Interannual Variation of Tropical Night Frequency in Beijing and Associated Large-Scale Circulation Background

This study examined the variability in frequency of tropical night occurrence （i.e., minimum air tem- perature 25℃） in Beijing, using a homogenized daily temperature dataset during the period 1960–2008. Our results show that tropical nights occur most frequently in late July and early August, which is consis- tent with relatively high air humidity associated with the rainy season in Beijing. In addition, year-to-year variation of tropical night occurrence indicates that the tropical nights have appeared much more frequently since 1994, which can be illustrated by the yearly days of tropical nights averaged over two periods： 9.2 days of tropical nights per year during 1994–2008 versus 3.15 days during 1960–1993. These features of tropical night variations suggest a distinction between tropical nights and extreme heat in Beijing. We further investigated the large-scale circulations associated with the year-to-year variation of tropical night occurrence in July and August, when tropical nights appear most frequently and occupy 95% of the annual sum. After comparing the results in the two reanalysis datasets （NCEP/NCAR and ERA-40） and considering the possible effects of decadal change in the frequency of tropical nights that occurred around 1993/94, we conclude that on the interannual time scale, the cyclonic anomaly with a barotropic structure centered over Beijing is responsible for less frequent tropical nights, and the anticyclonic anomaly is responsible for more frequent occurrence of tropical nights over Beijing.

Comparative Analysis of Energy Characteristics of Two Southwest Vortices in Sichuan Under Similar Circulation Backgrounds

Based on ERA5 reanalysis data,the present study analyzed the thermal energy development mechanism and kinetic energy conversion characteristics of two extreme rainstorm processes in relation to the shallow southwest vortex in the warm-sector during a“rain-generated vortex”process and the deep southwest vortex in a“vortex-generated rain”process.The findings were as follows:(1)During the extreme rainstorm on August 11,2020(hereinafter referred to as the“8·11”process),intense surface heating and a high-energy unstable environment were observed.The mesoscale convergence system triggered convection to produce heavy rainfall,and the release of latent condensation heat generated by the rainfall promoted the formation of a southwest vortex.The significant increase(decrease)in atmospheric diabatic heating and kinetic energy preceded the increase(decrease)in vorticity.By contrast,the extreme rainstorm on August 16,2020(hereinafter referred to as the“8·16”process)involved the generation of southwest vortex in a low-energy and highhumidity environment.The dynamic uplift of the southwest vortex triggered rainfall,and the release of condensation latent heat from rainfall further strengthened the development of the southwest vortex.The significant increase(decrease)in atmospheric diabatic heating and kinetic energy exhibited a delayed progression compared to the increase(decrease)in vorticity.(2)The heating effect around the southwest vortex region was non-uniform,and the heating intensity varied in different stages.In the“8·11”process,the heating effect was the strongest in the initial stage,but weakened during the vortex's development.On the contrary,the heating effect was initially weak in the“8·16”process,and intensified during the development stage.(3)The available potential energy of the“8·11”process significantly increased in kinetic energy converted from rotational and divergent winds through baroclinic action,and the divergent wind energy continued to convert into rotational wind energy.By contrast,the“8·16”process involved the conversion of rotational wind energy into divergent wind energy,which in turn converted kinetic energy back into available potential energy,thereby impeding the further development and maintenance of the southwest vortex.

MM5 Simulations of the China Regional Climate During the LGM.Ⅱ:Influence of Change of Land Area,Vegetation,and Large-scale Circulation Background

