Cold Air Activity Research Articles

Multiple-Time-Scale Variations of the Record-Breaking Pre-Summer Rainfall over South China in 2022

Abstract South China encountered an exceptionally heavy pre-summer rainy season in 2022 with the regional precipitation ranking first in the past 44 years. This study aims to analyze the multiple-time-scale variations of precipitation in this pre-summer rainy season to shed light on the complex dynamics influencing pre-summer precipitation over South China. The findings reveal that the variation of precipitation was dominated by the 10–20-day oscillation during April–May, while interannual variation and trend during May–June. The 10–20-day oscillation of precipitation in pre-summer rainy season in South China demonstrates a strong association with cold-air activity, which can be traced back to the propagation of disturbances along a teleconnection, which represents the dominant mode of intraseasonal atmospheric circulation over Eurasia in high latitudes during April–May. This teleconnection plays a crucial role in facilitating cold-air invasion and triggering precipitation over East China and South China. The interannual component of abnormal precipitation is strong during May–June of 2022. It is primarily attributed to the abnormal highs in the lower troposphere over the subtropical western North Pacific and Japan. These abnormal highs are likely stimulated by the combined influences of Eurasian teleconnection propagation and cooling sea surface temperature anomalies (SSTAs) over the tropical central and eastern Pacific in the third year of a consecutive La Niña event. However, the universality of the impact of Eurasian teleconnection propagation on the abnormal high over Japan on interannual scale necessitates further investigation. Furthermore, there is a significant upward trend in pre-summer rainfall over South China, accounting for 38% of the total anomaly observed in 2022.

Summertime ENSO potentially amplifies rainstorm and flood risk in the lower Yellow River Basin, China

Study regionThe Yellow River Basin (YRB) in China. Study focusEcological conservation and high-quality development of the YRB have been incorporated into the national strategy of China. Here we investigated into the impacts of El Nino-Southern Oscillation (ENSO) on rainstorm and flood risk in the YRB, paid special attention to disaster-induced losses and explained the mechanisms of ENSO affecting the risk. New hydrological insights for the regionENSO tends to amplify rainstorm and flood risk in the YRB due to the increase in rainstorm days (rainstorm amount) by 33.6 % (34.1 %) in the next year with the previous summertime El Nino, by 20.3 % (19.3 %) in the current year with summertime La Nina, and by 70.7 % (72.5 %) in the ENSO phase shift year, and leads to a marked increase in disaster-induced losses. The summertime La Nina (El Nino) can increase the annual extreme runoff amount by 25.8 % (decrease by 8.4 %) in the lower YRB in the current year. At 500hpa, the higher air pressure over Eastern Europe and Okhotsk Sea in July, and the higher air pressure over the northern Eurasian continent and the lower air pressure over Lake Baikal and Balkhash in August tend to amplify rainstorm and flood risk. At 850hpa, the increase of water vapor transport from the south and cold air activities in high latitudes tend to increase the risk.

Relationship between extensive and persistent extreme cold events in China and stratospheric circulation anomalies

This study examines the relationship between the extensive and persistent extreme cold events (EPECEs) in China and geopotential height anomalies in the stratosphere using daily mean fields of outgoing long wave radiation (OLR) produced by the NCAR and daily atmospheric circulations produced by the NCEP/NCAR. The OLR composite analysis for the EPECE in China demonstrates that the negative OLR height anomalies (cold air) originated from Siberia influence China progressively from north to south. The largest negative OLR height anomaly (cooling event) occurs in the region to the north of the Nanling Mountains. This suggests that the OLR height anomalies can be used to represent the temporal and spatial characteristics of extreme low temperatures and cold air activities in winter in China. The composite analysis of large-scale atmospheric circulations during the EPECE reveals characteristic evolutions of stratospheric and tropospheric circulations during the extreme cold event. We demonstrate the important role of atmospheric circulation anomalies in the outbreak and dissipation of the EPECE in China. We also demonstrate that significant perturbations in the stratospheric circulation occur more than 10 days prior to the outbreak of the EPECE, with positive height anomalies in the Arctic stratosphere. These positive anomalies propagate downward from the stratosphere and affect the formation and development of the high pressure ridge in the middle troposphere over the Ural Mountains. Significant changes also occur in the atmospheric circulation in the mid-latitude stratosphere. These changes propagate downward from the stratosphere and strengthen the low pressure trough in the troposphere in the region to the east of Lake Balkhash and Lake Baikal. Therefore, the changes in the stratospheric circulation during the EPECE in China occur prior to changes in the tropospheric circulation and are very useful for predicting extreme wintertime cold temperatures in China.

