The Roles of Low-level Jets in “21·7” Henan Extremely Persistent Heavy Rainfall Event

An extremely heavy rainfall event lasting from 17 to 22 July 2021 occurred in Henan Province of China, with accumulated precipitation of more than 1000 mm over a 6-day period that exceeded its mean annual precipitation. The present study examines the roles of persistent low-level jets(LLJs) in maintaining the precipitation using surface station observations and reanalysis datasets. The LLJs triggered strong ascending motions and carried moisture mainly from the outflow of Typhoon In-fa(2021). The varying directions of the LLJs well corresponded to the meridional shifts of the rainfall. The precipitation rate reached a maximum during 20-21 July as the LLJs strengthened and expanded vertically into double LLJs, including synoptic-weather-system-related LLJs(SLLJs) at 850–700 hPa and boundary-layer jets(BLJs)at ~950 hPa. The coupling of the SLLJ and BLJ provided strong mid-and low-level convergence on 20 July, whereas the SLLJ produced mid-level divergence at its entrance that coupled with low-level convergence at the terminus of the BLJ on21 July. The formation mechanisms of the two types of LLJs are further examined. The SLLJs and the low-pressure vortex(or inverted trough) varied synchronously as a whole and were affected by the southwestward movement of the WPSH in the rainiest period. The persistent large total pressure gradient force at low levels also maintained the strength of low-level geostrophic winds, thus sustaining the BLJs on the synoptic scale. The results based on a Du-Rotunno 1D model show that the Blackadar and Holton mechanisms jointly governed the BLJ dynamics on the diurnal scale.

Analysis of the Characteristics of the Low-level Jets in the Middle Reaches of the Yangtze River during the Mei-yu Season

Here,we analyze the characteristics and the formation mechanisms of low-level jets(LLJs)in the middle reaches of the Yangtze River during the 2010 mei-yu season using Wuhan station radiosonde data and the fifth generation of the European Centre for Medium-Range Weather Forecasts(ERA5)reanalysis dataset.Our results show that the vertical structure of LLJs is characterized by a predominance of boundary layer jets(BLJs)concentrated at heights of 900-1200 m.The BLJs occur most frequently at 2300 LST(LST=UTC+8 hours)but are strongest at 0200 LST,with composite wind velocities>14 m s^(-1).Synoptic-system-related LLJs(SLLJs)occur most frequently at 0800 LST but are strongest at 1100LST,with composite wind velocities>12 m s^(-1).Both BLJs and SLLJs are characterized by a southwesterly wind direction,although the wind direction of SLLJs is more westerly,and northeasterly SLLJs occur more frequently than northeasterly BLJs.When Wuhan is south of the mei-yu front,the westward extension of the northwest Pacific subtropical high intensifies,and the low-pressure system in the eastern Tibetan Plateau strengthens,favoring the formation of LLJs,which are closely related to precipitation.The wind speeds on rainstorm days are greater than those on LLJ days.Our analysis of four typical heavy precipitation events shows the presence of LLJs at the center of the precipitation and on its southern side before the onset of heavy precipitation.BLJs were shown to develop earlier than SLLJs.

Formation of Low-Level Jets over Southern China in the Mei-yu Season

This study examines the diversity of low-level jet(LLJ)formation and related physical processes over southern China.A total of 171 LLJ formation events with enhanced daily southwesterlies and early-morning maximum wind speeds were observed during the mei-yu seasons of 1989–2018.The LLJs can be further categorized into four types based on the increases in the daily mean and diurnal amplitude of the low-level winds.Analysis of the synoptic-scale disturbances shows that the two types of LLJ formation(Q1 and Q4),which feature large increases of daily southerly wind components,are mainly induced by west-east dipole patterns of pressure change,in association with the enhanced southwest vortex and/or the western Pacific subtropical high(WPSH).In contrast,the other two types(Q2 and Q3),which feature relatively large increases in their daily westerly components,are related to a northwest-southeast dipole pattern of pressure change due to the mid-latitude trough and the WPSH.We further analyze the considerable variations in the diurnal thermal forcing among the LLJ formation events.The strong(weak)daytime heating of solar radiation leads to relatively large(small)increases in the diurnal amplitude of low-level winds in Q1 and Q2(Q3 and Q4)types.Therefore,different combinations of synopticscale disturbances and diurnal thermal forcings are found to account for the diversity in LLJ formation and associated differences in downstream rainfall patterns.These results help to improve our understanding and prediction of the formation of LLJs.

