Seasonal Forecasts of Precipitation during the First Rainy Season in South China Based on NUIST-CFS1.0

Current dynamical models experience great difficulties providing reliable seasonal forecasts of regional/local rainfall in South China.This study evaluates seasonal forecast skill for precipitation in the first rainy season(FRS,i.e.,April–June)over South China from 1982 to 2020 based on the global real-time Climate Forecast System of Nanjing University of Information Science and Technology(NUIST-CFS1.0,previously known as SINTEX-F).The potential predictability and the practical forecast skill of NUIST-CFS1.0 for FRS precipitation remain low in general.But NUIST-CFS1.0 still performs better than the average of nine international models in terms of correlation coefficient skill in predicting the interannual precipitation anomaly and its related circulation index.NUIST-CFS1.0 captures the anomalous Philippines anticyclone,which transports moisture and heat northward to South China,favoring more precipitation in South China during the FRS.By examining the correlations between sea surface temperature(SST)and FRS precipitation and the Philippines anticyclone,we find that the model reasonably captures SST-associated precipitation and circulation anomalies,which partly explains the predictability of FRS precipitation.A dynamical downscaling model with 30-km resolution forced by the large-scale circulations of the NUIST-CFS1.0 predictions could improve forecasts of the climatological states and extreme precipitation events.Our results also reveal interesting interdecadal changes in the predictive skill for FRS precipitation in South China based on the NUIST-CFS1.0 hindcasts.These results help improve the understanding and forecasts for FRS precipitation in South China.

Temporal Variations of the Frontal and Monsoon Storm Rainfall during the First Rainy Season in South China

The temporal variations in storm rainfall during the first rainy season (FRS) in South China (SC) are investigated in this study. The results show that the inter-annual variations in storm rainfall during the FRS in SC seem to be mainly influenced by the frequency of storm rainfall, while both frequency and intensity affect the inter-decadal variations in the total storm rainfall. Using the definitions for the beginning and ending dates of the FRS, and the onset dates of the summer monsoon in SC, the FRS is further divided into two sub-periods, i.e., the frontal and monsoon rainfall periods. The inter-annual and inter-decadal variations in storm rainfall during these two periods are investigated here. The results reveal a significant out-of-phase correlation between the frontal and monsoon storm rainfall, especially on the inter-decadal timescale, the physical mechanism for which requires further investigation.

INFLUENCES OF LOW-FREQUENCY MOISTURE TRANSPORTATION ON LOW FREQUENCY PRECIPITATION ANOMALIES IN THE ANNUALLY FIRST RAINY SEASON OF SOUTH CHINA IN 2010

85-station daily precipitation data from 1961-2010 provided by the National Meteorological Information Center and the NCEP/NCAR 2010 daily reanalysis data are used to investigate the low-frequency variability on the precipitation of the first rain season and its relationships with moisture transport in South China,and channels of low-frequency water vapor transport and sources of low-frequency precipitation are revealed.The annually first raining season precipitation in 2010 is mainly controlled by 10-20 d and 30-60 d oscillation.The rainfall is more(interrupted) when the two low-frequency components are in the same peak(valley) phase,and the rainfall is less when they are superposed in the inverse phase.The 10-20 d low-frequency component of the moisture transport is more active than the 30-60 d.The10-20 d water vapor sources lie in the South India Ocean near 30° S,the area between Sumatra and Kalimantan Island(the southwest source),and the equatorial middle Pacific region(the southeast source),and there are corresponding southwest and southeast moisture transport channels.By using the characteristics of 10-20 d water vapor transport anomalous circulation,the corresponding low-frequency precipitation can be predicted 6 d ahead.

SSTA SIGNAL CHARACTERISTIC ANALYSIS OVER THE INDIAN OCEAN DURING RAINY SEASON IN CHINA

The teleconnection distribution characteristics of sea surface temperature (SST) over the India Ocean and the precipitation during rainy season in China were studied by using the methods of EOF and CCA. The results indicate that the change of SST field will affect the change of rain belt during rainy seasons in China, and greatly affect the precipitation in northwest and southwest China, the Yangzi and Yellow River downstream basins. Strong signal phenomena of SSTA over India Ocean were revealed that showed the anoma-lous distribution of drought and flood in China. It shows that the precipitation during rainy seasons in China may be forecast by analyzing SST distribution characteristics over the India Ocean.