The Spatiotemporal Characteristics and Driving Factors of Dust Emissions in East Asia (2000–2021)

The climate effect and environmental pollution caused by dust discharged into the atmosphere have attracted much attention. However, the driving factors of dust emissions have not been studied thoroughly. Here, spatiotemporal variations in dust emissions and the relationship between dust emissions and large-scale atmospheric circulation in East Asia from 2000 to 2021 were investigated using Modern-Era Retrospective Analysis for Research and Applications version 2, Cloud-Aerosol Lidar Pathfinder Satellite Observations, ERA5 reanalysis data, and climate indices. Results showed that the Taklimakan Desert in the Tarim Basin, the Gurbantonggut Desert in the Junggar Basin, the Turpan Basin, and the Gobi Desert in western Inner Mongolia and southern Mongolia are the main sources of dust emissions in East Asia. The period of strong dust emissions is from March to May, and emissions to the atmosphere were mainly distributed at 0–4 km in the troposphere. In the eastern and southwestern Tarim Basin, northern Junggar Basin, and parts of the Gobi Desert in southern Mongolia, dust emissions have significantly increased over the past 22 years, whereas in the southwestern Tibetan Plateau, southwestern Inner Mongolia, and a small part of the northern Mongolian Gobi Desert there was a significant decreasing trend. The winter North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) were significantly negatively correlated with East Asian dust emissions the following spring. The various phases of the AO/NAO coupling have clear different effects on East Asian dust emissions in the spring. When the AO/NAO coupling was negative (positive), the East Asian trough and Siberian High were strengthened (weakened), the frequency of cold air activity increased (weakened), 800 hPa wind speed strengthened (weakened), and East Asian emissions increased (decreased). In AO−/NAO+ years, the Asian polar vortex was stronger to the south and the East Asian trough was stronger to the west. The Lake Baikal trough was in the deepening phase, which caused more polar cold air to move into East Asia, aggravating the intensity of dust activity. In the AO+/NAO− years, the Siberian High and East Asian trough weakened, which was unfavorable to the southward movement of cold air from Siberia. Therefore, the frequency of windy weather in East Asia decreased, partly weakening dust emissions. However, a positive geopotential anomaly in northeast China and a negative geopotential anomaly in South Asia triggered an anomalous enhancement in easterly wind in the tropospheric area over northwest China. Strengthening of the Balkhash trough provides favorable conditions for gale weather in northwest China. The frequency of gale weather increased, and dust emissions were enhanced in northwest China.

Analysis of climatic features and major meteorological disasters over the Three Gorges Region of the Yangtze River Basin in 2021

