Southwesterly Winds Research Articles

Glacial Southern Ocean Expansion Recorded in Foraminifera‐Bound Nitrogen Isotopes From the Agulhas Plateau During the Mid‐Pleistocene Transition

AbstractThe emergence of 100‐Kyr glacial cycles (The Mid‐Pleistocene Transition [MPT]) is attributed in part to slower global overturning circulation and iron stimulation of biological carbon drawdown in the Southern Ocean. We present foraminifera‐bound nitrogen isotope values and polar planktic foraminifera abundances from the Agulhas Plateau that show that increases in biogenic sediment accumulation coincide with northward migrations of the Subtropical Frontal Zone (STFZ) and elevated foraminifera‐bound nitrogen isotope values during MPT glacial episodes. The nitrogen isotope values of two planktic foraminifera species, Globigerina bulloides and Globorotalia inflata show remarkable coherence amongst the sea surface temperature gradient between the STFZ and SAZ, and polar foraminifera abundances, indicating a strong relationship between nitrogen isotope dynamics above the Agulhas Plateau and migrations of the STFZ. Northward migration of the STFZ may have been essential to prolonging glacial intervals by increasing deep ocean carbon storage via a northward shift of the South Westerly Winds and a reduction in upwelling, delivery of fresher surface waters into the upper limb of global overturning circulation, or inhibiting heat and salt delivery to the Atlantic as Agulhas Leakage.

A Comparison of Different Station Data on Revealing the Characteristics of Extreme Hourly Precipitation Over Complex Terrain: The Case of Zhejiang, China

AbstractBoth long‐term but small number of national stations and short‐term but large number of regional stations have been frequently used to study the extreme hourly precipitation (EXHP) in China. However, few studies focus on the differences of the two for revealing the features of EXHP. In this study, the characteristics of EXHP in Zhejiang Province are investigated using three rainfall data sets at three threshold criteria. The comparison between different data sets shows that increasing the station density can better reflect the climatic spatial distribution of EXHP thresholds if long‐term data is absent. The majority of EXHP can be classified into four weather types: the southwesterly wind type (30.7%–48.5%), the trough type (12.2%–23.6%), the tropical cyclone (TC) type (11.4%–17.5%) and the easterly wind type (4.9%–17.9%). The selection of stations is more sensitive to the proportions of the four weather types than the statistical years and threshold criteria. The monthly and diurnal variations of EXHP, as well as their differences revealed by the three data sets, are varied by weather type. Only using national stations cannot distinguish the spatial differences between the TC type and the easterly wind type, and there is an underestimation for southwesterly wind type and trough type in the mountainous area of southwest Zhejiang. The statistical year and station height are the main reasons for the differences in the duration of EXHP events calculated by different data sets, with the TC type having the largest effect and the southwesterly wind type the smallest.

What Controlled the Low-Level Moisture Transport during the Extreme Precipitation in Henan Province of China in July 2021?

Abstract A record-breaking precipitation event occurred in the Henan province of China in July 2021 (217HP). To identify the moisture source of the event, ensemble experiments with 120 members were conducted in this study. Results show that the precipitable water during this extreme event was primarily contributed by the low-level southeasterly (LLSE) water vapor transport. The LLSE was largely enhanced by the pressure gradient force maintained by the western Pacific subtropical high and further amplified by the latent heat release in the rainfall system over Henan. The positive moisture advection by the LLSE and evaporative water occurred below 950 hPa and was redistributed into higher levels by the LLSE jet-enhanced subgrid vertical turbulent transport. As a result, the combination of the enhanced LLSE centered around 950 hPa and the increase of moisture below 850 hPa were the main drivers for the continuous strengthening of LLSE moisture transport, with the former playing the dominant role. It is also found that not only the presence but also the intensity of the LLSE jet were important for reproducing the extreme rainfall. The impact of binary tropical cyclones In-Fa and Cempaka on the low-level moisture transport was also examined. We found that In-Fa (2021) presented an uncertain impact on 217HP, while Cempaka (2021) was found to be unfavorable for 217HP. Different from the LLSE water vapor transport, Cempaka mainly acted to weaken the southwesterly wind to the southwest of Henan by reducing the pressure gradient and impeding the water vapor transport toward Henan.