Using a regional climate model MM5 nested to an atmospheric global climate model CCM3, a series of simulations and sensitivity experiments have been performed to investigate the relative LGM climate response to changes of land-sea distribution, vegetation, and large-scale circulation background over China. Model results show that compared with the present climate, the fluctuations of sea-land distribution in eastern Asia during the LGM result in the temperature decrease in winter and increase in summer. It has significant impact on the temperature and precipitation in the east coastal region of China. The impact on precipitation in the east coastal region of China is the most significant one, with 25%-50% decrease in the total precipitation change during the LGM. On the other hand, the changes in sea-land distribution have less influence on the climate of inland and western part of China. During the LGM, significant changes in vegetation result in temperature alternating with winter increase and summer decrease, but differences in the annual mean temperature are minor. During the LGM, the global climate, i.e., the large-scale circulation background has changed significantly. These changes have significant influences on temperature and precipitation over China. They result in considerable temperature decreases in this area, and direct the primary patterns and characteristics of temperature changes. Results display that, northeastern China has the greatest temperature decrease, and the temperature decrease in the Tibetan Plateau is larger than in the eastern part of China located at the same latitude. Moreover, the change of large-scale circulation background also controls the pattern of precipitation change. Results also show that, most of the changes in precipitation over western and northeastern parts of China are the consequences of changing large-scale circulation background, of which 50%-75% of precipitation changes over northern and eastern China are the results of changes in large-scale circulation background. Over China, the LGM climate responses to different mechanisms in order of strength from strong to weak are, the large-scale circulation pattern, sea- land distribution, vegetation, CO2 concentration, and earth orbital parameters.

CLIMATE PREDICTION EXPERIMENT FOR TROPICAL CYCLONE GENESIS FREQUENCY USING THE LARGE-SCALE CIRCULATION FORECAST BY A COUPLED GLOBAL CIRCULATION MODEL

Based on an analysis of the relationship between the tropical cyclone genesis frequency and large-scale circulation anomaly in NCEP reanalysis, large-scale atmosphere circulation information forecast by the JAMSTEC SINTEX-F coupled model is used to build a statistical model to predict the cyclogenesis frequency over the South China Sea and the western North Pacific. The SINTEX-F coupled model has relatively good prediction skill for some circulation features associated with the cyclogenesis frequency including sea level pressure, wind vertical shear, Intertropical Convergence Zone and cross-equatorial air flows. Predictors derived from these large-scale circulations have good relationships with the cyclogenesis frequency over the South China Sea and the western North Pacific. A multivariate linear regression(MLR) model is further designed using these predictors. This model shows good prediction skill with the anomaly correlation coefficient reaching, based on the cross validation, 0.71 between the observed and predicted cyclogenesis frequency. However, it also shows relatively large prediction errors in extreme tropical cyclone years(1994 and 1998, for example).

Large-scale circulation features associated with the heat wave over Northeast China in summer 2018

In late July and early August 2018,Northeast China suffered from extremely high temperatures,with the maxium temperature anomaly exceeding 6°C.In this study,the large-scale circulation features associated with this heat wave over Northeast China are analyzed using station temperature data and NCEP–NCAR reanalysis data.The results indicate that strong anomalous positive geopotential height centers existed from the lower to upper levels over Northeast China,and the related downward motions were directly responsible for the extreme high-temperature anomalies.The northwestward shift of the western Pacific subtropical high(WPSH)and the northeastward shift of the South Asian high concurrently reinforced the geopotential height anomalies and descending flow over Northeast China.In addition,an anomalous Pacific–Japan pattern in the lower troposphere led to the northwestward shift of the WPSH,jointly favoring the anomalous geopotential height over Northeast China.Two wave trains emanating from the Atlantic region propagated eastwards along high latitudes and midlatitudes,respectively,and converged over Northeast China,leading to the enhancement of the geopotential height anomalies.

Drought variability and its connection with large-scale atmospheric circulations in Haihe River Basin

Drought is one of the most widespread and devastating extreme climate events when water availability is significantly below normal levels for a long period.In recent years,the Haihe River Basin has been threatened by intensified droughts.Therefore,characterization of droughts in the basin is of great importance for sustainable water resources management.In this study,two multi-scalar drought indices,the standardized precipitation evapotranspiration index(SPEI)with potential evapotranspiration calculated by the PenmaneMonteith equation and the standardized precipitation index(SPI),were used to evaluate the spatiotemporal variations of drought characteristics from 1961 to 2017 in the Haihe River Basin.In addition,the large-scale atmospheric circulation patterns were used to further explore the potential links between drought trends and climatic anomalies.An increasing tendency in drought duration was detected over the Haihe River Basin with frequent drought events occurring in the period from 1997 to 2003.The results derived from both SPEI and SPI demonstrated that summer droughts were significantly intensified.The analysis of large-scale atmospheric circulation patterns indicated that the intensified summer droughts could be attributed to the positive geopotential height anomalies in Asian mid-high latitudes and the insufficient water vapor fluxes transported from the south.©2021 Hohai University.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