Based on daily observation data in the Three Gorges Region (TGR) of the Yangtze River Basin and global reanalysis data, the authors analyzed the climate characteristics and associated temporal variations in the main meteorological factors in 2021, as well as the year's climatic events and meteorological disasters. The 2021 average temperature was 0.2°C above the 1991–2020 average and the 13th-warmest year since 1961. Seasonally, winter and autumn were both warmer than usual. The annual mean precipitation was 12.8% above normal, and most regions experienced abundant rainfall throughout the year. The seasonal variation in precipitation was significant and the TGR had a wetter-than-normal spring and summer. The number of rainstorm days was higher than normal; the wind speed was above normal; and the relative humidity was higher than normal. In terms of rain acidity, 2021 was tied with 2020 as the lowest since 1999. From mid-September to early October 2021, the TGR experienced exceptional high-temperature weather, which was driven by abnormal activity of mid- and high-latitude atmospheric circulation over the Eurasian continent and the western Pacific subtropical high (WPSH). In addition, a strong blocking high over the Ural Mountains accompanied by intense mid-latitude westerly winds prevented cyclonic disturbances from extending to the subtropical region. As a result, under the combined effect of the weaker-than-normal cold-air activities and the anomalous WPSH, the TGR experienced extreme high-temperature weather during early autumn 2021.摘要本文分析了2021年长江三峡地区气候特征及主要天气气候事件. 2021年, 三峡地区年平均气温较常年偏高0.2°C, 降水量偏多12.8%, 年暴雨日数偏多, 年平均风速偏大, 年平均相对湿度偏高. 2021年与2020年同为有记录以来酸雨强度最弱年. 汛期区域强降水频发; 春季多低温阴雨; 秋冬寒潮过程降温幅度大影响广. 9月中旬至10月初, 三峡地区遭遇罕见“秋老虎”, 分析表明该时段内异常高温与中高纬度大气环流及西太平洋副热带高压活动异常等因子密切相关.

Jointly using the GLDAS 2.2 model and GRACE to study the severe Yangtze flooding of 2020

A severe flood occurred in the middle and lower reaches of the Yangtze River (MLYR) in 2020 and caused an obvious terrestrial water storage (TWS) increase. However, the process and cause of this flood have not been fully revealed. To address this problem, we use Global Land Data Assimilation System (GLDAS) −2.2 daily TWS data that assimilate Gravity Recovery and Climate Experiment (GRACE) observations to thoroughly investigate this flood event. Evaluation results show that the GLDAS TWS agrees well with GRACE TWS and water budget results around the MLYR mainstream, which provides confidence in using GLDAS TWS data to monitor floods. The daily GLDAS TWS anomalies and hydrological observations around the MLYR mainstream coincide well in terms of their temporal patterns. These two independent indicators together identify this severe flood event and its process. GLDAS and hydrologic results suggest that this flood event started south of the MLYR mainstream basin in early June and then moved northward, and in July it covered the whole MLYR mainstream basin and intensified along the MLYR mainstream and its north. This flood event reached its peak around the MLYR mainstream basin and north of it around late July, and then retreated progressively till September. GLDAS results show that the largest regional positive TWS anomaly was ∼ 130.8 mm and happened in late July around the MLYR mainstream, influencing a vast area of ∼ 9.2 × 105 km2 in the MLYR basin. The flood center first appeared along the MLYR mainstream, then moved northward, and finally stayed north of the MLYR mainstream. These results provide us a quantitative understanding of it. The water budget analysis demonstrates that the above-normal precipitation around the MLYR mainstream during June-July 2020 is the main cause of this flood. The weather analysis demonstrates that the precipitation increase is mainly due to anomalies in monsoon circulation systems, including the anomalous extension of the Western Pacific subtropical high and South Asia high, anomalous low-level southwesterly wind and upper-level westerly jet. And the precipitation increase is also closely associated with the anomalous blocking high and cold air activities in the middle and high latitudes.

Subseasonal Variation of 2020 Meiyu Rainfall With Extremely Long Duration Over the Yangtze–Huaihe River Basin in China