Characteristics of net ecosystem exchange and source distribution of Xilinhot grassland, China.

To understand carbon sequestration capacity of grasslands, the changes of CO2 flux in Xilinhot grasslands and the influence of environmental factors were analyzed by using the eddy data of Xilinhot National Climate Observatory in 2018-2021, and the distribution of flux source areas was analyzed. The results showed that the southwest wind prevailed in the study area throughout the year, the source area in the growing season was larger than that in the non-growing season, and the source area under stable atmospheric conditions was larger than that under unstable conditions. The maximum length of source region with a contribution rate of 90% was close to 400 m, which was consistent with the length estimated by the classical law. The net ecosystem exchange (NEE) of Xilinhot grasslands had obvious diurnal and seasonal dynamics, which was manifested as a carbon sink in the daytime and a carbon source at night during the growing season and weak carbon source in the non-growing season. From 2018 to 2021, the annual total NEE were -15.59, -46.28, -41.94, and -78.14 g C·m-2·a-1, respectively, with an average value of -45.49 g C·m-2·a-1, indicating that Xilinhot grassland had strong carbon sequestration capacity. Vapor pressure deficit and photosynthetically active radiation helped grasslands absorb atmospheric CO2. At night, when temperature was above 0 ℃, the increases in air and soil temperature promoted vegetation respiration to release CO2.

Contribution of PDO to Decadal Variations of Glaze Dipole Pattern in China

Abstract Based on the in situ observations, reanalysis, and model simulation, the variations in glaze dipole pattern in China and its underlying physical mechanism have been explored. The glaze dipole pattern features an out-of-phase relationship between winter glaze in the south of the Yangtze River valley (YRV) and northern China, accompanied by pronounced interdecadal variation around the late 1970s. The results from synoptic analyses suggest that cold air brought by the northerly winds and warm moist air by the southwesterly winds, as well as the occurrence of inversion layer are vital to the glaze weather in the south of YRV. Further analyses indicate that the interdecadal shift of the Pacific decadal oscillation (PDO) contributes largely to variations in glaze dipole pattern. Specifically, the warm PDO provides a beneficial environment for the occurrence of glaze dipole pattern by stimulating the tropical–extratropical circulation configuration with the deepened East Asian trough, strengthened East Asian westerly jet, anomalous anticyclone over the tropical western Pacific Ocean, and cyclone over the southern Tibetan Plateau at the decadal time scale. Consequently, the enhanced moisture transport brought by southwesterly and cold air intrusion induced by the deepened East Asian trough benefit the glaze weather in the south of YRV, while the decreased precipitation and a much lower temperature in northern China depress the generation of glaze. Moreover, the results from the CAM4 model simulation indicate the atmospheric circulation anomalies forced by PDO-like SST can roughly reproduce the extratropical configuration related to the glaze, but it has difficulties in capturing the tropical circulation anomalies. Significance Statement The purpose of this study is to better understand how the glaze dipole pattern in China responds to climate anomalies, which focuses on the spatial–temporal evolution of glaze days and its underlying physical mechanism. This study highlights the important role of Pacific Ocean sea surface temperature anomalies in the interdecadal variations in glaze dipole pattern, and it gives a clear clarification to demonstrate the impacts of Pacific decadal oscillation on the climatological background for the occurrence of glaze. Our results provide a guide on what drives the interdecadal variation in the glaze dipole pattern and help to understand the climate dynamic mechanism of variations in freezing events in China.