SnO_(2)/Co_(3)O_(4)nanofibers using double jets electrospinning as low operating temperature gas sensor

SnO_(2)/Co_(3)O_(4)nanofibers(NFs)are synthesized by using a homopolar electrospinning system with double jets of positive polarity electric fields.The morphology and structure of SnO_(2)/Co_(3)O_(4)hetero-nanofibers are characterized by using field emission scanning electron microscope(FE-SEM),transmission electron microscope(TEM),x-ray diffraction(XRD),and x-ray photoelectron spectrometer(XPS).The analyses of SnO_(2)/Co_(3)O_(4)NFs by EDS and HRTEM show that the cobalt and tin exist on one nanofiber,which is related to the homopolar electrospinning and the crystallization during sintering.As a typical n-type semiconductor,Sn O_(2)has the disadvantages of high optimal operating temperature and poor reproducibility.Comparing with Sn O_(2),the optimal operating temperature of SnO_(2)/Co_(3)O_(4)NFs is reduced from 350℃to 250℃,which may be related to the catalysis of Co_(2)O_(2).The response of SnO_(2)/Co_(3)O_(4)to 100-ppm ethanol at 250℃is 50.9,9 times higher than that of pure Sn O_(2),which may be attributed to the p–n heterojunction between the n-type Sn O_(2)crystalline grain and the p-type Co_(2)O_(2)crystalline grain.The nanoscale p–n heterojunction promotes the electron migration and forms an interface barrier.The synergy effects between Sn O_(2)and Co_(2)O_(2),the crystalline grain p–n heterojunction,the existence of nanofibers and the large specific surface area all jointly contribute to the improved gas sensing performance.

Thermodynamics and Microphysical Characteristics of an Extreme Rainfall Event Under the Influence of a Low-level Jet over the South China Coast

In this paper,the data of Automatic Weather Stations(AWSs),ERA5 reanalysis,sounding,wind profile radar,and dual-polarization radar are used to study an extreme rainfall event in the south China Coast on 11 to 12 May 2022 from the aspects of thermodynamics and microphysical characteristics under the influence of low-level jets(LLJs).Results show that:(1)The extreme rainfall event can be divided into two stages:the first stage(S1)from 0000 to 0600 LST on May 12 and the second stage(S2)from 0700 to 1700 LST on the same day.During S1,the rainfall is mainly caused by the upper-level shortwave trough and the boundary layer jet(BLJ),characterized by strong upward motion on the windward side of mountains.In S2,the combined influence of the BLJ and synoptic-system-related low-level jet(SLLJ)increases the vertical wind shear and vertical vorticity,strengthening the rainstorm.In combination with the effect of topography,a warm and humid southwest flow continuously transports water vapor to farther north,resulting in a significant increase in rainfall over the study area(on the terrain’s windward slope).From S1 to S2,the altitude of a divergence center in the upper air decreases obviously.(2)The rainfalls in the two stages are both associated with the mesoscale convergence line(MCL)on the surface,and the wind field from the mesoscale outflow boundary(MOB)in S1 is in the same direction as the environmental winds.Due to a small area of convergence that is left behind the MOB,convection moves eastward quickly and causes a short duration of heavy rainfall.In S2,the convergence along the MOB is enhanced,which strengthens the rainfall and leads to strong outflows,further enhancing the surface convergence near the MOB and forming a positive feedback mechanism.It results in a slow motion of convection and a long duration of heavy rainfall.(3)In terms of microphysics,the center of a strong echo in S1 is higher than in S2.The warm-rain process of the oceanic type characterizes both stages,but the convective intensity in S2 is significantly stronger than that in S1,featuring bigger drop sizes and lower concentrations.It is mainly due to the strengthening of LLJs,which makes small cloud droplets lift to melting levels,enhancing the ice phase process(riming process),producing large amounts of graupel particles and enhancing the melting and collision processes as they fall,resulting in the increase of liquid water content(LWC)and the formation of large raindrops near the surface.