An observed fact is proposed that the 2020 record-breaking Meiyu over the Yangtze–Huaihe River Basin (YHRB) is characteristic of significant subseasonal variation. The anomalously enhanced rainfall experiences a southward retreat from the northern YHRB in June to the southern YHRB in July. The meridional shift of abnormal rainbelt is closely related to the different anomalous Meiyu front circulations. The first stage features a warm Meiyu front with an extremely strong northward transport of warm-and-wet air, whereas a cold front forms with a convergence between the strong northeasterlies and southwesterlies in July. Another significant subseasonal variation of the 2020 Meiyu is that the Meiyu rainfall evolves in a periodic quasi-biweekly oscillation (QBWO). On the scale of the QBWO, the cold air activities over East Asia contribute significantly to Meiyu; each intense southward invasion of cold air corresponds well to the enhancement of Meiyu rainfall. However, the location of the western Pacific subtropical high, the subtropical East Asia summer monsoon (SEASM), and the intensity of the East Asia subtropical westerly jet exhibit opposite QBWO characteristics in June and July. For an increasing (decreasing) of Meiyu rainfall, the western Pacific subtropical high marches northward (retreats southward), the subtropical East Asia summer monsoon weakens (strengthens), and the East Asia subtropical westerly jet accelerates (decelerates) in June. The opposite holds true in July. Further analyses reveal that the filtered cyclone over Northeast China benefiting the southward intrusion of cold air is associated with the eastward propagation of a wave train over the mid-high latitudes in June, whereas it arises from both the westward propagation of a wave train over the mid-high latitudes and the northward propagation of cyclonic circulation from the tropics in July.

Can Arctic Sea Ice Influence the Extremely Cold Days and Nights in Winter over the Tibetan Plateau?

The Arctic, Antarctic, and Tibetan Plateau (TP) are the northernmost, the southernmost, and the highest places of the Earth, respectively, known as Earth’s “three poles”. The Arctic and TP are the “North Pole” and “Third Pole”, and they exert a significant influence on the regional and global climate. This study analyzed the changing characteristics of Arctic sea ice and explored relationships between extreme cold days on the TP and sea ice concentration in the Arctic. From 1979 to 2019, the sea ice concentration of August–October decreased significantly. The low concentration of sea ice leads to a warmer Arctic and causes cold air over the Arctic to be unstable and more likely to move into the southern. Over the TP, the frequent cold air activities lead to more extreme cold events. This article aims to investigate the response characteristics of atmospheric circulation via the NCAR–CAM5.1 model (National Center for Atmospheric Research Community Atmosphere Model, Version 5.1). In order to verify the mechanism of Arctic sea ice concentration impacts on the extreme low temperature of the TP, we designed three experimental plans with different sea ice concentrations and sea surface temperatures (SST). In the two sensitivity experiments, the decrease in sea ice concentration and the increase in SST from August to October in the key areas are amplified simultaneously. The simulation results show that the increases in atmosphere thickness of 950–500 hPa in the Arctic from November to the following February reduce the meridional thickness-gradients between the Arctic and the middle latitudes. The westerly flow in middle–high latitudes weakened. As a result, the polar vortex over the Arctic is more likely to move south. There are negative geopotential height anomalies at 500 hPa over the Arctic and TP and positive anomalies over Eurasia. The anticyclonic system at 500 hPa slightly strengthens in the high latitudes of Eurasia (northerly winds in the TP). Strongly negative anomalies of temperature in the northern parts of the TP generate the cold source. To the north of the TP, the strengthened meridional propagation in middle–high latitudes causes more cold extremes.

Analysis of a Cold Wave Weather Process of China from December 23rd to 26th, 2021

Cold wave weather process is a large-scale strong cold air activity process, will bring drastic cooling and wind, as well as rain and snow weather. This research mainly studies the evolution and cause of a cold wave weather process of China from December 23rd to 26th, 2021. The weather map from the National Climate Center and the geopotential height and sea level pressure reanalysis data from December 23rd to 26th, 2021 provided by National Centers for Environmental Prediction of United States are analyzed by the comparative analysis method and the high altitude situation forecast equation. As a result, this research found that this cold wave was considered a nationwide process. Besides, this cold wave process was mainly caused by the invasion of cold air from Siberia. In addition, under the joint action of the warm high pressure ridge in the Ural Mountains, the polar vortex split and presented a multi-polar distribution. One of them was located near West Siberia, which was conducive to the accumulation of cold air here. With the rotation of the transverse trough, cold air moved southward, and cold wave broke out.

