1998 in China

1. IntroductionDuring June and July, from the mid-lower reachesof the Yangtze River basin in China to southern Japan,a precipitation zone with intensive torrential rain ap-pears and lasts for two to three weeks. This phe-nomenon is called the "mei-yu" in …     

Typical Structure, Variety, and Multi-Scale Characteristics of Meiyu Front

Meiyu front plays an important role in summer rainfall in central China. Based on the GMS-5 satellite images, NCEP reanalyses （2.5°×2.5°） and final analyses （1°×1°） data, and meteorological conventional sounding observations, the horizontal and vertical structures of the Meiyu front were summarized using multiple diagnostic variables, including winds, temperature, jet stream, front, pseduo-equivalent potential temperature, divergence, vertical motion, static instability, etc. In this paper, four cases were selected and analyzed, two of which are in 26-28 June and 23 July 2002 during the Experiment on Heavy Rain in the Meiyu period in the lower reaches of the Yangtze River, and the others are in May and July 1998. The two cases in July 1998 and July 2002 are the secondary Meiyu front cases. The results show that the structures and characteristics of the Meiyu front are different for various cases, or at various places and time, or at various stages of one case, and the frontal characteristics can be converted from the polar front to the equatorial front. Because of the interaction of the different scale circulations in the high and low latitudes, the horizontal structure of the Meiyu front has various forms.
The results in this paper also show that the typical Meiyu front consists of a narrow band with a high gradient of potential equivalent temperature below 500 hPa, south of which is warm and moist air mass, and north of which is the transformed air mass from the midlatitude ocean or polar continent. Below the mid troposphere, south of the front blows southwesterlies, while north blows easterlies. The ascending motion and precipitation usually occur ahead of the Meiyu front. In the upper troposphere, the subtropical front is above the Meiyu front, but two fronts are separated. In addition, the upper westerly jet stream and the easterlies to the south of the Meiyu front result in the upper divergent flow field.
The multi-scale characteristics of the horizontal structure of the Meiyu front can     

Mesoscale Moist Adjoint Sensitivity Study of a Mei-yu Heavy Rainfall Event

The mesoscale moist adjoint sensitivities related to the initiation of mesoscale convective systems (MCSs) are evaluated for a mei-yu heavy rainfall event. The sensitivities were calculated on a realistic background gained from a four-dimensional variational data assimilation of precipitation experiment to make the sensitivity computation possible and reasonable within a strong moist convective event at the mesoscale. The results show that the computed sensitivities at the mesoscale were capable of capturing the factors affecting MCS initiation. The sensitivities to the initial temperature and moisture are enhanced greatly by diabatic processes, especially at lower levels, and these sensitivities are much larger than those stemming from the horizontal winds, which implies that initiation of MCSs is more sensitive to low-level temperature and moisture perturbations rather than the horizontal winds. Moreover, concentration of sensitivities at low levels reflects the characteristics of the mei-yu front. The results provide some hints about how to improve quantitative precipitation forecasts of mei-yu heavy rainfall, such as by conducting mesoscale targetted observations via the adjoint-based method to reduce the low-level errors in the initial temperature and moisture.     

梅雨锋的典型结构、多样性和多尺度特征

在天气尺度梅雨锋的天气学定义基础上,利用GMS-5静止卫星红外云图、常规气象探空资料、NCEP再分析与最终分析资料对2002年长江流域典型梅雨期6月26—28日和二度梅期间7月23日、1998年5月梅雨与7月二度梅共4个梅雨锋个例进行了分析与比较,归纳了梅雨锋结构多样性;并着重对典型梅雨期的梅雨锋发展过程、水平以及垂直结构进行了多种物理量场(包括风场、温度场、急流、锋区、假相当位温、散度、垂直速度、静力稳定度等)的综合分析。结果表明,不同的个例,不同的地区和时期,一次梅雨过程的不同阶段,梅雨锋的结构和性质都有可能不同,它可以从比较接近极锋的性质过渡到接近赤道锋的性质。在水平结构上梅雨锋是在高、低纬度不同尺度的环流系统共同作用下形成的,从而造成了梅雨锋结构具有丰富的多样性。对典型梅雨锋结构进行综合分析表明梅雨锋对流层中下层锋面由强假相当位温水平梯度形成;梅雨锋南侧为暖湿气团、北侧为变性气团;梅雨锋南面为西南季风、北面为偏东气流;梅雨锋的上升运动和强降水主要发生在梅雨锋的前沿;梅雨锋上方对流层上半部存在与副热带高空急流相配合的高空副热带锋;对流层上部的高空热带东风与副热带高空西风急流构成了梅雨锋降水的高空辐散流场。根据典型期梅雨锋以及二度梅倾斜型梅雨锋的对流层上、中、下水平环流特征,给出了梅雨锋的多尺度概念模型,主要包括中低纬度系统相互作用、对流层高层的行星尺度的环流系统副热带高空西风急流、高空热带东风急流与南亚高压、对流层中层的副热带高压与北方的短波槽以及对流层低层的行星尺度季风和切变线。     