The interannual periodicity of precipitation and its links to the large-scale circulations over the Third Pole

To understand the spatio-temporal variability of precipitation(P)in the Third Pole region(centered on the Tibetan Plateau-TP),it is necessary to quantify the interannual periodicity of P and its relationship with large-scale circulations.In this study,Morlet wavelet transform was used to detect significant(p<0.05)periodic characteristics in P data from meteorological stations in four climate domains in the Third Pole,and to reveal the major large-scale circulations that triggered the variability of periodic P,in addition to bringing large amounts of water vapour.The wavelet transform results were as follows.(1)Significant quasiperiodicity varied from 2 to 11 years.The high-frequency variability mode(2 to 6 years quasi-periods)was universal,and the low-frequency variability mode(7 to 11 years quasi-periods)was rare,occurring mainly in the westerlies and Indian monsoon domains.(2)The majority of periods were base periods(53%),followed by two-base periods.Almost all stations in the Third Pole(95%)showed one or two periods.(3)Periodicity was widely detected in the majority of years(84%).(4)The power spectra of P in the four domains were dominated by statistically significant high-frequency oscillations(ie.,with short periodicity).(5)Large-scale circulations directly and indirectly influenced the periodic P variability in the different domains.The mode of P variability in the different domains was influenced by interactions between large-scale circulation features and not only by the dominant circulation and its control of water vapour transport.The results of this study will contribute to better understanding of the causal mechanisms associated with P variability,which is important for hydrological science and waterresourcemanagement.

Differences in Variations of Long-Lived and Short-Lived Summer Heat Waves during 1981-2020 over Eastern China and Their Corresponding Large-Scale Circulation Anomalies

Using daily maximum temperature(Tmax)data from 516 observation stations in eastern China from 1981 to 2020,this study employed a relative threshold method to define short-and long-lived heat waves(HWs)by considering regional climate differences to investigate the spatial characteristics and evolution of large-scale circulation during summer HWs.The results demonstrated spatial disparities in the frequency distribution of HWs of different durations and differences in the magnitude of duration and intensity between short-and long-lived HWs.Empirical orthogonal function analysis revealed three dominant spatial modes for both short-and long-lived HWs.The first mode showed that short-lived HWs occur prominently in both northern and southern regions,whereas long-lived HWs mainly occur in the northern region.The second mode was characterized by a meridional dipole pattern in both cases.The third mode exhibited a quadrupole pattern for short-lived HWs and a tripole pattern for long-lived HWs.Differences in the center locations of anomalies in the 500-hPa geopotential height and 850-hPa wind fields significantly influenced the temperature and precipitation anomaly distribution of typical HWs by affecting the warm column in the lower troposphere,cloud distribution,and moisture transport.Moreover,the atmospheric circulation evolution processes of typical HWs associated with the different modes of long-and short-lived HWs were linked to distinct teleconnection patterns.During the three modes of long-lived(short-lived)HWs,there was stronger(weaker)wave flux activity with multiple(single)propagation paths.Stronger westward Atlantic wave train activity at 300 hPa triggered the synergistic action of meridional and zonal wave fluxes,favoring the strengthening and maintenance of positive anomalies in geopotential height of 500 hPa.This may have contributed to the formation of long-lived HWs.These findings provide valuable insights to enhance our understanding and prediction of summer HWs.