Detection of the synoptic southeastward-extending Siberian cold high during 1978–2017

To better understand the relationship between anticyclones in Siberia and cold-air activities and temperature changes in East Asia, this study proposes a 2D anticyclone identification method based on a deep-learning model, Mask R-CNN, which can reliably detect the changes in the morphological characteristics of anticyclones. Using the new method, the authors identified the southeastward-extending Siberian cold high (SEESCH), which greatly affects wintertime temperatures in China. This type of cold high is one of the main synoptic systems (45.7%) emerging from Siberia in winter. Cold air carried by SEESCH has a significant negative correlation with the temperature changes in the downstream area, and 52% of SEESCHs are accompanied by cold-air accumulation in North and East China, which has a significant impact on regional cooling. These results provide clues for studying the interconnection between SEESCHs and extreme cold events.摘要为了更好地研究西伯利亚地区反气旋与冷空气活动,东亚地区气温变化之间的关联, 本文提出一种基于Mask R-CNN的反气旋识别方法, 能够较为准确地刻画反气旋形态特征变化. 使用该方法能够识别对中国冬季气温具有较大影响的东南延伸型西伯利亚冷高压(SEESCH), 这种冷高压是冬季出现在西伯利亚地区的主要天气系统之一(45.7%). SEESCH携带的冷空气与下游地区温度变化呈显著负相关, 52%的SEESCH伴随着华北华东地区冷空气聚集, 对区域降温有显著影响. 这些结果为研究 SEESCH 与极端寒冷事件之间的联系提供线索.

Variation Characteristics Mathematical Calculation of O3 and Its Relationship with Meteorological Factors by Big Data Technology

Based on the data of environmental monitoring stations and meteorological stations in Qinhuangdao from May 2017 to May 2020, the variation characteristics of O3 and precursors (NO2 and CO) as well as their relationship with meteorological elements were analyzed. The results showed that the daily average concentration of O3-8 h in Qinhuangdao increased year by year. The monthly average concentration of O3-8 h was high in summer and low in winter, and the peak appeared in June. The diurnal variation of O3 concentration was unimodal structure, and the concentration increased in the afternoon, but it decreased at night. The concentration of NO2 and CO was inversely correlated with O3, and the peak value of NO2 in March could be related to frequent cold air activity and increased burning of loose coal. The meteorological elements favorable for the occurrence of ozone pollution weather in Chengde were total solar radiation irradiance greater than 1000W/m2, the daily maximum temperature greater than 33 °C, and the daily minimum relative humidity less than 40% and 65%∽80%, southerly wind or southwest wind.

Analysis of the Continuous Heavy Pollution Process in the Winter of 2016 in Beijing, Tianjin, and Hebei

From December 16 to 21, 2016, continuous heavy haze occurred in the Beijing-Tianjin-Hebei region of China. To further understand the development mechanism of haze and improve the forecasting of haze events, a variety of data, background circulation, and the meteorological elements of this severe haze process were comprehensively analyzed. The results show that the process lasted for a long time, pollution intensity was high, influence area was wide, visibility was low, and external transport was the main cause. Aerosols were mainly distributed at a height of 600 m, and there was a certain extreme anomaly. The static weather index and air quality index showed a strong correlation. The air in the Beijing-Tianjin-Hebei region is controlled by zonal circulation in front of a high-pressure ridge, which maintains the westerly airflow. Cold-air activity is weak, and downflow is dominant. The water vapor content is low, the cloud cover is limited at high altitudes, and a warm ridge extends northwards at low altitudes. Limited static wind on the ground, high relative humidity, and low mixing-layer height are not conducive to the horizontal and vertical diffusion of pollutants.