1991年江淮流域持续暴雨的大范围平均云量特征

利用ＧＭＳ－４卫星的ＩＲ数据提取的大范围平均云量和平均ＴＢＢ（亮温）资料及部分常规资料，对１９９１年５－７月发生在江淮地区的特大暴雨过程进行分析，结果表明，梅雨锋的建立及所带来的强降水与印度西南季风及南海季风的活动有密切的关系。      

Summertime atmospheric teleconnection pattern associated with a warming over the eastern Tibetan Plateau

By using a surface air temperature index (SATI) averaged over the eastern Tibetan Plateau (TP), investigation is conducted on the short-term climate variation associated with the interannual air warming (or cooling) over the TP in each summer month. Evidence suggests that the SATI is associated with a consistent teleconnection pattern extending from the TP to central-western Asia and southeastern Europe. Associated rainfall changes include, for a warming case, a drought in northern India in May and June, and a stronger mei-yu front in June. The latter is due to an intensified upper-level northeasterly in eastern China and a wetter and warmer condition over the eastern TP. In the East Asian regions, the time-space distributions of the correlation patterns between SATI and rainfall are more complex and exhibit large differences from month to month. Some studies have revealed a close relationship between the anomalous heating over the TP and the rainfall anomaly along the Yangtze River valley appearing in the summer on a seasonal mean time-scale, whereas in the present study, this relationship only appears in June and the signal's significance becomes weaker after the long-term trend in the data was excluded. Close correlations between SATI and the convection activity and SST also occur in the western Pacific in July and August: A zonally-elongated warm tone in the SST in the northwestern Pacific seems to be a passive response of the associated circulation related to a warm SATI. The SATI-associated teleconnection pattern provides a scenario consistently linking the broad summer rainfall anomalies in Europe, central-western Asia, India, and East Asia.     

The Extreme Mei-yu Season in 2020: Role of the Madden-Julian Oscillation and the Cooperative Influence of the Pacific and Indian Oceans※

The middle and lower reaches of the Yangtze River in eastern China during summer 2020 suffered the strongest mei-yu since 1961. In this work, we comprehensively analyzed the mechanism of the extreme mei-yu season in 2020, with focuses on the combined effects of the Madden-Julian Oscillation (MJO) and the cooperative influence of the Pacific and Indian Oceans in 2020 and from a historical perspective. The prediction and predictability of the extreme mei-yu are further investigated by assessing the performances of the climate model operational predictions and simulations. 　 It is noted that persistent MJO phases 1?2 during June?July 2020 played a crucial role for the extreme mei-yu by strengthening the western Pacific subtropical high. Both the development of La Ni?a conditions and sea surface temperature (SST) warming in the tropical Indian Ocean exerted important influences on the long-lived MJO phases 1?2 by slowing down the eastward propagation of the MJO and activating convection related to the MJO over the tropical Indian Ocean. The spatial distribution of the 2020 mei-yu can be qualitatively captured in model real-time forecasts with a one-month lead. This can be attributed to the contributions of both the tropical Indian Ocean warming and La Ni?a development. Nevertheless, the mei-yu rainfall amounts are seriously underestimated. Model simulations forced with observed SST suggest that internal processes of the atmosphere play a more important role than boundary forcing (e.g., SST) in the variability of mei-yu anomaly, implying a challenge in quantitatively predicting an extreme mei-yu season, like the one in 2020.     

