Nocturnal Low-levelWinds and Their Impacts on Particulate Matter over the Beijing Area

Three-month wind profiles, 260 m PM_1 concentrations [i.e., particulate matter(PM) with an aerodynamic diameter ≤1μm], and carrier-to-noise ratio data at two Beijing sites 55 km apart(urban and suburban) were collected to analyze the characteristics of low-level nocturnal wind and PM in autumn and winter. Three mountain-plain wind events with wind shear were selected for analysis. The measurements indicated that the maximum wind speeds of the northerly weak low-level jet(LLJ) below 320 m at the suburban site were weaker than those at the urban site, and the LLJ heights and depths at the suburban site were lower than those at the urban site. The nocturnal 140 m mean vertical velocities and the variations in vertical velocity at the urban site were larger than those at the suburban site. A nocturnal breeze with a weak LLJ of ～3 m s^(-1) noticeably offset nocturnal PM transport due to southerly flow and convergence within the northern urban area of Beijing. Characteristics of the nocturnal LLJ, such as start-up time, structure, intensity, and duration, were important factors in determining the decrease in the nocturnal horizontal range and site-based low-level variations in PM.

STATISTICAL ANALYSIS OF LOW-LEVEL JET STREAMS IN NANJING AREA BASED ON WIND PROFILER DATA

In order to understand the activity characteristics of low-level jets in the Nanjing area,statistical analysis and comparative study are carried out on their monthly and diurnal variations,characteristics of their cores and accompanying weather conditions using wind profile data in 2005-2008 collected by two wind profilers.The results show that low-level jets have significant monthly and diurnal variations.They occur more frequently in spring and summer than in autumn and winter and are more active in early morning and at night,with the maximum wind speed usually occurring at midnight.The central part of the low-level jet occurs mainly at the height of less than 1400 meters,and the enhancement of central speed is beneficial to the appearance of precipitation.Meanwhile,when the low-level jet appears in summer,it helps cause heavy rain.The statistical results of the boundary wind profiler are well consistent with those of the tropospheric wind profiler.Two kinds of wind profilers also have the capability of continuously detecting the development of low-level jets.

QBO Features of Tropical Pacific wind Stress Field with the Relation to El Nino

Analysis has been implemented of 1970-1992 tropical Pacific wind stress anomaly and sea surface temperature anomaly (SSTA) datasets, indicating that quasi-biennial oscillation (QBO) of the tropical Pacific WS and SSTA is featured both by a standing and a progressive form, the former emerging in the most intense centers of action and the latter travelling east- or west-ward out of the SSTA sources. Results show that the SSTA is in the warm (cold) phase as zonal component of euqatorial wind stress anomaly gets weakened (reinforced) and the QBO of wind stress anomaly is well related to the El Nino cycle.

Change of Low-level Jet in a Heavy Rainstorm Process of Xiangtan in July 2016

Based on the previous research on the model of rainstorm weather with low-level jet in Xiangtan,using the classification result of radar echo characteristics,the wind profile data provided by new generation of weather radar in Changsha and hourly rainfall data,a thorough study of the heavy rainfall from 2 to 5 July 2016 in Xiangtan was conducted. It was concluded that heavy precipitation had the characteristics of the WPSH pattern of rainstorm with low-level jet at early stage,and then it converted to cold shear jet pattern in latter stage. When low-level southwest jet began to have momentum download,that is to say,there was more than 12 m/s of southwest jet below 1 km,and it rapidly strengthened and expanded downward,it was conducive to the occurrence of short-term rainstorm. The low-level jet would not immediately cause a strong precipitation when it reached the station,with a certain lag. A positive correlation existed between the increase of low-level jet index and precipitation intensity,and low-level jet index could predict the occurrence of heavy rainfall and rain intensity.