Analysis of large-scale transmission line icing and galloping event in the Jilin province during November 2020

A large-scale transmission line icing and galloping event occurred in Jilin province during November 2020. First of all, the circulation background and the evolution characteristics of key meteorological factors of the icing and galloping event are analysed, which provide basis for the prediction and early warning of the galloping event in the future. It is found that, the warm and wet air transported northward to the central part of Northeast China due to the abnormal northward position of the western pacific subtropical high, combined with the strong cold air activity in the north, the cold and warm air converged in the middle of Northeast China, resulting in the co-occurrence of temperature decline, precipitation, gale and high humidity, which is the direct cause of this large-scale galloping event. Then, the prediction and actual situation of the disaster event are analysed. The results show that the power gird icing and galloping numerical prediction system accurately predicted the influence area and disaster intensity three days in advance, which can provide effective support for future disaster response. Finally, some relevant suggestions are put forward according to the characteristics of this disaster.

Heavy air pollution with a unique “non-stagnant” atmospheric boundary layer in the Yangtze River middle basin aggravated by regional transport of PM<sub>2.5</sub> over China

Abstract. The regional transport of air pollutants, controlled by emission sources and meteorological factors, results in a complex source–receptor relationship of air pollution change. Wuhan, a metropolis in the Yangtze River middle basin (YRMB) of central China, experienced heavy air pollution characterized by hourly PM2.5 concentrations reaching 471.1 µg m−3 in January 2016. To investigate the regional transport of PM2.5 over central eastern China (CEC) and the meteorological impact on wintertime air pollution in the YRMB area, observed meteorological and other relevant environmental data from January 2016 were analyzed. Our analysis presented noteworthy cases of heavy PM2.5 pollution in the YRMB area with unique “non-stagnant” meteorological conditions of strong northerly winds, no temperature inversion, and additional unstable structures in the atmospheric boundary layer. This unique set of conditions differed from the stagnant meteorological conditions characterized by near-surface weak winds, air temperature inversion, and stable structure in the boundary layer that are typically observed in heavy air pollution over most regions in China. The regional transport of PM2.5 over CEC aggravated PM2.5 levels, thus creating heavy air pollution in the YRMB area. This demonstrates a source–receptor relationship between the originating air pollution regions in CEC and the receiving YRMB region. Furthermore, a backward trajectory simulation using a Flexible Particle dispersion (FLEXPART) Weather Research and Forecasting (WRF) model to integrate the air pollutant emission inventory over China was used to explore the patterns of regional transport of PM2.5 governed by the strong northerly winds in the cold air activity of the East Asian winter monsoon season. It was estimated that the regional transport of PM2.5 from non-local air pollutant emissions contributes more than 65 % of the PM2.5 concentrations to the heavy air pollution in the YRMB region during the study period, revealing the importance of the regional transport of air pollutants over China as a causative factor of heavy air pollution over the YRMB area.

The Influence of Multi-Scale Atmospheric Circulation on Severe Haze Events in Autumn and Winter in Shanghai, China

Severe haze events have many adverse effects on agricultural production and human activity. Haze events are often associated with specific patterns of atmospheric circulation. Therefore, studying the relationship between atmospheric circulation and haze is particularly important for early warning and forecasting of urban haze events. In order to study the relationship between multi-scale atmospheric circulation and severe haze events in autumn and winter in Shanghai, China, we used a T-mode objective classification method to classify autumn and winter atmospheric circulation patterns into six types based on sea level pressure data from 2007 to 2016 in the Shanghai area. For the period between September 2016 and February 2017, we used the Allwine–Whiteman method to classify the local wind in Shanghai into three categories: stagnation, recirculation, and ventilation. By further studying the PM2.5 concentration distribution, visibility distribution, and other meteorological characteristics of each circulation type (CT) and local wind field type, we found that the Shanghai area is most prone to severe haze when exposed to certain circulation patterns (CT1, CT2, and CT4), mainly associated to the cold air activity and the displacement of the high pressure system relative to Shanghai. We also found that the local wind fields in the Shanghai area are dominated by recirculation and stagnation events. These conclusions were further verified by studying a typical pollution process in Shanghai in November 2016 and the pollutant diffusion path using the HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory model) simulation model.