Analysis of Paths and Sources of Moisture for the South China Rainfall during the Presummer Rainy Season of 1979–2014

The paths and sources of moisture supplied to South China during two periods of the presummer rainy season （April-June） of 1979-2014, i.e., before and after the onset of the summer monsoon over the South China Sea （SCS）, are investigated by using the Hybrid Single-Particle Lagrangian Integrated Trajectory （HYSPLIT） model. During the premonsoon-onset period, the moisture transport trajectories are clustered into 6 groups, with four ocean-originating paths providing 83.9% and two continent-originating paths （originating over Lake Baikal and the Persian Gulf） con- tributing the remaining 16.1% of the total moisture. The two Pacific-originating paths, from the western Pacific Ocean and the East China Sea, combined account for about 46%, the SCS-originating path contributes about 24.3%, while the Bay of Bengal-originating path accounts for 13.6% of the total moisture over South China. The trajectories during the postmonsoon-onset period are clustered into 4 groups, with three southwesterly paths （from the Arabian Sea, the central Indian Ocean, and the western Indian Ocean, respectively） accounting for more than 76% and the sole Pacific-originating path accounting for 23.8% of the total moisture. The formation of the moisture transport tra-jectories is substantially affected by the topography, especially the Tibetan Plateau and the Indian and Indo-China Peninsulas. The SCS region contributes the most moisture during both periods （35.3% and 31.1%）. The Pacific Ocean is ranked second during the former period （about 21.0%） but its contribution is reduced to 5.0% during the lat-ter period, while the contribution from the Bay of Bengal and the Indian Ocean combined increases from 17.1% to 43.2%.

NUMERICAL SIMULATION OF A MESOSCALE CONVECTIVE SYSTEM(MCS)DURING THE FIRST RAINY SEASON OVER SOUTH CHINA

Based on the NCEP/NCAR reanalysis data and observations collected during the SCSMEX,a mesoscale convective system (MCS) occurring over South China during 23-24 May 1998 has been studied with a numerical simulation using the Fifth Generation Penn-State/NCAR Mesoseale Modeling System (MMS).The successful simulations present us some interesting findings.The simulated MCS was a kind of meso-β scale system with a life cycle of about 11 hours.It generated within a small vortex along a cold front shear line.The MCS was characterized by severe convection.The simulated maximum vertical velocity was greater than 90 cm s^(-1),and the maximum divergence at about 400 hPa.The rainfall rate of MCS exceeded 20 mm h^(-1).To the right of the simulated MCS,a mesoscale low-level jet (mLLJ) was found.A strong southwesterly current could also be seen to the right of MCS above the mLLJ.This strong southwesterly current might extend up to 400 hPa.A column of cyclonic vorticity extended through most part of the MCS in the vertical direction.Additionally,the simulated MCS was compared favorably with the observational data in terms of location,precipitation intensity and evolution.