THE ANALYSIS OF THE RELATIONSHIP BETWEEN PULSE OF LLJ AND HEAVY RAIN USING WIND PROFILER DATA

Detailed analysis and comparisons are made on the data of Hong Kong wind-profiler and the weather/precipitation every hour during the HUAMEX and the experiment for the monsoon of the South China Sea (SCSMEX) in 1998. It is found that the wind-profiler data could reveal the meso-scale phenomena in the PBL of Southwest Monsoon, which was closely related to rainstorms. The center of the low-level jet under the altitude of 2 km, which corresponded to the appearance of heavy rain, appeared 1 to 2 hours after the center of low-level jet (LLJ) did above the 2-km altitude. An index I is designed to represent the intensity and height of the LLJ. This index can express clearly the close relationship between the precipitation and the LLJ. The results show that the wind-profiler is somewhat predictive to a rainstorm.

Secondary Product Inversion of Wind Profile Data and Its Application in Weather Forecast

Using wind field data of boundary-layer wind profile of Changsha national comprehensive meteorological observation test base,the inversion of atmospheric temperature advection was studied.Moreover,four typical heavy precipitation processes in Changsha region during 2018-2019 were analyzed in detail,and low-level jet index product was calculated.The results showed that the retrieved temperature advection can well reflect the change details of atmospheric temperature advection,and there was a good corresponding relationship between change of the cold and warm advection and the heavy precipitation process;low-level jet index generally had an explosion and enhancement process before precipitation,but the time was not consistent with prior research result,and a large number of samples need to be further studied.

Oscillation of Surface PM2.5 Concentration Resulting from an Alternation of Easterly and Southerly Winds in Beijing： Mechanisms and Implications

We used simultaneous measurements of surface PM_（2.5） concentration and vertical profiles of aerosol concentration,temperature, and humidity, together with regional air quality model simulations, to study an episode of aerosol pollution in Beijing from 15 to 19 November 2016. The potential effects of easterly and southerly winds on the surface concentrations and vertical profiles of the PM_（2.5） pollution were investigated. Favorable easterly winds produced strong upward motion and were able to transport the PM_（2.5） pollution at the surface to the upper levels of the atmosphere. The amount of surface PM_（2.5） pollution transported by the easterly winds was determined by the strength and height of the upward motion produced by the easterly winds and the initial height of the upward wind. A greater amount of PM_（2.5） pollution was transported to upper levels of the atmosphere by upward winds with a lower initial height. The pollutants were diluted by easterly winds from clean ocean air masses. The inversion layer was destroyed by the easterly winds and the surface pollutants and warm air masses were then lifted to the upper levels of the atmosphere, where they re-established a multi-layer inversion. This region of inversion was strengthened by the southerly winds, increasing the severity of pollution. A vortex was produced by southerly winds that led to the convergence of air along the Taihang Mountains. Pollutants were transported from southern–central Hebei Province to Beijing in the boundary layer. Warm advection associated with the southerly winds intensified the inversion produced by the easterly winds and a more stable boundary layer was formed. The layer with high PM_（2.5） concentration became dee-per with persistent southerly winds of a certain depth. The polluted air masses then rose over the northern Taihang Mountains to the northern mountainous regions of Hebei Province.

Low-Level Wind Shear Characteristics and Lidar-Based Alerting at Lanzhou Zhongchuan International Airport,China

Lanzhou Zhongchuan International Airport[International Civil Aviation Organization(ICAO)code ZLLL]is located in a wind shear prone area in China,where most low-level wind shear events occur in dry weather conditions.We analyzed temporal distribution and synoptic circulation background for 18 dry wind shear events reported by pilots at ZLLL by using the NCEP final(FNL)operational global analysis data,and then proposed a lidar-based regional divergence algorithm(RDA)to determine wind shear intensity and location.Low-level wind shear at ZLLL usually occurs in the afternoon and evening in dry conditions.Most wind shear events occur in an unstable atmosphere over ZLLL,with changes in wind speed or direction generally found at 700 hPa and 10-m height.Based on synoptic circulations at 700 hPa,wind shear events could be classified as strong northerly,convergence,southerly,and weak wind types.The proposed RDA successfully identified low-level wind shear except one southerly case,achieving94%alerting rate compared with 82%for the operational system at ZLLL and 88%for the ramp detection algorithm(widely used in some operational alert systems)based on the same dataset.The RDA-unidentified southerly case occurred in a near neutral atmosphere,and wind speed change could not be captured by the Doppler lidar.