Volatile organic compounds measured by proton transfer reaction mass spectrometry over the complex terrain of Quintero Bay, Central Chile

This research provides new evidence regarding the different kinds of air quality episodes, and their underlying mechanisms, that frequently impact the urban area of Quintero Bay in Central Chile, which is located along complex coastal terrain and is surrounded by industries. The monitoring campaign was carried out in January 2022 and encompassed two distinctive meteorological regimes. The first part of the month was dominated by a coastal low centered to the south of Quintero, which resulted in prevailing northerly flow (or weak southerlies) and a deep cloud-topped marine boundary layer. After a 2–3-day transition, the latter collapsed, and a clear-sky regime ensued, which was characterized by a shallow boundary layer and strong southerly winds during the daytime that lasted until the end of the campaign.By using proton transfer reaction time of flight mass spectrometry (PTR–TOF–MS) at a high temporal resolution (1 s), we measured high levels of volatile organic compounds (VOCs) during air quality episodes in real time. The episodes detected were associated with different prevailing meteorological regimes, suggesting that different point sources were involved. In the first episode, propene/cyclopropane, butenes, benzene, toluene and ethylbenzene/xylenes were associated with north and northwesterly weak winds. Complaints associated with hydrocarbon odor were reported. The pollution originated from industrial and petrochemical units located to the north of Quintero, which transport and store natural gas, liquified petroleum gas and oil. The second episode was linked to an oil refinery located south of our measurement site. In this case, high levels of phenol, furan and cresols occurred under strong southwesterly winds. During this event, headaches and dizziness were reported. By contrast, the levels of other aromatic compounds (benzene, toluene, ethylbenzene/xylenes) were lower than in the first air pollution episode.

Future Drying Subtropical East Asia in Winter: Mechanism and Observational Constraint

Abstract Coupled climate models project robust wintertime wetting trend over midlatitude East Asia under global warming scenarios, but the projected change in precipitation shows large intermodel uncertainty over subtropical East Asia from southern China to southwestern Japan. Based on an ensemble of climate models participating in CMIP6, this study shows that the weakened southern branch westerly jet (SWJ) on the southern side of Tibetan Plateau (TP) plays a key role in suppressing subtropical East Asian precipitation. The SWJ is deflected into southwesterly wind on the southeastern side of TP, bringing ascent and precipitation to subtropical East Asia primarily through isentropic gliding. As a result of the poleward and upward shift of the planetary-scale westerly jet under global warming, the SWJ becomes weaker and it acts to suppress subtropical East Asian precipitation by weakening the southwesterly wind and ascent. The SWJ–precipitation linkage also exists on interannual time scales, but the sensitivity of precipitation to interannual SWJ variability is systematically underestimated by the models compared with observation. The combined effects of the change in SWJ strength and the sensitivity of precipitation to SWJ explain about 40% of the intermodel spread of the projected precipitation changes. Observational constraint on the SWJ–precipitation relationship amplifies the projected drying trend and narrows the intermodel spread. It shows that the regional-averaged precipitation over subtropical East Asia decreases by 3.3% per degree of warming, and the amplitude of precipitation reduction over subtropical East Asia (southern China) is about 1.4 (3.4) times the raw projection.

Role of Topography in Triggering Elevated Thunderstorms Associated With Winter Cold Fronts Over the Eastern Yunnan‐Guizhou Plateau

AbstractUsing observations and reanalysis data, the circulation characteristics of elevated thunderstorms behind winter cold fronts (ETCFs) over the eastern Yunnan‐Guizhou Plateau are summarized, and a new triggering mechanism in the cold season related to topography is proposed. ETCFs always occur on the ridge and eastern side of the Dalou Mountain and are triggered at night. Pattern classifications using obliquely rotated T‐mode principal component analysis show that low‐level large‐scale circulation patterns have the following two characteristics: a southwestern airflow at 850 hPa, changing from southwest wind to east wind; and a northeast airflow at 900 hPa, changing from northeast wind to east wind. ETCFs occur in the area of this low easterly wind. A case study of 21 January 2020, shows that it is the intersection of the topography and cold front that provides unique geographical and circulation conditions for triggering elevated thunderstorms. Lateral friction along the mountains causes horizontal wind shear to flow parallel to them, forming positive and negative vortices above and below the front, respectively. This pair of vortices is strengthened by inertial oscillation at night, producing a stronger easterly wind. In the combined effects of frontal lift and topographical lift of the easterly flow, the elevated convection is triggered in the convergence zone above the front. These results provide a new insight into how topography helps to trigger convection in the cold season.