Comparison of ECMWF Significant Wave Height Forecasts in the China Sea with Buoy Data

Abstract This paper focuses on a comprehensive comparison of the European Centre for Medium-Range Weather Forecasts (ECMWF) significant wave height (SWH) forecasts with buoy data in the China Sea, and analysis of accuracy characteristics varying with related variables (SWH, water depth, and distance from shore) and different scenarios (each month, different sea area, and typhoon- and cold air activity–induced waves). This is the first time that observations from the Chinese Ocean Monitoring Network have been used to verify ECMWF wave forecasts in the China Sea. Two years’ worth of data from 24 hydrometeorological buoys are used. The comparison shows good accuracy and predictive stability of SWH forecasts for the Chinese buoys, which is consistent with Korean and global buoys. However, the accuracy in the Bohai Sea and Taiwan Strait is worse than that in the South and East China Sea. SWH forecasts for QF206 in the southern Taiwan Strait have systematically underestimated observations, which may be mainly due to the coarse resolution of wave forecasts. Besides, forecasts underestimate observations when 1.5 m < SWH ≤ 6.5 m. The accuracy and predictive stability in spring and summer are worse than those in winter, especially in April, which is the worst in 12 months. The accuracy increases with water depth and distance from shore. During typhoon conditions, the accuracy is worse than during cold air conditions.

Quantifying the Impacts of Cold Airmass on Aerosol Concentrations Over North China Using Isentropic Analysis

AbstractWinter air quality can vary markedly due to cold air activities. However, the impacts of the polar cold airmass on aerosol variations has not been clarified in a quantitative manner. Using an isentropic analysis method, we investigated changes in the cold airmass and sulfate aerosol distributions over North China during the winter of 2014/2015. The cold airmass and sulfate concentrations exhibited pronounced out‐of‐phase variations, with comparable amplitudes at subseasonal (30–60 days) and synoptic (4–6 days) scales. Subseasonal sulfate variations were closely associated with distribution of the polar cold airmass, as it shifted between zonal and meridional patterns, regulating cold airmass fluxes over North China. Typically, subseasonal surges in the cold airmass consisted of several synoptic disturbances, including one cold air outbreak event and 3–6 renewal/decay oscillations. These synoptic inflows of clean cold airmass repeatedly pushed the warm polluted airmass away from North China. Thus, spatiotemporal variations in sulfate better reflect cold airmass distributions and fluxes than northerly wind regimes. Using diagnostic equations derived herein, we estimated the contribution of various physical processes to aerosol variations during cold air anomalies. We found that the local changes in sulfate concentrations were mainly attributed to advection of warm airmasses, as well as advection and vertical displacement by horizontal convergence of the polar cold airmass. The latter two processes contributed most to sulfate decreases, leading to a relatively strong aerosol reductions during cold air outbreaks, while first two processes led to weaker aerosol reductions during cold air oscillations.

Observed Decadal Transition in Trend of Autumn Rainfall over Central China in the Late 1990s

Abstract This article presents a decadal transition from a decreasing trend to an increasing trend in the late 1990s for autumn rainfall in central China. The atmospheric and oceanic background underlying this regime shift is also addressed. Accompanying the above decadal transition, the moisture convergence and ascending motion averaged in central China both switch from a weakening trend to a strengthening trend. Meanwhile, after the late 1990s, the declining of the Asian sea level pressure (SLP) and the deepening of the Lake Balkhash trough may induce more cold air from high latitudes to break out southward. The strengthening of the low-level southerly in East Asia could transport more moisture northward from low latitudes. More cold air and more warm-moist airflow encountering in the targeted region contribute to the increasing trend of local rainfall. The situation before the late 1990s is generally reversed, which accounts for the decreasing trend of autumn rainfall. The movement of the East Asian jet (EAJ) from southward to northward also has a contribution via its influence on dynamic condition. Additionally, changes in the sea surface temperatures (SSTs) in the western Indian and Atlantic Oceans from a cooling trend to a warming trend in the late 1990s play significant roles through their modulations on moisture transport and cold-air activities.