THE VARIATION OF THE SPRING PRECIPITATION IN GUANGZHOU AND ITS PRECURSORY SIGNALS

Guangzhou spring rainfall mainly exhibits interannual variation of Quasi-biannual and interdecadal variation of 30 yrs, and is in the period of weak rainfall at interdecadal time scale. SST anomalies （SSTA） of Nino3 are the strongest precursor of Guangzhou spring rainfall. They have significant positive correlation from previous November and persist stably to April. Nino3 SSTA in the previous winter affects Guangzhou spring rainfall through North Pacific subtropical high and low wind in spring. When Nino3 SSTA is positive in the previous winter, sprirg subtropical high is intense and westward, South China is located in the area of ascending airflow at the edge of the subtropical high, and water vapor transporting to South China is intensified by anticyclone circulation to the east of the Philippines. So Guangzhou spring rainfall is heavy. When Nino3 SSTA is negative, the subtropical high is weak and eastward, South China is far away from the subtropical high and is located in the area of descending airflow, and water vapor transportirg to South China is weak because low-level cyclonic circulation controls areas to the east of the Philippines and north wind prevails in South China. So Guangzhou spring rainfall is weak ard spring drought is resulted.

Assessment of the Impacts of Tropical Cyclones Idai to the Western Coastal Area and Hinterlands of the South Western Indian Ocean

Tropical Cyclones (TCs) are among the atmospheric events which may trigger/enhance the occurrence of disasters to the society in most world basins including the Southwestern Indian Ocean (SWIO). This study analyzed the dynamics and the impacts of the Tropical Cyclone (TC) Idai (4th-21st March, 2019) which devastated most of the SWIO countries. The study used the Reanalysis 1 products of daily zonal (u) and meridional (v) winds, Sea Surface Temperatures (SSTs), amount of Precipitable Water (PRW), and relative humidity (Rh). The dynamics and movements of Idai were analyzed using the wind circulation at 850, 700, 500 and 200 mb, where the TC dynamic variables like vertical wind shear, vorticity, and the mean zonal wind were calculated using u and v components. Using the open Grid Analysis and Display System (GrADS) software the data was processed into three-time epochs of pre, during and post;and then analyzed to feature the state of the atmosphere before (pre), during and post TC Idai using all datasets. The amount of precipitable water was used to map the rainfall on pre, during, and post Idai as well as during its landfall. The results revealed that dynamics of TC Idai was intensifying the weather (over Mozambique) and clearing the weather equatorward or southward of 12°S, with low vertical wind shear over the landfall areas (-3 to 3 m/s) and higher shear values (10 - 40 m/s) northward and southward of the Mozambican channel. Higher moisture content (80 - 90%) and higher PRW (40 - 60 mm/day) mapped during Idai over the lowland areas of Mozambique propagating westward. Higher low-level vorticity values were also mapped over the landfall areas. More results revealed that countries laying equatorward of 12°S, e.g., the northern coastal areas of Kenya (Turkana and Baringo) and Tanzania, Idai disrupted the 2019 March to May (MAM) seasonal rainfall by inducing long dry spell which accelerated the famine over the northeastern Kenya (Turkana). Moreover, results revealed that the land falling of Idai triggered intensive flooding which affected a wide spectrum of socio-economic livelihoods including significant loss of lives, injuries, loss of material wealth, infrastructure;indeed, people were forced to leave their houses for quite a longtime;water-borne diseases like malaria, cholera among others were experienced. Furthermore, results and reports revealed that a large amount of funds were raised to combat the impacts of Idai. For instance, USAID/OFDA used about $14,146,651 for human aid and treatment of flood-prone diseases like Cholera in Mozambique ($13,296,651), Zimbabwe ($100,000), and Malawi ($280,000), respectively. Also a death toll of about 602 in Mozambique and 344 in Zimbabwe, and more than 2500 cases of injured people were reported. Conclusively the study has shown that TCs including Idai and other are among the deadliest natural phenomenon which great affects the human and his environments, thus extensive studies on TCs frequency, strength, tracks as well as their coast benefit analysis should be conducted to reduce the societal impacts of these TCs.

