A bulk theory of airflow over a mountain ridge allowing for a stable overlying atmosphere

A steady-state, one-dimensional, mesoscale, non-linear problem of the airflow over a mountain ridge is considered in the framework of the bulk theory in an attempt to develop a meteorological generalisation of the hydraulic problem formulated and solved previously by Houghton and Kasahara. In essence, only three modifications of their formulation are made in the present paper: (a) inversion strength is introduced and considered as an additional dependent variable; (b) the atmosphere above the flow is assumed to be stably stratified with a constant vertical potential temperature gradient; and (c) the condition requiring the flow to return to its initial state after descending and moving away from the ridge is removed.The problem is reduced to a transcendental algebraic equation, the solution of which describes all the possible types of flow, including a discontinuous one. A classification and meteorological criterion graphically presented in the form of a map is proposed for the realisation of each type of flow.If the ridge is higher than some threshold, a jump arises above the windward slope and propagates against the flow. As a result, the initial parameters change to new values for which a secondary steady-state solution exists. An iterative method has been developed to calculate these new initial parameters.The general features of the different types of flow, including those with a jump and secondary flows, are discussed. It is shown that if the ridge height exceeds some second threshold there are no solutions at all, which means that the flow is totally blocked. Concrete calculated examples are presented of all possible types of steady-state flows including secondary ones.It is shown that after introduction of these modifications, the hydraulic model of Houghton and Kasahara acquires more real meteorological meaning.     

On the Two Successive Supercold Waves Straddling the End of 2020 and the Beginning of 2021

Two supercold waves straddling 2020 and 2021 successively hit China and caused record-breaking extremely low temperatures.In this study,the distinct features of these two supercold waves are analyzed on the medium-range time scale.The blocking pattern from the Kara Sea to Lake Baikal characterized the first cold wave,while the large-scale tilted ridge and trough over the Asian continent featured the second cold wave.Prior to the cold waves,both the northwest and hyperpolar paths of cold air contributed to a zonally extensive cold air accumulation in the key region of Siberia.This might be the primary reason why strong and extensive supercold waves occur even under the Arctic amplification background.The two cold waves straddling 2020 and 2021 exhibited distinct features:(1)the blocking circulation occurred to the north or the east of the Ural Mountains and was not confined only to the Ural Mountains as it was for the earlier cold waves;(2)the collocation of the Asian blocking pattern and the polar vortex deflection towards East Asia preferred the hyperpolar path of cold air accumulation and the subsequent southward outburst;and(3)both high-and low-frequency processes worked in concert,leading to the very intense cold waves.The cold air advance along the northwest path,which coincides with the southeastward intrusion of the Siberian High(SH)front edge,is associated with the high-frequency process,while the cold air movement along the hyperpolar path,which is close to the eastern edge of the SH,is controlled by the low-frequency process.     

东北地区冬季气温与北极涛动年代际关系研究

利用中国160站气温资料、北极涛动指数资料及关国NCEP／NCAR再分析资料中月平均海平面气压场、高度场、风场资料，分析了东北地区冬季气温、冬季北极涛动的年代际特征及其关系。结果表明：在年代际时间尺度上，两者之间存在显著正相关。冬季北极涛动处于低（高）指数期，东北冬季气温为持续冷冬（暖冬）期。可能影响机制是：在地面，冬季北极涛动处于低（高）指数期时，西伯利亚高压增强（减弱），亚洲大陆偏北冬季风增强（减弱），东北为持续冷冬（暖冬）期；在对流层中层，冬季北极涛动处于低（高）指数期时，东亚大槽加深（减弱），贝加尔湖以西以北脊增强（减弱），环流呈经向（纬向）型发展，东北对流层中层偏北风增强（减弱），东北为持续冷冬（暖冬）期。     