Preliminary investigation of soil organic carbon distribution and turnover patterns, and potential pollution sources in and around a typical coking plant in North China

The variation characteristics of soil organic carbon (SOC) in and around the coking plant area are still unclear. In this work, the concentration and stable carbon isotope composition of SOC in coke plant soils were investigated to preliminarily identify the sources of SOC in and around the plant area, and to characterize soil carbon turnover. Meanwhile, the carbon isotopic technique was used to initially identify the soil pollution processes and sources in and around the coking plant area. The results demonstrate that the SOC content (12.76 mg g−1) of the surface soil in the coking plant is about 6 times higher than that outside the coking plant (2.05 mg g−1), and the variation range of δ13C value of the surface soil in the plant (−24.63～-18.55‰) is larger than that of the soil outside the plant (−24.92～-20.22‰). The SOC concentration decreases gradually from the center of the plant outward with increasing distance, and the δ13C in the middle and north of the plant tends to be positive compared with the δ13C in the west and southeast of the plant. As the increase of soil depth, the SOC content and δ13C value in the plant increases. On the contrary, δ13C value and SOC content outside the plant decreases, with a minor variation. Based on the carbon isotope method, the SOC in and around the coking plant area is mainly from industrial activities (e.g., coal burning and coking), and partly from C3 plants. Notably, organic waste gases containing heavy hydrocarbons, light oils, and organic compounds accumulated in the northern and northeastern areas outside the plant due to south and southwest winds, which may pose an environmental health risk.

Thermodynamic and dynamic effects of anomalous dragon boat water over South China in 2022

In 2022, persistent extreme precipitation events occurred during the Dragon Boat Water (DBW) period (May to June) over South China, with rainfall-breaking historical extremes at many stations. In this study, the relative importance of the thermodynamic and dynamic effects on this anomalous event was explored through budget equations of moisture and moist static energy (MSE). The moisture budget analysis suggests that the thermodynamic effect contributes to the precipitation anomaly by less than 10% through the advection of abnormal moisture by mean horizontal and vertical motion. However, the dynamic effect, associated with the mean moisture advection by anomalous vertical motion, makes the largest contribution. This seems to imply that the increase in water vapor due to global warming does not play a critical role in this extreme event. Nevertheless, further analysis of the MSE budget equation reveals that the anomalous vertical motion is governed by both the dynamical effect of atmospheric circulation changes and the thermodynamic effect of atmospheric thermal state changes. The thermodynamic effect, which accounts for 47.21% of the total effect (sum of the thermodynamic and dynamic effects), is caused by anomalous moist enthalpy (mainly dry enthalpy) advection due to the mean horizontal motion. In contrast, the dynamical effect is related to the warm and humid air advection caused by the anomalous southwest wind caused by the anomalous anticyclone over the northwest Pacific. Simultaneously, the anomalous northeast wind brings dry cold air, resulting in the convergence of cold and warm air over the Pearl River basin and persistent heavy rainfall events. The results of this study provide valuable insights into the direct and indirect physical mechanisms leading to anomalous DBW period over South China in 2022.