Two climate factors in May that affect Korean rainfall in September

This study revealed a high positive correlation between rainfall in Korea during September and the trade wind （TW）/Arctic Oscillation （AO） index in May that combines two climate factors, low-level TWs and the AO. This correlation was identified on the basis of the difference in the 850 hPa streamline analysis between the positive and negative phases selected using the combined TW/AO index. In May, the spatial pattern of the anomalous pressure systems is similar to that in the positive AO phase. These anomalous pressure sys- tems continue in June to August （JJA） and September, but the overall spatial distribution shifts a little to the south. Particularly in September, a huge anomalous anticyclone centered over the southeast seas of Japan strengthens in most of the western north Pacific region and supplies a large volume of warm and humid air to the region near Korea. This characteristic is confirmed by the facts that during the positive TW/AO phase, the subtropical western north Pacific high （SWNPH） is more developed to the north and that the continu- ous positioning of the upper troposphere jet over Korea from May to September strengthens the anomalous upward flow, bringing warm and humid air to all layers. These factors contribute to increasing September rainfall in Korea during the positive TW/AO phase. Because the SWNPH develops more to the north in the positive phase, tropical cyclones tend to make landfall in Korea frequently, which also plays a positive role in increasing September rainfall in Korea.The above features are also reflected by the differences in average rainfall between the six years that had the highest May Nifio 3.4 indices （El Niflo phase） and the six years that had the lowest May Niflo 3.4 indices （La Nifia phase）.

Objective Analysis of Circulation Extremes During the 21 July 2012Torrential Rain in Beijing

It has been reported that the heaviest rain event since 1951 hit Beijing on 21 July 2012 （henceforth referred to as the 721 case）.The frequency and extreme attributes of the large-scale circulation patterns observed during the 721 case are explored by using obliquely rotated T-mode principle component analysis （PCA） and reanalysis data from NCEP/NCAR.The occurrence frequency of the 721-type circulation during the summers of 1951-2012 is 10.9%,while the frequency of torrential rain under this type of circulation is 4.51%.Relative to other rainstorms with similar large-scale circulations during the study period,the 721 case is characterized by a more westward extension of the subtropical high over the western North Pacific,a stronger low-level jet in the lower troposphere over the south of Beijing,a larger amount of ambient precipitable water,and a stronger vertical wind shear over Beijing.Among the 621 days with the 721-type circulation during the study period,the 721 case ranks the 54th in terms of the 925-hPa low-level jet south of Beijing,the 209th in terms of the local vertical wind shear,and the 8th in terms of the local precipitable water.The 721 case is particularly extreme with respect to the 925-hPa low-level jet south of Beijing and local precipitable water.Cases with similar circulations and equal or greater values of the 925-hPa low-level jet south of Beijing and local precipitable water have occurred thrice during the summers of 1951-2012 （i.e.,once every 21 years）.

Effect of terrain and building structures on the airflow in an airport

The aim of this study was to perform computational fluid dynamics (CFD) simulations on the airflows at the Hong Kong International Airport (HKIA). In particular, the effects of hangar buildings and terrain were studied to explore the effects of turbulence on flying aircraft, especially during landing. The CFD simulation showed that significant differences in wind speeds may occur between the north and the south runways on the western part of the HKIA under typhoon conditions with a strong north to northwesterly wind. Simulation also showed that the hanger buildings between the two runways on the western side and the nearby terrain could be causing the observed difference in the wind speeds. The results also indicated that these obstacles could cause significant wind speed variations at the western end of the south runway. This may affect the operation of landing aircraft. The CFD results for a typical typhoon case were analyzed and found to match the wind data recorded by an aircraft landing that day.