Experimental Investigation on Space-dispersed Double-wall Jet Combustion System for DI Diesel Engine

A space-dispersed double-wall jet combustion system was developed by adopting the wall-guiding spray method and the stratification theory.The experimental test was carried out to optimize the structural parameters of the diesel-engine combustion system,including chamber structure,swirl ratio of cylinder head,included angle of jet orifice,number and diameter of jet orifice,fuel injection pressure and timing.The effect of double-wall jet combustion system on combustion and engine performance was tested to obtain the best performance indexes,and the double-wall jet combustion system was compared to the prototype.The results show that NOx is reduced from 712 PPm to 487 PPm at 2 100 r/min,and from 593 PPm to 369 PPm at 3 000 r/min,which are reduced by 31.6% and 37.7%,respectively.The smoke intensity was reduced form 3.67 BSU to 2.1 BSU,and the oil consumption was reduced from 240.5 g/(kW·h) to 225.4 g/(kW·h),which was decreased by 6.3% at low speed.The pressure in the cylinder was obviously reduced from 115 bar to 108 bar,which was reduced by 6%.

Impact of the Complex Terrain in Beijing on Formation of Low-Level Jets

This study investigated how the Taihang Mountains and the Yanshan Mountains affect low-level jets(LLJs)in the Beijing area,based on conventional radiosonde observations from Nanjiao Observatory(2016–2017)and high-resolution Weather Research and Forecasting–Advanced Research WRF(WRF-ARW)model simulations.Analysis of radiosonde observations indicated that LLJs in the study area are mainly from the southwest and northwest directions,with occurrence frequency of 44.6%and 33.0%,respectively.Southwest(northwest)LLJs are aligned parallel(perpendicular)to the orientation of the Taihang Mountain Range.Terrain sensitivity experiments using the WRF-ARW model were then conducted to examine the effects of terrain forcing on the northwest and southwest LLJs,with adopted terrain heights of 100%and 50%.The results showed that for northwest LLJs,reduction in the elevation of the Taihang Mountain Range led to weakening of jet intensity by approximately 20%and reduction in jet maximum height by approximately 250 m;lowering the Yanshan Mountain Range had minor influence on the northwest LLJs,with only a 5.2%reduction in intensity and no substantial change in jet maximum height.For southwest LLJs,reduction in the elevation of both the Taihang and Yanshan Mountain ranges resulted in minor changes in the intensity and height of the jets.Further analysis revealed that the topography in the Beijing area could modulate the height and intensity of the stable layer by altering the inversion structure within the boundary layer.The LLJs can develop rapidly within the stable layer,and both the location and the scale of the jet core exhibited reasonable agreement with the extent of the stable layer.

Influence of Coastal Marine Boundary Layer Jets on Rainfall in South China

Coastal marine boundary layer jets(CMBLJs)play an important role in coastal and inland rainfall in South China.Using 21 years of ERA5 and CMORPH rainfall data,two main CMBLJs are found,one on each side of Hainan Island(named BLJ-WEST and BLJ-EAST),which are always strengthened jointly.Both CMBLJs often occur in the pre-summer rainy season and exhibit an evident diurnal cycle with a maximum at night.With the emergence of the CMBLJs,rainfall is significantly enhanced in South China,particularly downstream of each CMBLJ.The response of rainfall to the CMBLJs is mainly attributed to convergence at the terminus of each CMBLJ,terrain-induced lifting,and relevant atmospheric stratification.Coastal rainfall downstream of the BLJ-WEST is much weaker than that downstream of the BLJ-EAST because of higher CIN over the Beibu Gulf,which is caused by lower temperature lapse rates and adiabatic heating in the lee of the Annamite Range.The inland rainfall increases along with CMBLJ intensity,whereas coastal rainfall reaches a maximum in the presence of moderate CMBLJs rather than stronger CMBLJs.Stronger CMBLJs induce stronger dynamic lifting but higher CIN near the coastal area.Additionally,CAPE near the coast does not become highest with strongest CMBLJs,because the CAPE generation contributed by coastal dynamic lifting can be offset by the negative contribution caused by the horizontal advection of cold and dry air from the Indochina Peninsula.Therefore,anomalous dynamic lifting,moisture flux convergence,and CAPE/CIN associated with CMBLJ intensity jointly result in anomalous rainfall.

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.

A Study of the Relationship between Low-level Jet and inversion Layer over an Agroforest Ecosystem in East China Plain

The relationship between the super–low–level jet (LLJ) and inversion layer over an agroforest ecosystem on the Huang–Huai–Hai plain in the eastern China is studied by means of a time–independent K–closure model. It is found that the intensified inversion near the surface of a luxuriantly growing agroforest ecosystem leads to the formation and development of the LLJ, the more intense the inversion, the stronger is the LLJ. The critical value of inversion intensity index for the LLJ formation is 0.75°C/ 100 m, which relates to the necessary geostraphic wind velocity of 6.0 to 10 m / s at the top level of the model. The numerical calculations show that the roughness length of the underlying surface has considerable effects on the LLJ structure. Key words Low?level jet - Temperature inversion - Agroforest ecosystem The project was supported by the National Natural Science Foundation of China (NSFC) (49975016).