IOD Modoki事件对华南后汛期降水异常的影响及可能机理

利用1979—2019年CN05.1中国区域高分辨率降水格点数据、英国Hadley中心观测海温数据、ERA5逐月大气再分析资料及大气环流模式,研究了华南后汛期期间(7—9月)IOD Modoki事件与华南后汛期降水异常的关系及可能机理。观测资料结果表明,华南后汛期降水异常与热带印度洋中部(东和西部)海温异常呈显著正(负)相关关系,表现为印度洋IOD Modoki或印度洋三极子事件的空间分布型。滤除ENSO信号影响后,华南后汛期降水异常仍和IOD Modoki存在较为密切的联系。IOD Modoki正异常对华南后汛期降水异常的影响有以下途径,一方面,异常水汽从热带印度洋东部向西输送至热带中印度洋后,在北半球受科氏力作用向东输送至华南地区,为华南地区提供了充足的水汽条件,并且对华南地区降水正异常的主要水汽辐合贡献为平均水汽的水平扰动散度项和扰动引起的平均水汽垂直平流项。另一方面,热带东南印度洋海温负异常,通过Mastuno-Gill响应引起对流层低层自热带东南印度洋至热带中印度洋有东南风异常,增强了70°E附近的越赤道气流,在北半球向东输送至西北太平洋,这引起了华南地区对流层低层气旋式环流异常。另外,热带东印度洋对流层低(高)层异常辐散(辐合),华南地区低(高)层异常辐合(辐散)增强了东亚地区的局地Hadley环流,有利于华南地区降水的产生。再者,IOD Modoki引起南亚季风区受异常下沉运动控制,并通过季风-荒漠机制引起副热带北大西洋东部、北非荒漠区及地中海西部周围正涡度异常,激发了沿急流向下游传播的准静止Rossby波,增强了日本海高压异常和华南及邻近地区对流层低层气旋式环流异常。上述原因均有利于华南地区降水的产生,反之亦然。上述结果在数值模式中亦得到了验证。

东、黄海海表面温度季节内变化特征的EOF分析

基于1998—2004年的TRMM/TMI卫星遥感海面温度(SST)数据,在初步分析东、黄海SST的季节分布特征的基础上,采用EOF方法分析了SST的季节内变化特征,进而对SST季节内变化的可能机制进行了探讨。EOF分析获得的前4个模态的累积方差贡献率为57.07%,其结果基本反映了东、黄海SST变化的主要物理过程。其中,EOF的第一模态的方差贡献率占30.17%,其空间模态揭示了以东海北部为中心的、整个海域SST变化趋于一致的特征,这一模态的显著变化周期为6.3周;第二模态的方差贡献率占14.36%,其空间模态呈现东南海域与西北海域SST的反相变化趋势,显著变化周期为8.7周和10.6周;第三模态的方差贡献率占7.02%,其空间SST变率最大的区域位于黄海海域,显著变化周期为6.8,8.7,10.2周等;第四模态的方差贡献率占5.52%,其空间SST变率最大的区域位于东、黄海近海,显著变化周期为6.8周。东、黄海SST季节内变化与此海区大气中的季节内振荡是紧密相关的。

南海海面高度年际变化的S-EOF分析

应用依赖于季节的经验正交函数(S-EOF)分析,探讨了最近15a南海海面高度随季节演变的年际变化。S-EOF分析得出南海海面高度异常各模态不同季节的空间结构以及时间演变过程,证实了季风强盛期冬季和夏季基本模态的结构,还分离出了季风转换期(春季和秋季)海面高度的分布格局。结果表明,南海海面高度随季节演变的年际变化与厄尔尼诺和拉尼娜事件密切相关。S-EOF1的结果表明,南海海面高度的变化具有明显而稳定的季节振荡,但在ENSO年海面高度的季节振荡相对减弱;S-EOF2模态显示了1998—2001年间冬季吕宋岛西侧存在一个较强的正异常,且能一直持续到春季;S-EOF3模态主要体现了南海西部一系列中尺度涡状结构的年际差异,包含1997/1998年厄尔尼诺对南海环流的影响。

基于EOF分析华南前汛期降水的时空分布特征

基于广西、广东、湖南和福建62个台站1979—2005年的前汛期平均总降水量,利用EOF分解方法,分析了华南前汛期降水的时空分布特征和时间分布特征.EOF分解显示前3个方差贡献比例为54.74%;第一特征向量场空间分布均为正值,方差贡献比例为32.49%,说明华南前汛期平均总降水量变化具有一致性,但是各特征向量场之间的差异明显;第一时间系数的变化等同于华南前汛期降水量距平的变化;第二、第三时间系数的变化有明显的年际变化特征.