Mesoscale Analysis of a Heavy Rainfall Event Along the Huaihe River Valley During 8-9 July 2007

During 8-9 July 2007,several successively developed rainstorms along the Meiyu front produced heavy rainfall in the Huaihe River Valley,which led to the most catastrophic flooding in this region since 1954. Through mesoscale analysis of both conventional and intensive observations from upper air and surface stations,automatic weather stations,Doppler radars,and the FY-2C satellite,the current study examines the developing style and environmental conditions of the mesoscale convective systems（MCSs） that led to the development of the rainstorms. Our analysis showed that this event went through three phases.The first phase of the heavy rainfall （PhaseⅠ） was caused by a meso-α-scale wind shear in the lower troposphere during 0200-1700 BT（Beijing Time） 8 July.PhaseⅡwas characterized by a reduction in rain rate and the formation of a low-level vortex between 1700 BT 8 and 0200 BT 9 July.In PhaseⅢ,the well-organized mature meso-a-scale low-level vortex brought about intensified rains during 0200-0800 BT 9 July.Satellite and radar observations showed a backward development of MCSs（new convective cells were generated at the back of the system） in PhaseⅠ,a forward development in PhaseⅢ,and a spiral organization of the convective lines in PhaseⅡ. The heavy rainstorm systems were initiated continuously along a surface mesoscale dew-point front with a horizontal scale of～300 km（as part of the Meiyu front） in the upper reaches of the Huaihe River Valley near Fuyang City,Anhui Province and then gradually decayed in the middle and lower reaches.It is hypothesized that lifting by strong low-level convergence is sufficient to trigger convection in the high CAPE（convective available potential energy） environment.

The unusual 2014–2016 El Nino events: Dynamics, prediction and enlightenments

The 2014–2016 El Ni?o events consist of a stalled El Ni?o event in the winter of 2014/2015 and a following extreme El Ni?o event in the end of 2015.Neither event was successfully predicted in operational prediction models.Because of the unusual evolutions of these events that rarely happened in the historical observations,few experience was ready for understanding and predicting the two El Ni?o events when they occurred.Also due to their specialties,considerable attention were attracted with aims to reveal the hidden mechanisms.This article reviews the recent progresses and knowledge that were obtained in these studies.Emerging from these studies,it was argued that the key factor that was responsible for the stalled El Ni?o in 2014 was the unexpected summertime Easterly Wind Surges(EWSs)or the lack of summertime Westerly Wind Bursts(WWBs).Most operational prediction models failed to reproduce such stochastic winds and thus made unrealistic forecasts.The two El Ni?o events awakened the research community again to incorporate the state-of-the-art climate models to simulate the stochastic winds and investigate their roles in the development of El Nino.

Numerical Simulation on the Formation of Mesoscale Vortex in Col Field

There exist typically two kinds of low-level col fields over the middle and lower reaches of the Yangtze River of China during summer. ＇One is associated with the mesoscale vortex embedded in the Meiyu front; the other is related to tropical cyclones making landfall over eastern or southern China. The first one is the focus of this study. The meso scale vortex （MβV） causing heavy rainfall usually forms in a col field or within a shear line associated with the mesoscale low-level jet （mLLJ）. The MβV, triggered by mesoscale wind perturbation in a col field, is simulated by using a three-dimensional q-coordinate mesoscale model. This col field represents the circumstance of the 98.7 heavy rainfall event over eastern Hubei Province. The results show that the MβV triggered by wind perturbation was weak and maintained only several hours if the latent heat feedback was switched off. The wind perturbation also weakened rapidly. However, when the latent heat feedback was included, precipitation became more intense and the mLLJ and MβV quickly developed. The MβV maintained quasi-stationary during its life cycle under the stable col field. The MβV triggered by the southwesterly perturbation was located closely to that by the northeasterly perturbation. They were both located in the weak wind region near the col point. The stronger the perturbation was, the more intense and longer the dynamic MβV lived. The 24-h accumulated precipitation in different experiments showed a similar pattern, which indicates that the relatively stable intensity and range of precipitation were the intrinsic characters of the stable col field. Furthermore, it is found that mesoscale perturbations had some impacts on the location and intensity of the rainfall. The fluctuation of large-scale LLJ to the south of the col field might produce a perturbation, causing instable stratification and rainfall within the low-level col field or shear line. The mLLJ near the rainfall was enhanced due to the latent heat feedback and then caused formation of the MβV. The stable col field provided favorable thermodynamic conditions for the formation and development of the MβV. Therefore, it is an ＂incubator＂ for MβVs and the associated consecutive heavy rainfall.