Water Vapor Budget Features and Its Precipitation Effects over the Key Area of Sichuan-Tibet Railway in Summer

In this study, we investigate the variation of characteristics of summer precipitation with different magnitudes and the water vapor budget in different key areas of the Sichuan-Tibet Railway by using the daily precipitation data from meteorological stations and the monthly mean ERA-Interim reanalysis data from 1979 to 2018. The results show that the summer heavy precipitation anomaly in the Brahmaputra valley and mountain (BVM) area is generated by the multi-scale interactions between the unique topography and different water vapor transports at low latitudes. When an intense anticyclonic circulation occurs over the western Pacific and the South China Sea, there is also an anomalous anticyclonic circulation extending from the northern Indian Peninsula to the Bay of Bengal. This circulation pattern is conductive to the increase of the water vapor inflow from the southern boundary and the net water vapor budget in the BVM area. The strong southwesterly wind and water vapor convergence over this area thus leading to heavy precipitation in the summer. The summer heavy precipitation amount, frequency and intensity in the western parts of the steep terrain (ST) area on the east slope of the plateau are positively correlated with the water vapor inflow from its western boundary. However, the correlations of the water vapor budget to the summer heavy precipitation amount and frequency are opposite in the eastern parts of the ST area. The unique terrain and circulation patterns lead to the localization and diversity of heavy precipitation in the ST area.

Changing patterns of the East Asian monsoon drive shifts in migration and abundance of a globally important rice pest.

Numerous insects including pests and beneficial species undertake windborne migrations over hundreds of kilometers. In East Asia, climate-induced changes in large-scale atmospheric circulation systems are affecting wind-fields and precipitation zones and these, in turn, are changing migration patterns. We examined the consequences in a serious rice pest, the brown planthopper (BPH, Nilaparvata lugens) in East China. BPH cannot overwinter in temperate East Asia, and infestations there are initiated by several waves of windborne spring or summer migrants originating from tropical areas in Indochina. The East Asian summer monsoon, characterized by abundant rainfall and southerly winds, is of critical importance for these northward movements. We analyzed a 42-year dataset of meteorological parameters and catches of BPH from a standardized network of 341 light-traps in South and East China. We show that south of the Yangtze River during summer, southwesterly winds have weakened and rainfall increased, while the summer precipitation has decreased further north on the Jianghuai Plain. Together, these changes have resulted in shorter migratory journeys for BPH leaving South China. As a result, pest outbreaks of BPH in the key rice-growing area of the Lower Yangtze River Valley (LYRV) have declined since 2001. We show that these changes to the East Asian summer monsoon weather parameters are driven by shifts in the position and intensity of the Western Pacific subtropical high (WPSH) system that have occurred during the last 20 years. As a result, the relationship between WPSH intensity and BPH immigration that was previously used to predict the size of the immigration to the LYRV has now broken down. Our results demonstrate that migration patterns of a serious rice pest have shifted in response to the climate-induced changes in precipitation and wind pattern, with significant consequences for the population management of migratory pests.

Diurnal and Seasonal Variability of the Atmospheric Boundary-Layer Height in Marseille (France) for Mistral and Sea/Land Breeze Conditions

Marseille (France) is a city on the Mediterranean coast characterized by two specific wind patterns: mistral (northwesterly wind blowing above 10 m/s) and sea/land breezes (southwesterly wind during daytime/northeasterly wind during the nighttime, blowing below 6 m/s). For the first time, this study investigates the diurnal and seasonal variability in the atmospheric boundary-layer height (ABLH) in Marseille for both wind patterns. A 532 nm aerosol lidar was installed in the urban center in the summer of 2021. The lidar installed in the winter of 2021–2022 had an additional near-infrared channel at 808 nm. The ABLH was extracted from the lidar datasets using a Haar wavelet method. For well-established mistral conditions, the ABLH reached to about 1000 m and showed a diurnal amplitude of ~650 m in winter and 740 m in summer, with a morning growth rate limited by turbulence. During sea breeze situations, the ABLH maxima were lower in both seasons (300–600 m) due to the sea’s thermal inertia. During land breeze situations, ABLH minima were estimated to be lower than 150 m. In summer, the Haar method was unable to calculate them because of unpronounced aerosol layers. While the near-infrared channel gives better results, the polarization of the green channel allows us to understand the type of aerosols and thus the origin of the air mass; a combination of the two gives complementary information.