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.

ENERGETIC DIAGNOSIS FOR TWO KINDS OF LOW LEVEL JETS

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Excitation of Low-level Jet as Seen by GOES (I-O) Satellite off the Somali Coast

The intensification of a low-level jet off the Somali coast, as observed through GOES (I-O) satellite during Indian summer monsoon 1979 has been studied. Excitation of Low-level cross-equatorial flow in the western Indian ocean results from an interaction between extratropical perturbations moving eastward across the South African-Malgassy region of the Southern Hemisphere. This excitation occurs 2-3 days after the first appearance of a northward propagation cold front across the South African-Malgassy region. Inten-sification of cross-equatorial flow is followed by an increase in rainfall activity along the west coast of India after 3-4 days. The study reveals that this association can be used to forecast an increase in rainfall activity along the west coast of India 5-7 days in advance.

燃烧室百叶与双排孔冷气对透平端壁冷却性能影响研究

针对燃烧室与透平冷气掺混导致实际二次流发展及端壁冷却性能偏离设计值的问题,综合考虑燃烧室百叶冷气尾迹与双排孔射流,数值研究了多源冷气射流在燃烧室与透平交界区的掺混机制,分析了其对透平静叶端壁气膜冷却与流动特性的影响规律。研究结果表明:空腔涡卷吸大部分百叶冷气,并跨越双排孔冷气向下游发展,马蹄涡卷吸限制大部分双排孔冷气,并向空腔涡外侧发展;当双排孔冷气吹风比M_h=0.5,1.0较小时,马蹄涡导致冷气形成楔形高冷效区域,空腔涡的附着侧和分离侧分别产生了高、低冷效区,随着百叶冷气吹风比M_l的增加,空腔涡强度显著降低,导致位于楔形中心的分离侧冷却效率有所提升;当双排孔冷气吹风比M_h为1.5时,双排孔冷气部分跨越马蹄涡,静叶端壁可实现完全冷气覆盖,此时百叶冷气吹风比M_l从0.5增加到1.5,静叶端壁平均冷却效率可从0.4提升到0.63。该研究为考虑燃烧室与透平交界区实际流动掺混情况的静叶端壁冷却结构与布局设计提供理论基础。

高压气体双孔浸没射流振荡频率特性研究

在沸水堆或小型堆中一般将抑压水池作为气体排放的热阱以实现安全壳压力的有效控制。基于这一背景,本文在不同过冷度下对高压气体双孔浸没射流振荡主频特性进行了一系列实验研究,其中不凝性气体份额为0.0~0.4,孔间距范围为1.5 d~3 d。实验结果表明,无论是纯蒸汽还是混合气,在双孔射流时两股气羽均表现出向中间汇聚的形态,甚至汇聚到一起形成联合射流,相较单孔射流而言双孔射流气羽尺度增大冷凝时间延长,所以其振荡主频更低。分析射流参数影响发现,纯蒸汽射流振荡主频随过冷度和孔间距的增加而增加。同时本文总结振荡主频影响规律提出了纯蒸汽射流主频的预测关系式,预测精度在-15%~20%之间。混合气体射流时(纯蒸汽+不凝性气体)振荡主频随过冷度和孔间距的增加而增加,与纯蒸汽射流主频变化规律一致。值得注意的是,混合气射流相较纯蒸汽而言振荡主频更低,并且主频随不凝气份额增加而降低。

ATLAS 2.76 TeV单喷注实验数据对CT18NNLO部分子分布函数的影响

通过ePump(error PDF(parton distribution function)updating method package)研究了大型强子对撞机(large hadron collider,LHC)在质心能量√s=2:76TeV时,来自ATLAS(a toroidal LHC apparatus)探测器单喷注的双微分散射截面的实验数据对CT18NNLO(next-to-next-to-leading order)部分子分布函数(PDF)的影响.首先,使用CT18NNLO部分子分布函数计算了单喷注的双微分散射截面,观察到理论预测与实验数据符合得较好;其次,计算了单喷注双微分散射截面的理论预测值与CT18NNLO胶子部分子分布函数的关联余弦cosϕ;最后,使用ePump更新了CT18NNLO部分子分布函数,并研究了实验数据与全局拟合数据间的冲突.通过比较在Q=100GeV时CT18NNLO胶子部分子分布函数和ePump更新的胶子部分子分布函数,发现ATLAS 2.76 TeV单喷注双微分散射截面实验数据在大和小的动量分数区域对CT18NNLO胶子部分子分布函数的约束较小.