An Eddy-Permitting Oceanic General Circulation Model and Its Preliminary Evaluation

An eddy-permitting, quasi-global oceanic general circulation model, LICOM (LASG/IAP (State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics) Climate System Ocean Model), with a uniform grid of 0.5? × 0.5? is established. Forced by wind stresses from Hellerman and Rosenstain (1983), a 40-yr integration is conducted with sea surface temperature and salinity being restored to the Levitus 94 datasets. The evaluation of the annual mean climatology of the LICOM control run shows that the large-scale circulation can be well reproduced. A comparison between the LICOM control run and a parallel integration of L30T63, which has the same framework but a coarse resolution, is also made to con?rm the impact of resolution on the model performance. On account of the reduction of horizontal viscosity with the enhancement of the horizontal resolution, LICOM improves the simulation with respect to not only the intensity of the large scale circulations, but also the magnitude and structure of the Equatorial Undercurrent and South Equatorial Current. Taking advantage of the ?ne grid size, the pathway of the Indonesian Through?ow (ITF) is better represented in LICOM than in L30T63. The transport of ITF in LICOM is more convergent in the upper layer. As a consequence, the Indian Ocean tends to get warmer in LICOM. The poleward heat transports for both the global and individual basins are also signi?cantly improved in LICOM. A decomposed analysis indicates that the transport due to the barotropic gyre, which primarily stands for the barotropic e?ect of the western boundary currents, plays a crucial role in making the di?erence.

Relationships between the Oxygen Isotopes in East Asian Stalagmites and Large-Scale Atmospheric and Oceanic Modes

The stalagmite δ18O record is known to be associated with the climate, but the specifics of the relationship remain unclear. It may not represent variation in air temperature or precipitation, but instead reflect integral changes of monsoon circulation, especially water vapor sources(the so-called "circulation effect"). Since large-scale atmospheric-oceanic modes such as the Atlantic Multidecadal Oscillation(AMO), Pacific Decadal Oscillation(PDO), and North Atlantic Oscillation(NAO) exert significant effects on Asian monsoon, in this paper the authors investigate the relationships of the East Asian stalagmite δ18O record with these modes. The last three centuries form the focus of our study, for which the authors use reconstructed as well as instrumental data. Considering the impacts of human activity, our analysis is conducted with respect to two periods—the pre- and post-industrial periods. The results show significant lead-lag connections: a positive correlation peaks when the PDO leads East Asian stalagmite δ18O by 3 years, which is persistent over the past 300 years; while the relationships of the AMO and NAO with the East Asian stalagmite δ18O record show significant differences in the post-industrial relative to the pre-industrial period. This implies that the East Asian stalagmite δ18O record may primarily reflect the PDO signal.

Circulation-Enhanced Tank Heating Using Shallow Profile Coil Bundles

Heavy fuel oils require heated tanks to facilitate their transportation and processing.This paper proposes and investigates threeand four-level heating coil bundles.Numerical study revealed that powerful large-scale circulation of the heated fluid enhances heat transfer,delivering 16.7%and 23%improvements to the average heat transfer coefficient for the three-and four-level bundles,respectively.Furthermore,this circulation improves oil mixing and limits the variation in bulk oil temperature to-0.3 to+1.3°C from the average.The study also quantified oil flow velocity near the bottom of the tank.The time-averaged horizontal components of velocity,estimated 25 mm and 50 mm above the bottom of the tank,exceed 2 mm/s and 4 mm/s,respectively.The proposed heating coil bundles feature a compact design that reduces the material and labor costs of construction and that,by occupying only a small portion of the bottom of the tank,improves accessibility,maintenance,and cleaning.

Evolving Perspectives on Abrupt Seasonal Changes of the General Circulation

Professor Duzheng YE（Tu-cheng YEH） was decades ahead of his time in proposing a model experiment to investigate whether abrupt seasonal changes of the general circulation can arise through circulation feedbacks alone, unrelated to underlying inhomogeneities at the lower boundary. Here, we introduce Professor YEH＇s ideas during the 1950 s and 1960 s on the general circulation and summarize the results and suggestions of Yeh et al.（1959） on abrupt seasonal changes. We then review recent advances in understanding abrupt seasonal changes arising from model experiments like those proposed by Yeh et al.（1959）. The model experiments show that circulation feedbacks can indeed give rise to abrupt seasonal transitions.In these transitions, large-scale eddies that originate in midlatitudes and interact with the zonal mean flow and meridional overturning circulations in the tropics play central roles.