Wind–Human Resonance in a Polluted City: The Case of Dalinpu in Kaohsiung, Taiwan

Air pollution creates significant challenges, particularly in countries undergoing rapid industrialization and urbanization. Wind, a strong agency in moving polluting to and away, however, does not attract sufficient attention from the social science literature on urban health and air pollution. This article fills this gap by proposing the concept of wind–human resonance, referring to both the capacity and intensity of the wind as two kinds of human–wind relationships in the weather-world. On the one hand, certain cleaning and blocking practices are carried out by residents, corresponding to the wind’s carrying capacity of air pollutants; fishing activities are altered according to the wind capacity in the atmo-oceanic dynamic. On the other hand, the intensity of the wind envelops an industrialized coastal village where community members engage with the oceanic wind that shapes the community identity (and affect). This framework of an entangled human–wind relationship is empirically examined through the case of the coastal Dalinpu area of Kaohsiung in southwestern Taiwan, a community that has successfully organized a campaign in the name of “southwest wind preservation” to terminate an industrial zone construction project. By revealing how wind is physically and affectively entangled into urban politics, this article aims to foreground the air flow study in volume geography in particular and the human–environment relationship in general.

Spatial Distribution and Trends of Wind Energy at Various Time Scales over the South China Sea

In this study, the spatial distribution and trends of wind energy (as measured by wind and wind power density) were investigated from 1979 to 2021 across various time scales over the South China Sea (SCS)by utilizing ERA5 reanalysis data. The results indicate that the SCS possesses abundant wind energy. In addition, due to the fact that the East Asian monsoon dominates the SCS, the wind energy exhibits obvious seasonal changes. It is in winter and autumn that the winter monsoon (i.e., the northeast wind) prevails over the SCS. Here, the wind energy is abundant and reaches its maximum in December. In summer, the summer monsoon (i.e., the southwest wind) prevails over the SCS. Here, the wind energy is abundant over the southwestern SCS. In spring, however, the wind energy is poor. The annual mean wind energy shows a decreasing trend along the northern coast and an increasing trend over the central SCS. The trends of seasonal mean wind energy in winter, spring, and summer demonstrate a similar pattern to the annual mean wind energy. With respect to the intensity of the trends, they are strongest in winter, followed by spring and autumn, and weakest in summer. The trend of wind energy in autumn almost demonstrates the opposite pattern in comparison with the other seasons, i.e., both decreasing and increasing trends over the northern and southern SCS, respectively. The decreasing trend of wind energy along the northern coast of the SCS occurs in February, April, July, September, and November, whereas the increasing trend over the central SCS appears from the period of December to June. The spatial distribution and trends of wind energy over the SCS can help with issuing a more informed recommendation with respect to offshore wind energy planning.

The characteristics and evolution process of presummer intense regional rainfall over the Pearl River Delta metropolitan region

AbstractThe presummer intense regional rainfall events (RREs) over the Pearl River Delta (PRD) metropolitan region have been defined using rainy stations, maximum instantaneous regional rainfall coefficient (RRCt) and maximum rainfall intensity based on the rain gauge data from 2008 to 2017. Most of intense RREs with the maximum rainfall intensity about 20–60 mm·h−1 and the lifetime shorter than 24 h have the primary rainfall peak from noon to afternoon. Based on the rainfall pattern at 3 h prior to the events start, the intense RREs can be classified into two groups that one is southeastward propagation mode originating from the northwestern mountains of PRD and another is coastal mode from South China Sea (SCS). During the premonsoon‐onset period, the first type of RREs is the main mode causing the heavy rain in the cities of the inland region owing to the anomalous cold air from north confronting the wet and warm air from south with the increased southwesterly winds resulting in the strengthening frontal zone and moisture convergence. After the summer monsoon onset, the main rainfall pattern changes to the coastal mode having the significant impact on the coastal cities. The obvious anomalous cyclonic circulation related with the increased moisture convergence in the coastal regions providing the favourable conditions of the convection initiation and development. The study of the characteristics and evolution process of preceding intense RREs can improve the forecasting skills and reduce the risk of urban inundation.