特厚煤层孤岛煤柱水力扩孔防冲卸压技术研究

砚北煤矿2502采区大巷保护煤柱两侧采空,为孤岛煤柱,应力集中程度高,由于煤层较厚,传统大直径钻孔卸压无法取得较好的防冲效果,针对该问题,提出了煤层水力扩孔卸压的技术。通过理论分析研究了水射流破煤和扩孔的卸压机理,水力扩孔通过增加钻孔孔径扩大了单孔的卸压范围;确定了砚北煤矿特厚煤层水力扩孔的主要技术参数,采用数值模拟分析了钻孔内分段扩孔的卸压模式;开发了相应的配套设备,采用双层高压钻杆实现煤体的“钻-割”一体化卸压,提高了卸压效率。通过砚北煤矿现场试验表明,煤层水力扩孔在水压40 MPa和高压喷嘴作用下,扩孔半径能够达到0.35~0.5 m,钻屑量在扩孔区域明显降低,表明水力扩孔能够显著降低煤体内应力集中,达到冲击地压防治的目的。

Assessment of Wet Season Precipitation in the Central United States by the Regional Climate Simulation of the WRFG Member in NARCCAP and Its Relationship with Large-Scale Circulation Biases

Assessment of past-climate simulations of regional climate models(RCMs)is important for understanding the reliability of RCMs when used to project future regional climate.Here,we assess the performance and discuss possible causes of biases in a WRF-based RCM with a grid spacing of 50 km,named WRFG,from the North American Regional Climate Change Assessment Program(NARCCAP)in simulating wet season precipitation over the Central United States for a period when observational data are available.The RCM reproduces key features of the precipitation distribution characteristics during late spring to early summer,although it tends to underestimate the magnitude of precipitation.This dry bias is partially due to the model’s lack of skill in simulating nocturnal precipitation related to the lack of eastward propagating convective systems in the simulation.Inaccuracy in reproducing large-scale circulation and environmental conditions is another contributing factor.The too weak simulated pressure gradient between the Rocky Mountains and the Gulf of Mexico results in weaker southerly winds in between,leading to a reduction of warm moist air transport from the Gulf to the Central Great Plains.The simulated low-level horizontal convergence fields are less favorable for upward motion than in the NARR and hence,for the development of moist convection as well.Therefore,a careful examination of an RCM’s deficiencies and the identification of the source of errors are important when using the RCM to project precipitation changes in future climate scenarios.

120 t转炉用双马赫数氧枪喷头射流特性研究

针对转炉吹炼过程中传统氧枪寿命短、喷溅黏枪烧枪、吹炼时间长等问题,设计了一款双马赫数氧枪,在传统氧枪喷头上增设中心孔。利用数值仿真建立数学模型,分析不同马赫数的超音速射流特性。结果表明,设计的3种方案中,当中心孔马赫数为1.98、外围孔马赫数为2.08时,可以获得更好的吹炼效果。双马赫数氧枪较传统氧枪在有效冲击面积上更具优势,中心孔可以抵消传统周边5孔布置引起的枪下负压区,从而减少黏枪概率,双马赫数氧枪在提高喷头寿命和增强吹炼的稳定性方面具有优势。

高铁站房地下车库基坑复合支护施工技术

为保障临近既有建筑基坑开挖时的安全性,本文通过实际工程案例,针对基坑周边环境复杂、基坑开挖范围内存在深厚杂填土层等工程特点,采用双排桩+旋喷桩+注浆复合支护结构,在双排桩之间采用二重管法打入咬合桩来加固支护结构,并与前排灌注桩形成一道止水帷幕相连,以确保具备良好的止水性能并对双排桩之间的杂填土进行注浆加固。本文对钻孔灌注桩、高压旋喷桩、杂填土注浆技术的结构特点及施工工艺进行了详细分析,探讨并给出了处理措施,以解决场地复杂、工序繁杂等施工难点。结果表明,双排桩+旋喷桩+注浆复合支护结构具有较大的抗侧移刚度,能够保障基坑开挖的安全性和稳定性。