Quantifying the effects of different cold air events on latent and sensible heat fluxes of Lake Taihu

作为冷季主要的天气事件，冷空气过境会改变湖泊上方的气团性质，对湖泊的水热通量产生影响，进而影响湖泊的生物物理和化学过程.以亚热带大型浅水湖泊——太湖为研究对象，基于2012-2017年5个冷季（11月-翌年3月）的太湖中尺度通量网观测数据，量化不同强度冷空气（寒潮、强冷空气和较强冷空气）对太湖水热通量的影响.结果表明：在5个冷季中，寒潮、强冷空气和较强冷空气发生的总次数分别为4、11和33次，累积持续天数分别为14、31和78天.冷空气过境显著增强太湖的水热通量，3种冷空气过境使太湖的感热通量分别增至无冷空气时的10.3、6.0和4.3倍，潜热通量分别增至无冷空气时的4.0、2.1和2.7倍.虽然冷空气影响天数仅占冷季天数的16.4%，但对整个冷季的潜热和感热通量贡献分别为34.9%和51.7%，以较强冷空气贡献最大.冷空气影响时，水-气界面的温度梯度是太湖感热通量的主控因子，而潜热通量的主控因子为风速.与深水湖泊相比，太湖等浅水湖泊对冷空气过境的响应更快，寒潮过境时尤为明显.;As the major synoptic system, cold air events influence the water vapor and heat exchanges between lake and atmosphere by changing the meteorological conditions of air masses over the lake. Then biophysical and biogeochemical cycles in the lake ecosystem would be moderated by cold air passage. Based on dataset of the Taihu Eddy Flux Network observed during the five cool seasons (2012-2017), the effects of different cold air events (cold wave, severe cold air events and strong cold air events) on latent and sensible heat fluxes were quantified on the large subtropical shallow Lake Taihu. The results showed that cold wave, severe cold air events and strong cold air events totally happened 4, 11 and 33 times, and lasted for 14, 31 and 78 days, respectively. The sensible and latent heat exchanges between lake and atmosphere were accelerated significantly by the passage of cold air. The sensible heat flux increased by 10.3, 6.0 and 4.3 times during cold wave, severe cold air events and strong cold air events, respectively. The latent heat flux was increased by 4.0, 2.1 and 2.7 times, respectively. Although cold air passage only occupied 16.4% of entire cool season, the cold air events contributed 34.9% and 51.7% of the total latent and sensible heat fluxes, respectively. Moreover, the strong cold air events were the biggest contributor. During cold air events, the temperature gradient between the air and water is the most significant factor governing the sensible heat exchange rate. While, the latent heat flux is mostly dominated by wind speed. Compared to deep lakes, shallow lakes response faster to cold air activities. Therefore, the latent and sensible heat fluxes of shallow lakes increase much more, especially during cold waves.

Interannual variation in the number and severity of autumnal haze days in the Beijing–Tianjin–Hebei region and associated atmospheric circulation anomalies

In contrast with the considerable number of studies on the variability of winter haze pollution over eastern China, there have been few studies of autumnal haze in this region. The interannual variability of autumnal (September–November) haze days in the Beijing–Tianjin–Hebei region (AHDBTH) is significantly correlated with the Scandinavia–central Siberia–Western Pacific (SCSWP) teleconnection pattern, a quasi-barotropic atmospheric circulation anomaly. The SCSWP teleconnection pattern may therefore be a useful indicator for forecasting AHDBTH. The positive phase of the SCSWP teleconnection pattern enhances the anomalous anticyclonic circulation centered over the Western Pacific via the southeastward propagating Rossby wave train emanating from Scandinavia. The positive SCSWP teleconnection pattern, together with the northward shift of the East Asian jet stream induced by low-level atmospheric baroclinicity, could provide the favorable local meteorological circumstances (i.e., suppressed cold air activity, higher surface temperatures and low-level atmospheric inversion, and lower planetary boundary layer height and decreased surface wind speed) required to explain the recent increased incidence of AHDBTH.