The Record-breaking Mei-yu in 2020 and Associated Atmospheric Circulation and Tropical SST Anomalies※

The record-breaking mei-yu in the Yangtze-Huaihe River valley (YHRV) in 2020 was characterized by an early onset, a delayed retreat, a long duration, a wide meridional rainbelt, abundant precipitation, and frequent heavy rainstorm processes. It is noted that the East Asian monsoon circulation system presented a significant quasi-biweekly oscillation (QBWO) during the mei-yu season of 2020 that was associated with the onset and retreat of mei-yu, a northward shift and stagnation of the rainbelt, and the occurrence and persistence of heavy rainstorm processes. Correspondingly, during the mei-yu season, the monsoon circulation subsystems, including the western Pacific subtropical high (WPSH), the upper-level East Asian westerly jet, and the low-level southwesterly jet, experienced periodic oscillations linked with the QBWO. Most notably, the repeated establishment of a large southerly center, with relatively stable latitude, led to moisture convergence and ascent which was observed to develop repeatedly. This was accompanied by a long-term duration of the mei-yu rainfall in the YHRV and frequent occurrences of rainstorm processes. Moreover, two blocking highs were present in the middle to high latitudes over Eurasia, and a trough along the East Asian coast was also active, which allowed cold air intrusions to move southward through the northwestern and/or northeastern paths. The cold air frequently merged with the warm and moist air from the low latitudes resulting in low-level convergence over the YHRV. The persistent warming in the tropical Indian Ocean is found to be an important external contributor to an EAP/PJ-like teleconnection pattern over East Asia along with an intensified and southerly displaced WPSH, which was observed to be favorable for excessive rainfall over YHRV.     

A modified moist ageostrophic <Emphasis Type="Italic">Q</Emphasis> vector

The quasi-geostrophic Q vector is an important rainfall associated with large-scale weather systems diagnostic tool for studying development of surface and is calculated using data at single vertical level. When ageostrophic Q vector was introduced, it required data at two vertical levels. In this study, moist ageostrophic Q vector is modified so that it can be calculated using data at a single vertical level. The comparison study between the original and modified moist ageostrophic Q vectors is conducted using the data from 5 to 6 July 1991 during the torrential rainfall event associated with the Changjiang-Huaihe mei-yu front in China. The results reveal that divergences of original and modified moist ageostrophic Q vectors have similar horizontal distributions and their centers are almost located in the precipitation centers. This indicates that modified moist ageostrophic Q vector can be used to diagnose convective development with reasonable accuracy.     

Investigation of the mei-yu front using a new deformation frontogenesis function

A new frontogenesis function is developed and analyzed on the basis of a local change rate of the absolute horizontal gradient of the resultant deformation. Different from the traditional frontogenesis function, the newly defined deformation frontogenesis is derived from the viewpoint of dynamics rather than thermodynamics. Thus, it is more intuitive for the study of frontogenesis because the compaction of isolines of both temperature and moisture can be directly induced by the change of a flow field. This new frontogenesis function is particularly useful for studying the mei-yu front in China because mei-yu rainbands typically consist of a much stronger moisture gradient than temperature gradient, and involve large deformation flow. An analysis of real mei-yu frontal rainfall events indicates that the deformation frontogenesis function works remarkably well, producing a clearer mei-yu front than the traditional frontogenesis function based on a measure of the potential temperature gradient. More importantly, the deformation frontogenesis shows close correlation with the subsequent(6 h later) precipitation pattern and covers the rainband well, bearing significance for the prognosis or even prediction of future precipitation.     

Atmospheric Rivers and Mei-yu Rainfall in China:A Case Study of Summer 2020※

Atmospheric rivers (ARs) are long, narrow, and transient filaments of strong horizontal water vapor transport that can lead to extreme precipitation. To investigate the relationship between ARs and mei-yu rainfall in China, the mei-yu season of 2020 in the Yangtze-Huaihe River basin is taken as an example. An adjusted AR-detection algorithm is applied on integrated water vapor transport (IVT) of the ERA5 reanalysis. The JRA-55 reanalysis and the data from Integrated Multi-satellite Retrievals for GPM (IMERG) are also utilized to study the impacts of ARs on mei-yu rainfall in 2020. The results reveal that ARs in East Asia have an average length of 5400 km, a width of 600 km, a length/width ratio of 9.3, and a northeastward orientation of 30°. ARs are modulated by the western North Pacific subtropical high. The IVT core is located at the south side of low pressure systems, moving eastward with a speed of 10° d^?1. For the cross sections of ARs in the Yangtze-Huaihe River basin, 75% of the total flux is concentrated below 4 km with low-level jets near AR cores. Moreover, ARs occur mainly in the mei-yu period with a frequency of 20%–60%. The intensity of AR-related precipitation is 6–12 times that of AR-unrelated precipitation, and AR-related precipitation contributes about 50%–80% to total mei-yu precipitation. As shown in this case study of summer 2020, ARs are an essential part of the mei-yu system and have great impacts on mei-yu rainfall. Thus, ARs should receive more attention in research and weather forecast practices.     