"05.6"广东致洪暴雨过程的500 hPa环流场及低频特征

分析2005年6月15~25日广东出现的连续暴雨过程及结束期的500 hPa候平均环流场,并对4~9月暴雨中心区域的降水低频振荡特征进行了分析。结果表明:连续暴雨期欧亚中高纬度维持稳定的两槽一脊形势;中低纬度非洲与西太平洋副热带高压强大,西太平洋副高西伸到广东沿海,青藏高原为稳定的高压脊,高原下游为西风槽活动区,使冷空气沿着高原频繁扩散南下,并与西太平洋副高边缘的暖湿气流相遇,产生连续的暴雨过程。当巴尔喀什湖-贝加尔湖的高压脊东移而转为宽槽区,青藏高原高压脊减弱消失,高原以东为平直的西风环流,我国东北到日本为高压脊控制时,西北太平洋副热带高压加强北抬,华南降水减弱,连续暴雨过程结束。小波分析表明,连续暴雨期存在准20 d的周期振荡。     

2008年4月天山北坡一次强寒潮天气成因分析

利用T213 0场预报资料对2008年4月18日新疆天山北坡带中部石河子垦区发生的50 a不遇强寒潮天气过程的环流背景、影响系统及物理量场进行了综合分析。结果表明：“2008.04.18”强寒潮天气主要成因是：里黑海脊发展东移与东欧脊同位相叠加并顺转, 脊前东北风带建立并加强,诱导泰米尔半岛冷空气沿脊前东北气流西南下到横槽中, 迫使横槽转向强烈发展东南压,并形成高空急流,强锋区和强冷高压;由于高压脊部分东南垮,导致中高纬度强冷空气大举东南下,造成强寒潮天气过程;西南急流与冷空气的交汇产生了较强的动力辐合和水汽辐合,对大暴雪的形成起到了重要作用;大暴雪发生在较强的能量锋区、高湿区和水汽通量辐合区内。     

2011年9月华西秋雨特征及成因分析

本文分析了2011年9月华西秋雨的时空分布特征和大尺度环流形势,并对秋雨形成的主要物理机制进行了诊断分析。结果表明:2011年9月华西地区北部降雨日数多、强度大、持续时间长、落区集中。9月上中旬500hPa高度场上巴尔喀什湖以北的高压脊稳定维持,脊前西北气流携带的冷空气与副高外围的东南暖湿气流和来自孟加拉湾北上的西南暖湿气流交汇于华西地区北部,造成了该地区长时间的持续性强降雨。华西地区北部处于高低能量之间的强能量锋区中,东路干冷空气的汇入,激     

A numerical study of barotropic vortex motion near a large-scale mountain range with application to the motion of tropical cyclones approaching the Sierra Madre

Summary The motion of an idealized vortex near a large-scale mountain range is examined by numerically integrating the single-layer shallow-water equations on an equatorial beta plane. Modification of the asymmetric circulation by divergent vorticity generation as the air is forced across the ridge greatly affects the motion of the vortex. Attention is focused on the role of the vortex structure, the mountainrange width and orientation, and the stratification of the model atmosphere in influencing the motion of the vortex. A scale analysis is used to explain the sensitivity of the vortex track to changes in the model's external parameters. The sensitivity of the model vortex track to the slope and orientation of the mountain range is used to help explain some features of the observed tracks of hurricanes that make landfall on the east coast of Mexico.With 14 Figures     

2008年雪灾过程高原上游关键区水汽输送机制及其前兆性“强信号”特征

利用NCEP/NCAR再分析资料、中国气象台站降水量资料、青藏高原及周边地区GPS水汽观测站网大气可降水量资料,采用统计诊断方法,分析了2008年中国南方雨雪冰冻灾害期间4次暴雪过程的动力、热力特征,水汽变化及其输送特征.研究结果表明,灾害发生期间,中国北方中低层大气盛行偏北气流,使较强的偏北冷空气"楔入"中国东南部低层大气,构成低层"冷垫",同时偏南暖湿气流源源不断地将大量水汽输送到中国东南部,冷暖气流交汇及其垂直切变导致强烈的上升运动,构成了"冷垫"上空的"暖盖"及"南槽北脊"反位相环流汇合锋区的垂直环流结构.反位相环流的偏南水汽主要来自南海和印度洋,两者在中南半岛和中国西南地区合并,构成经过云南及周边关键区的强西南水汽流,形成了长江流域特大雪灾发生的关键水汽通道.研究结果亦证实高原及周边地区JICA项目GPS水汽监测网信息可为中国长江中下游地区暴雪的发生提供具有预报实用意义的重要分析工具及数值模式同化初始信息源.云南及周边区域水汽含量的多少对下游地区后期降水的发生具有一定的指示意义,即水汽输送上游关键区大气可降水量的变化可作为此次南方雨雪冰冻灾害期间降水发生的"前兆性"信号,而云南则是西南水汽输送通道中尤为重要的关键区域.     