Dynamic Field Retrieval and Analysis of Structural Evolution in Offshore Core Area of Typhoon Higos Based on Ground-Based Radar Observation

Three ground-based radars in the Pearl River Delta successfully observed Typhoon Higos (2020), which traveled over the offshore area in the South China Sea. During the observation period, the stratiform region of the outer rainband of HIGOS became active while swirling inward, merging into an unclosed eyewall and spreading outward, but its structure was asymmetric between upwind and downwind. To understand the dynamic mechanism of the asymmetry of the stratiform region in detail, the refined wind speed distributions in the inner core of Higos was retrieved by using the radar observation data and a three-dimensional, variational, direct, data assimilation, Dual-Doppler analysis (DDA). In addition, an Observing System Simulation Experiment (OSSE) was conducted with the numerical simulations by the Weather Research and Forecasting (WRF) model and numerical emulations by Cloud Resolving Model Radar SIMulator (CR-SIM) software to validate the retrieved data. From the OSSE, the emulated retrieved data were comparable with the WRF-out data. The analysis shows that the dynamic mechanisms are different between upwind and downwind in the stratiform rainband. In the former, the inflow sinks in the middle troposphere. In addition, there is an inflow in the lower troposphere, with an outflow aloft the inflow. In the latter, however, the stratiform rainband is primarily influenced by outflow from inside the rainband and inflow from outside the monsoon-related southwesterly winds. The vertical velocity characteristics in the stratiform rainband downwind also differ from those upwind. The upwind updraft was distinct in the middle troposphere, whereas the downwind updraft was caused by the convergence of the outflow from inside the stratiform rainband and the monsoon-related southwesterly inflow in the lower troposphere.

Impacts of biomass burning in Southeast Asia on aerosols over the low-latitude plateau in China: An analysis of a typical pollution event

From March to April, widespread forest fires and agro-residue burning frequently occur in Southeast Asia, which release large amounts of gas species and aerosols and impact air quality over the wide source and downwind regions. In this study, we investigated the impact of biomass burning (BB) over Southeast Asia on particulate matter concentrations and aerosol properties in downwind areas of the low-latitude plateau from 1 March to 30 April 2019, with a focus on a typical pollution event in Kunming (KM), the capital of Yunnan Province, by using a wide variety of observations from the Chenggong ground monitoring station in Yunnan University, an air quality network in China, satellite retrievals and ERA-5 reanalysis data and numerical simulation. A regional pollution event contributed by BB pollutants from Southeast Asia and the India-Myanmar trough occurred in Yunnan Province on 31 March to 1 April 2019, which was the only typical pollution event that pollution transmission ran through central Yunnan Province from south to north since 2013, when the Airborne Pollution Action Plan was unveiled by China government. The daily mean PM2.5, PM1, and black carbon concentrations increased by 73.3 μg m−3(78%), 70.5 μg m−3 (80%), and 7.7 μg m−3 (83%), respectively, and the scattering and absorbing coefficients increased by 471.6 Mm−1 and 63.5 Mm−1, respectively, at the Chenggong station. The southwest winds exceeding 2 km vertically thick appeared in front of the India-Myanmar trough over the fire regions, pushing BB plumes northward into Yunnan Province. The model results show that 59.5% of PM2.5 mass produced by BB in Yunnan Province was sourced from the Myanmar-Thailand border, and 29.3% was from western Myanmar at a lower altitude (<4.9 km), which indicated that BB in the Myanmar-Thailand border was the dominant contributor.