基于EOF的降尺度法在福建雨季降水预测中的应用

本文介绍了统计降尺度法（Downscaling）的基本原理并详细介绍了基于EOF（empirical orthogonal function）分析的降尺度方法，利用NCEP再分析资料的北半球雨季500hPa高度场和850hPa湿度场预报产品，采用基@EOF分析的降尺度方法对福建省的雨季降水进行预测，取得了较好的结果，表明用降尺度法预测福建的雨季降水是可行的，具有较高的技巧，在业务中有一定的应用价值。

Mesoscale Observational Analysis of Lifting Mechanism of a Warm-Sector Convective System Producing the Maximal Daily Precipitation in China Mainland during Pre-Summer Rainy Season of 2015

A long-lived, quasi-stationary mesoscale convective system （MCS） producing extreme ramtall （maximum of 542 mm） over the eastern coastal area of Guangdong Province on 20 May 2015 is analyzed by using high-resolution surface observations, sounding data, and radar measurements. New convective ceils are continuously initiated along a mesoscale boundary at the surface, leading to formation and maintenance of the quasi-linear-shaped MCS from about 2000 BT 19 to 1200 BT 20 May. The boundary is originally formed between a cold dome generated by previous convection and southwesterly flow from the ocean carrying higher equivalent potential temperature （θe） air. The boundary is subsequently maintained and reinforced by the contrast between the MCS-generated cold outflow and the oceanic higher-θe air. The cold outflow is weak （wind speed ≤ 5 m s-1）, which is attributable to the characteristic environmental conditions, i.e., high humidity in the lower troposphere and weak horizontal winds in the middle and lower troposphere. The low speed of the cold outflow is comparable to that of the near surface southerly flow from the ocean, resulting in very slow southward movement of the boundary. The boundary features temperature contrasts of 2-3℃ and is roughly 500-m deep. Despite its shallowness, the boundary appears to exert a profound influence on continuous convection initiation because of the very low level of free convection and small convection inhibition of the near surface oceanic air, building several parallel rainbands （of about 50-kin length） that move slowly eastward along the MCS and produce about 80% of the total rainfall. Another MCS moves into the area from the northwest and merges with the local MCS at about 1200 BT. The cold outflow subsequently strengthens and the boundary moves more rapidly toward the southeast, leading to end of the event in 3 h.

Assimilating Doppler radar observations with an ensemble Kalman filter for convection-permitting prediction of convective development in a heavy rainfall event during the pre-summer rainy season of South China

This study examines the effectiveness of an ensemble Kalman filter based on the weather research and forecasting model to assimilate Doppler-radar radial-velocity observations for convection-permitting prediction of convection evolution in a high-impact heavy-rainfall event over coastal areas of South China during the pre-summer rainy season. An ensemble of 40 deterministic forecast experiments(40 DADF) with data assimilation(DA) is conducted, in which the DA starts at the same time but lasts for different time spans(up to 2 h) and with different time intervals of 6, 12, 24, and 30 min. The reference experiment is conducted without DA(NODA).To show more clearly the impact of radar DA on mesoscale convective system(MCS)forecasts, two sets of 60-member ensemble experiments(NODA EF and exp37 EF) are performed using the same 60-member perturbed-ensemble initial fields but with the radar DA being conducted every 6 min in the exp37 EF experiments from 0200 to0400 BST. It is found that the DA experiments generally improve the convection prediction. The 40 DADF experiments can forecast a heavy-rain-producing MCS over land and an MCS over the ocean with high probability, despite slight displacement errors. The exp37 EF improves the probability forecast of inland and offshore MCSs more than does NODA EF. Compared with the experiments using the longer DA time intervals, assimilating the radial-velocity observations at 6-min intervals tends to produce better forecasts. The experiment with the longest DA time span and shortest time interval shows the best performance.However, a shorter DA time interval(e.g., 12 min) or a longer DA time span does not always help. The experiment with the shortest DA time interval and maximum DA window shows the best performance, as it corrects errors in the simulated convection evolution over both the inland and offshore areas. An improved representation of the initial state leads to dynamic and thermodynamic conditions that are more conducive to earlier initiation of the inland MCS and longer maintenance of the offshore MCS.