The role of dynamic and diabatic processes in the generation of cut-off lows over Northwest Africa

Summary The present observational study addresses the role of dynamic and diabatic processes leading to the generation of four deep upper-level troughs/cut-offs, involved in two extreme precipitation episodes over West Africa during the cool season. The elongated potential vorticity (PV) streamers associated with the observed troughs form as a result of an equatorward transport of high-PV air downstream of a large ridge over the central North Atlantic. Strong deformation along the eastern side of the ridge leads to a thinning of the PV streamers. In some situations the tips of the streamers break up and form distinct and long-lived stationary cut-offs near West Africa, in particular if the presence of another PV ridge downstream allows a complete isolation from the midlatitude westerlies. In other situations a prior anticyclonic wave-breaking event over Europe leads to an advection of high-PV towards the Iberian Peninsula that merges with the streamer and impedes a complete cut-off. The observations presented here suggest that the rapid amplification of the PV ridges over the North Atlantic and thus the subsequent streamer formation are related to upstream latent heating through non-conservative diabatic reduction of upper-level PV and through the strong divergent outflow near the tropopause that support large negative isentropic PV advection. The intense latent heat release is promoted by cyclo- and frontogenesis, and the transport of warm, moist air by a low-level jet ahead of the surface cold front (often called a warm conveyor belt; WCB). Diabatic PV tendencies are highest where the WCB rises over the surface warm front to the northeast or east of the cyclone centre. In most cases the distinct heating occurs in connection with a strong upper-level jet and a rapid deepening of the involved surface cyclone. More quantitative dynamical and statistical studies of the suggested relation are needed to better understand the relative contributions of single factors to the large and synoptic scale evolution that leads to PV streamers/cut-offs near West Africa.     

2011年我国夏季降水动力统计预测与异常成因

该文对2011年我国夏季降水情况进行简要回顾,并对3月制作的夏季降水动力统计客观定量化和动力统计-诊断预测结果进行检验。以长江中下游地区为例,对比说明了两种预测方法选取因子的差异及动力统计-诊断较动力统计客观定量化预测结果有一定提高的原因。在此基础上,分析了导致2011年夏季主雨带较预测结果偏南的影响因素,并进一步探讨了大气环流特别是中高纬度阻塞高压和低纬度西太平洋副热带高压异常的可能成因。结果表明:2011年夏季主雨带偏南主要是中高纬度阻塞形势与低纬度副热带高压的季节内异常振荡及二者逐月不同配置的产物,而中高纬度阻塞形势与低纬度副热带高压的季节内异常振荡是由海温、积雪等外强迫及东亚环流系统内部成员相互作用所致。     

中国大范围持续性低温事件与中国南方降水异常

本文利用1951~2009年的数据资料,探讨了与冬季大范围持续性低温事件对应的中国南方降水多寡的原因以及不同降水类型事件的不同前兆信号。主要结果如下:(1)欧亚大陆中高纬大型斜脊斜槽是中国大范围持续性低温事件的关键环流特征。降水偏多型和偏少型低温事件的中高纬环流系统存在显著差异:当大型斜脊的纬向尺度较大且其正高度距平中心偏强时,斜槽的负距平中心靠近东亚大槽区,西伯利亚高压大范围加强,南侵冷空气活动偏强,使中国南方地区降水偏少。相反,当大型斜脊的纬向尺度相对小时,斜槽位置偏西,西伯利亚高压加强范围偏小,南侵冷空气活动也偏弱,有利于中国南方地区降水偏多。(2)在低温事件中我国南方地区的降水多寡,也取决于西太副高和孟加拉湾南支槽。当西太副高和孟加拉湾南支槽加强时,来自西太平洋、南海和孟加拉湾的三支暖湿气流到达我国南方地区,导致南方降水偏多,反之亦然。(3)不同降水类型低温事件的前兆环流信号(之前1~3侯)显著不同。降水偏多型低温事件前期,源自北大西洋地区的波列向东亚地区传播。降水偏少型事件前期,极涡在欧亚大陆一侧的次极区显著减弱,同时极涡呈两极型,一极伸向南欧及地中海地区,另一极伸向阿留申群岛。在中期—延伸期时间尺度上,这一结果可为持续性低温事件降水形势的预测提供依据。     