An unusual Northwest–Southeast oriented Meiyu rain belt in 2021

Affected by the East Asian summer monsoon, summer floods occur frequently in eastern China, causing huge economic losses and social impacts. In 2021, the precipitation anomaly during the Meiyu period in Yangtze-Huaihe River Valley (YHRV) was abnormally high, with a value of 188.0 mm, about 1.7 standard deviations above normal. However, it did not cause serious flood disasters. This was mainly due to the existence of two precipitation anomaly centers in YHRV, which shared the impact of the excessive precipitation. These two centers were located at the junction of Anhui Province and Henan Province and the junction of Jiangsu Province, Anhui Province and Jiangxi Province, respectively. And more notably, the 2021 Meiyu precipitation anomalies were distributed in a Northwest-Southeast oriented band that is very rare. During the 2021 Meiyu period, the western Pacific subtropical high (WPSH) weakened and extended westward. An anticyclonic circulation anomaly in the lower troposphere was observed over eastern China, which was found to be related to the quasi-stationary wave trains propagating eastward from Hainan Island to the northwestern Pacific by vorticity budget analysis. And the Rossby wave source term played a critical role. In addition, the tropical Atlantic anomalous warming produced convergence anomaly over the eastern Mediterranean and excited quasi-stationary wave trains propagating downstream at the mid-latitudes in upper layers through a monsoon-desert-like teleconnection. It raised the geopotential height over eastern China and thus favored the westward extension of WPSH. These large-scale circulation anomalies generated the anomalous easterly winds along the eastern coast of China and the anomalous southwesterly winds from Guangxi Province to the middle and lower reaches of the Yangtze River. Similar features can be found in the anomalous water vapor transport and the water vapor converged in the YHRV. The above-mentioned reasons jointly caused the abnormally high precipitation anomaly and its unusual pattern in the 2021 Meiyu season. This study might provide a scientific basis and clues for understanding Meiyu precipitation anomalies and disaster prevention and mitigation.

Characteristics of Ozone Pollution and High-impact Meteorological Factors in Urban Cities: A Case of Suzhou

The problem of urban ozone (O3) pollution has become prominent in recent years. However, the meteorological factors associated with O3 pollution remain unclear. Analyzing the characteristics of O3 pollution in Suzhou, as a typical urban city, and exploring the high-impact meteorological factors with O3 pollution are crucial to the prevention and control of air pollution in this region. This study used correlation analysis and machine learning methods to analyze the variation in O3 concentration and the relationship between meteorological driving factors in Suzhou based on the O3 concentration data provided by Suzhou Environmental Monitoring Center and the contemporaneous meteorological observation data in Suzhou from April to September in 2015 to 2020. The results showed that: ① O3 pollution exceeding the standard rate was more than 20% in ozone seasons during the past six years; further, pollution days of O3 and the number of pollution days of O3 as the primary pollutant increased yearly. Evidently, the problem of O3 pollution has become increasingly prominent. ② The diurnal variations in O3 were unimodal with the valley point at 07:00 and the highest peak between 15:00 and 16:00. Similar trends were found in diurnal variations of both air temperature and solar radiation, but the daily highest peak came earlier than that of O3. The results also showed an apparent weekend effect of O3 concentration in 2017 and 2019 and a significant correlation between O3 concentration and solar irradiance during the week. In addition, the monthly variation in O3 concentration and pollution exceeding the standard rate was bimodal. ③The occurrence of ozone pollution was affected by various meteorological conditions. The maximum number of days appeared when daily sunshine hours lasted longer than 7 hours, with a daily maximum air temperature around 30℃, solar irradiance ranging from 350 to 440 kW·m-2, and relative humidity ranging from 50% to 75%, at which time the intensity of pollution was the strongest. When the wind speed of easterly wind was less than 1.5 m·s-1, or the wind speed of southwest wind was less than 3.5 m·s-1, moderate ozone pollution occurred. ④ An optimal prediction model of O3 concentration was established based on machine learning, which had good predictive ability for O3 concentration in April, May, July, and September but did not perform well when O3 concentration exceeded 200 μg·m-3. Meanwhile, it was found that solar radiation had the most obvious effect on O3 concentration, followed by relative humidity, whereas the temperature and wind were less important than the former two factors.