Variations in the starting date of the pre-summer rainy season in South China, 1736-2010

The starting dates of the pre-summer rainy season during historical times （1736- 1911） in Fuzhou and Guangzhou of South China, were determined and reconstructed on the basis of historical documents in the Yu-Xue-Fen-Cun archive, together with observed features of precipitation during the pre-summer rainy season. In addition, starting dates of the pre-summer rainy season from 1953 in Fuzhou and from 1952 in Guangzhou were reconstructed for the instrumental period. These data allowed for analyses of inter-annual and inter-decadal changes in the starting dates of the pre-summer rainy season in South China over the past 300 years. Results show that the mean starting date of the pre-summer rainy season in South China was the first pentad of May; in addition, periodicities in the starting dates of 2-3 years, 10 years, and 40 years were detected during the period 1736-1911, and of 2-3 years, 10 years, and 22 years during the instrumental period. From 1736 to 1911, the earliest starting dates at Fuzhou and Guangzhou both occurred at the fourth pentad of April, while the latest starting dates were at the sixth pentad of May in Fuzhou and the first pentad of June in Guangzhou. During the instrumental period, the earliest and latest starting dates were at the fourth pentad of April and the first pentad of June, respectively, in both Fuzhou during 1953-2010 and Guangzhou during 1952-2010. The maximum difference between neighboring decades during 1736-1911 was 2.2 and 1.6 pentads in Fuzhou and Guangzhou, respectively, and during the instrumental period it was 2.5 and 2.4 pentads in Fuzhou and Guangzhou, respectively.

热带北太平洋海温异常对2018年华南前汛期降水负异常事件的影响

本文利用1979—2018年中国地面气象台站1767个站点降水资料、HadISST海温数据及NCEP/NCAR再分析资料,滤除同期ENSO影响后,分析了2018年华南前汛期降水负异常与热带北太平洋海温异常之间的联系。研究结果显示,2018年华南前汛期降水负异常事件和热带北太平洋(Tropical Northern Pacific,TNP)海温偏暖有关。热带北太平洋海温正异常,通过Mastuno-Gill响应引起西北太平洋气旋式环流异常,中国南海东风异常,减弱了南海夏季风,华南至中国南海地区对流层低层存在异常反气旋环流。水汽存在由中纬度北太平洋经黑潮海区-华南地区-中国南海-菲律宾群岛向热带太平洋的异常输送,且华南地区为水汽的异常辐散区域。另一方面,TNP区域暖海温异常引起了该区域低层异常辐合、高层异常辐散、异常上升运动,使得华南地区高层异常辐合、低层异常辐散、异常下沉运动。这样的环流配置对华南地区降水的产生有不利影响,引起了2018年华南前汛期降水异常偏少。2018年TNP区域暖海温异常引起华南前汛期降水负异常的物理机制还在ECHAM5模式30个成员集合平均的试验结果中得以验证。

青藏高原冬季积雪与华南前汛期降水年际变率的联系

利用1979—2018年青藏高原(简称高原,下同)卫星积雪数据集、华南地区261站逐日降水及ERA5再分析资料,探讨了高原冬季积雪与华南前汛期降水的联系。结果表明:1)高原西部积雪与华南前汛期降水的正相关关系最为稳定,其主要影响前汛期的锋面降水,对夏季风降水的影响较小;2)华南前汛期在高原西部积雪偏多年比偏少年偏早20 d,使得前汛期降雨日数偏多,持续时间偏长,总降水量偏多,而降水强度受积雪的影响较小;3)高原积雪偏多年,积雪的冷却作用形成了低层异常反气旋环流,而东亚沿岸为“+-+”的位势高度异常,中纬度“西高东低”的环流配置有利于中高纬冷空气南侵,使得华南上空温度偏低,同时偏强偏南的西太平洋副热带高压加强了低纬地区偏南气流和水汽输送。3—4月锋面在华南北部南北摆动,4月初偏北干冷空气南侵和偏南暖湿气流的持续北推使得锋面加强,触发了前汛期的较早建立;积雪偏少年冷空气和偏南暖湿气流均较弱,华南北部锋面在4月初中断,4月中下旬华南北部锋面在偏北弱冷空气和偏南暖湿气流的共同作用下重新建立,从而华南前汛期开始偏晚。