Circulation Characteristics Analysis on A Rare Light Rain Changing to Snowstorm Weather

By using the ground,high-altitude conventional observation data and NCEP global reanalysis data,the circulation background and the snowfall causes of light rain changing to snowstorm weather process in Panjin during February 12-13 in 2009 were analyzed in detail.Focusing on the rain and snow phase state conversion time,the effects of precipitation phase state transformation time on this precipitation forecast were discussed.The results showed that the circulation adjustment,the eastward shift and down south of polar vortex and the frequent activity of cold air were the main factors which caused the precipitation was obviously more in February in Panjin.The direct reasons which caused the strong snowfall in Panjin were that the warm and cold air evenly matched for a long time,and the southwest jet provided the sufficient water vapor condition.

Changes in Persistent Precipitation in Northwest China and Related Large-Scale Circulation Features

Based on China's daily precipitation data of 2415 stations and ERA5 hourly reanalysis data from 1961 to 2019, the station-based and regional precipitation events over Northwest China(NWC) are identified and sorted into persistent precipitation(PP, duration ≥ 2 days) events and non-persistent precipitation(NPP, duration = 1 day) events;and then changes in the persistence structure of the PP and NPP events over NWC and the long-term mean adjustment of the related large-scale circulation configuration are analyzed. The results show that PP and NPP both witness an increasing trend over most parts of NWC. In terms of the total precipitation at most stations and the regional mean, contributions from PP have been increasing, while those from NPP have been decreasing. This demonstrates that the wetting trend in NWC is mainly caused by the increase in PP. Through analyzing the large-scale circulation corresponding to regional PP events at several representative levels, we found that the westerly jet at 200 hPa, the ridge/trough systems at 500 hPa, and the Mongolian low at sea level are the key circulation systems responsible for regional PP events over NWC. As for long-term mean changes after and before 1990(a shifting point recognized by previous studies), it is found that the extent of the South Asian high becomes larger and the westerly jet shifts northward by approximately 1.5 degrees in the upper troposphere. The ridge near the Ural Mountains and the ridge downstream of NWC strengthen by approximately 10–30 hPa at 500 hPa. Furthermore, the difference between the Mongolian low trough and its surrounding high pressure increases by approximately 2 hPa at the sea level. The combinations of circulation changes from upper to lower levels facilitate the strengthening of ascending motions. These adjustments in circulations create more favorable conditions for PP to occur over NWC in the last three decades.

宁夏2018年8月20—21日区域性暴雨环流背景和物理量因子分析

【目的】利用常规观测资料、探空资料和NCEP/NCAR 1°×1°的逐6 h再分析资料,对2018年8月20—21日宁夏区域性暴雨产生的环流背景和物理量因子进行分析,为当地暴雨预报提供参考。【方法】通过环流分析和物理量诊断方法,对此次暴雨产生的环流背景、影响因子、水汽、热力、动力条件等物理量因子进行分析。【结果】结果表明:此次降水过程是在欧亚“两槽两脊”形势背景下,配合中低层低涡、切变线产生的。700 hPa水汽输送是主要水汽条件。700 hPa以下,36°~39°N为强辐合区,强辐合区上空对应辐散区,且辐合中心略向北倾斜,有利于上下抽吸,增强上升运动。水平方向上辐合区、辐散区成对紧邻出现,有利于增强水平辐合辐散,以及上升运动的维持。西北-东南向高能舌的持续存在,为强对流天气提供了充足不稳定能量。【结论】有利的环流背景,700 hPa充足的水汽,配合一定的动力、热力条件造成了此次暴雨天气的生成、发展。