Study on a Mesoscale Convective Vortex Causing Heavy Rainfall during the Mei-yu Season in 2003

The strong heavy rainfall on 3-5 July 2003 causing the severe flooding in Huaihe River basin (HRB), China is studied. It is noted that there are sometimes mesoscale convective vortex (MCV) in East Asia during the mei-yu season. Simulation results from the ARPS (Advanced Regional Prediction) data analysis system (ADAS) and WRF model were used to study the development of the mesoscale convective system (MCS) and mesoscale convective vortex (MCV). It is confirmed that the MCV formed during the development of a...     

Mesoscale structures of two types of cold-air outbreaks over the East China Sea and the effect of coastal sea surface temperature

Two types of cold-air outbreaks over the Yellow and East China Seas are investigated using a regional mesoscale model. Distinct patterns of surface turbulent heat fluxes and precipitation are identified between the two cases. The sea surface heat flux and frictional velocity are strongly influenced by mesoscale differences between high- and low-resolution datasets of sea surface temperature (SST). The influence of the SST difference on atmospheric water is spread to the downstream area of the outbreak with the phase transition of water. The air mass transformation around 800?hPa over the Kuroshio is partly influenced by the upstream SST difference. In particular, the SST difference near the mouth of the Yanzi River strongly modifies the air mass around Taiwan. Thus, in addition to the Kuroshio front, the mesoscale Chinese coastal SST structure is also important in the air mass transformation over the East China Sea.     

1991年梅雨锋云系分析

利用一小时一次的卫星资料和降水资料，分析了１９９１年江淮梅雨期第三段（６月２９日－７月１３日）梅雨锋云系的特征，包括梅雨锋云系的建立和重建过程、它的走向、分布特征、日变化特征，以及它和其他云系之间的关系等，梅雨锋云系的建立是来自高原东北部的华西盾状云系和西南季风云系相互作用的结果。外围不同性质的云系对梅雨锋云系的影响是造成江淮梅雨锋云系强烈发展，继而产生暴雨天气的重要原因之一。      

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.     

2011年夏季气候异常及主要异常事件成因分析

本文对2011年夏季的中国气候及大气环流异常特征进行分析,发现我国总体气温偏高,降水偏少。西北西部、华北南部、江淮至江南一带,西南地区东部等地出现了阶段性的较大范围极端高温天气过程。西南地区东部和广西等地出现严重干旱;而长江下游地区降水显著偏多。进一步对中国气候异常事件的成因分析表明：异常高压的长期维持,孟加拉湾的向北水汽输送偏弱及西太平洋副热带高压位置偏东使其西侧的东南和偏南水汽输送对我国西南地区影响小是导致西南地区严重干旱的大气环流因素;2010年秋季出现的中部型拉尼娜事件可能是西南干旱的一个重要外强迫条件。2011年夏季亚洲极涡偏弱偏小,欧亚中高纬地区经向环流偏强,有利于冷空气南下;同时,中纬度西太平洋地区海温持续偏低而激发反气旋性环流产生,造成西太平洋副高偏大偏强,冷暖气流在长江下游地区交汇造成降水显著偏多。     

春季格陵兰海冰与夏季中国气温和降水的关系

采有英国Hadley中心的GISST海冰面积资料，NCFP／NCAR再分析资料以及中国160站气温和降水资料，分析了春季格陵兰海冰面积与夏季中国区域气温和降水的关系。初步研究表明，春季格陵兰海冰面积变化和随后夏季我国黄河长江中下游之间地区气温以及8月份华北和西南地区降水呈明显正相关，而和6月黄河中上游地区降水则具有明显的负相关。同时，春季格陵兰海冰异常时期对应着北半球大气环流的明显主为化，表明海冰与我国气温及降水之间的联系具有一定的环流背景。