SEASONAL VARIATIONS OF CONVECTIVE ACTIVITIES OVER THE SOUTH CHINA SEA AND ITS NEIGHBORHOOD AND THEIR COMPARATIVE ANALYSES IN THE STRONG AND WEAK CONVECTION YEARS

The seasonal variations of convective activities over the South China Sea(SCS)and its neighborhood.as well as the similarities and differences of convection in the different key regions during the strong and weak convection years are analyzed by using the pentad data of TBB from 1980 to 1993.The results show that in winter and summer the seasonal variations of the convective activities are synchronous over the SCS and its neighborhood,the anomalous convection amplitudes are obviously different in different regions.The significant extents of convective activities have somewhat seasonal differences in the strong and weak convection years.In the strong convection years,it is in winter,spring and autumn that the convection anomaly is more evident than that in the normal years,however,after the summer monsoon onset the convection is sustained.stable and similar to that in the normal years.In the weak convection years.the convection weakens greatly in each season.but the primary weakening occurs in spring.summer and autumn.No matter in the strong or the weak convection years.the convective activities are somewhat of difference in the Bay of Bengal.the Indochina Peninsula.the SCS and the Philippines.In addition. the convective activities are also different over the south and the north parts of the SCS.the convection variation in the strong year is similar to that in the weak year over the north part of the SCS.but over the south part there are great differences.

汛期我国主要雨季进程成因及预测应用进展

汛期内我国中东部地区的雨季是东亚夏季风推进过程中的重要产物,主要包括华南前汛期、梅雨、华北雨季和华西秋雨等,各地雨季决定了我国中东部地区汛期的旱涝布局和旱涝演变,是我国汛期预测和服务的重点.该文回顾了4个雨季特征及影响因子方面的研究进展,在此基础上梳理物理概念预测模型.研究显示:海温异常是影响各区域雨季的重要先兆信号,但不同雨季的年际和年代际变化特征不同,海温作为外强迫信号的影响程度和时空形式也有差异.利用热带太平洋东西海温差指标能更好地解释华南前汛期降水的年际变化.而与梅雨的年际变化分量相关联的海温关键区主要分布于热带,与年代际或多年代际变化分量相联系的海温关键区则来自中高纬度.华北雨季降水的强弱不仅与ENSO循环的位相有关,更多受到ENSO演变速率的影响.而影响华西秋雨的海温关键区随着年代际背景的变化发生了改变,需要重新诊断和建模.

华南前汛期区域持续性暴雨的分布特征及分型

利用广东和广西两省共175个台站的日降水观测资料,采用计算机检索的方法,对1961-2005年间华南前汛期区域持续性暴雨进行了定义。对南海夏季风爆发前后区域持续性暴雨的气候分布特征的分析发现,季风爆发前持续性暴雨频数从60年代至今呈现出正态分布的年代际变化特征,而季风爆发后的区域持续性暴雨频数变化则几乎相反;广东省前汛期区域持续性暴雨降水明显比广西强。此外,通过EOF方法和相关分析得到了夏季风爆发前后出现频率较高的几种分布雨型,它们能较好地代表季风爆发前后华南降水分布的特点。

华南前汛期的锋面降水和夏季风降水II.空间分布特征

利用中国730站降水资料和第I部分(郑彬等,2006)得到的华南前汛期锋面降水和季风降水的划分日期,计算出1958-2000年华南前汛期锋面降水量(强度)和季风降水量(强度)的序列,采用EOF和扩展EOF分析方法,得到华南前汛期降水的几个主要分布型,并探讨锋面降水与季风降水的可能联系。分析结果表明:华南前汛期的锋面降水和季风降水分布主要有三种类型——全区旱涝型、西南涝(旱)东北旱(涝)型、东南涝(旱)西北旱(涝)型。各分布型的时间系数与850 hPa风场的相关结果表明不同的分布对应着不同的低层环流形势。统计结果显示华南前汛期锋面降水的分布形式与季风降水的分布形式有一定的对应关系。