宁夏两次夏季典型暴雨过程成因对比分析

利用自动气象站站点实况和ECMWF ERA5的逐时再分析资料,通过环流分析和物理量诊断方法,对比了2017年6月4日和2019年8月2日宁夏区域性暴雨强降水区域、环流背景、影响系统和物理量因子的共性和差异。结果表明:后者累积降水量和最大小时雨强均比前者大;2017年6月4日,宁夏处于东高西低的环流形势中,强降水出现在低空急流的左前方、低空切变线的右前方(即宁夏中北部),稳定维持的地面中尺度辐合线触发了局地强降水;2019年8月2日暴雨过程出现在宁夏南北两头,是两槽一脊环流背景下,由500 hPa冷涡、冷槽和700 hPa低槽、切变、低空急流共同造成,是一次典型的强对流单体导致的暴雨过程;2019年8月2日短时强降水过程比湿、CAPE、ZH 0℃高度、-20℃层高度数值较大;两次短时强降水过程沙氏指数为正;2017年6月4日LI为正值,2019年8月2日LI为负值;2017年6月4日850 hPa与500 hPa假相当位温差为负值,2019年8月2日为正值;二者200 hPa、700 hPa为辐散,850 hPa为辐合,2019年8月2日200 hPa辐散中心强度大于2017年6月4日,而700 hPa辐散中心强度和850 hPa辐合中心强度反之。

中国春季复合极端低温多雨事件的年代际变化及成因分析

利用1961—2020年中国553站逐日最低气温、降水资料和ERA5再分析资料,探讨了我国春季复合极端低温多雨事件的年代际变化特征及可能成因。结果表明,春季复合极端低温多雨事件整体呈现出南多北少的分布型,并且高频次中心在西南和华南地区,年平均超过了5 d。全国绝大部分地区春季复合极端低温多雨事件近60年呈现减少趋势,在1990年代末期出现了由多转少的全国范围的年代际突变,并以我国东南部季风区的减少最为显著。进一步分析发现,我国北方至白令海峡地区的异常气旋性环流和我国东南侧西北太平洋上的异常反气旋性环流是引起年代际突变的两个关键环流系统。

2022年台风“梅花”引起吉林地区暴雨天气过程的诊断分析

研究选择自动气象站资料、NCEP再分析资料等相关观测资料诊断分析2022年台风“梅花”在吉林引发暴雨的天气过程。结果显示:在本次天气出现之前,“梅花”引导气流沿副热带高压后部西南方向急剧向北移动。吉林省大多区域西风带出现小槽波动,在其后方增加了一些冷空气,给“梅花”系统暖湿空气带来较大影响,为本次极端降水天气创造了环流形势。

2019年盛夏最大降水环流成因分析

利用1961-2020年黑龙江省盛夏(7-8月)降水量、NCAR/NCEP大气再分析和Nino34 SST资料,分析2019年盛夏黑龙江降水异常与ENSO事件背景下大尺度环流特征。结果表明:2019年盛夏黑龙江省降水处于多水时段,ENSO暖事件多发期降水偏多发生的概率增加。指出了2019年降水特点主要受东北冷涡与3次台风北上相互作用,发生6次重大降水过程;500 hPa高度距平场为西北-东南向呈“+、-、+”形势;西太平洋副热带高压明显偏东,脊区向北伸展有利台风北上;低纬主要有3支强南风距平气流北上进入气旋环流,为维持台风暴雨提供充足水汽;黑龙江中部和东部地区最大上升气流速度出现在600-300 hPa高度,也是台风活跃区,辐合明显,黑龙江省强降水出现在东部最大降水中心,落区位于三江平原北部鹤岗至萝北。

哈尔滨机场一次极端冻雨天气成因及气候背景分析

基于观测资料和ERA5再分析资料,从环流背景、水汽追踪以及气候异常的角度,应用合成分析等方法,研究了2021年11月8日哈尔滨太平国际机场冻雨灾害天气的形成原因。结果表明:此次冻雨天气发生时哈尔滨机场上空温度具有显著的“冷-暖-冷”特点,是典型的“融化机制”。850 hPa低空暖湿东南急流、低空逆温层的稳定维持和地面东北风控制下地表气温偏低2~4℃为此次过程的关键因素。通过拉格朗日轨迹模式(Hysplit),发现影响哈尔滨机场冻雨天气的水汽通道主要有3支,起源于鄂霍兹克海上空850 hPa高度的水汽通道2为此次降水过程的主要水汽通道,日本海为重要水汽源地。此次天气过程发生在双峰型拉尼娜事件气候背景下,高低空环流形势场和地面气压场、温度场较往年同期均表现出一定的异常特征。双峰拉尼娜年表现出低层暖湿气流偏强和地面温度偏低的异常特征,与此次过程相似。