Gustiness of the Novorossiysk bora

The development of the Novorossiysk bora on February 5–6, 2010 is simulated with high spatial and temporal resolution using the regional model of atmospheric circulation. Considered are typical features of the wind speed and air temperature fields. Singled out are two regimes defining the type of the air flowing around the mountain ridge and the temporal variability (gustiness) of the surface wind speed.     

A COMPARATIVE STUDY ON CHARACTERISTICS AND THERMO- DYNAMIC DEVELOPMENT MECHANISMS OF TWO TYPES OF WARM-SECTOR HEAVY RAINFALL ALONG THE SOUTH CHINA COAST

Warm-sector heavy rainfalls along the south China coast from April to June during 2009–2014 can be divided into two main types based on their low-level circulations. Type I is the southerly pattern with meridional convergence line at the west of the Pearl River estuary, which is formed by the convergence of southeasterly, southerly, and southwesterly flows. Type II is the southwesterly pattern with a latitudinal convergence line at the east of the Pearl River estuary, which is formed by the convergence of westerly and southwesterly flows. Statistics on 6-hourly heavy rainfall events indicates that, during the afore-mentioned 6 years, there were on average 73.2 occurrences of the southerly pattern and 50.3 occurrences of the southwesterly pattern per year. After the onset of summer monsoon in the South China Sea, the occurrence frequencies of both patterns increase remarkably. The synthetic diagnosis of pattern circulation shows that, at 500 hPa, for the southerly pattern, there is a broad warm high ridge, and a temperature ridge is behind the high ridge, which causes an obvious warm advection at the high ridge area. There is no frontal region. For the southwesterly pattern, the circulation is a weak trough with a temperature trough behind it. The position of the frontal region is near Yangzi River, and the south China coast is in the warm-sector of the frontal region. At the vertical cross-section of each of the two categories of heavy rainfall, there is a strong vertical motion center stretching to 400 hPa, where the convergence layer in the rainfall region is deep and with several vertical convergence centers overlapping one another. Both types of heavy rainfalls are with abundant water vapor, accompanied with deep convective instability energy layers, and with strong release of latent heat caused by condensation of water vapor. The release of latent heat leads to the warming-up and stretching of the air column, thus strengthens deep convergence and vertical velocity upward. There is a stronger latent heat-release in the southwesterly pattern than in the southerly pattern, while in the southerly pattern, the warm advection at middle and upper levels is stronger than the latent head release. To study the thermo-dynamic development mechanisms, weather research and forecasting model (WRF) numerical simulations are made and the results show that, in the two rainstorm regions, latent heat release warms up the air column, hence significantly increase the depth and strength of the vertical velocity. Moreover, the release of latent heat strengthens convergent circulation at lower levels and weakens divergent circulation at middle levels, whose influence can be as strong as 30%–50% of the wind circulation strength of the two types of the warm-sector heavy rainfall over the south China coast, and further enhances deep convection, promoting warm-sector storm development.     

南昌市一次连续空气污染过程的气象条件分析

2004年12月8—16日南昌市出现了一次连续空气污染过程。利用城市空气污染观测资料和气象常规观测资料,从天气形势和主要气象要素两个方面,对此次空气污染过程进行了分析。结果表明,此次连续空气污染事件都是出现在风速小、无雨和有雾或霾的气象条件下,高空主要为高压脊的形势或是处在西风槽底的平直气流中,低层大气稳定,中层大气增温明显;地面形势主要为地面高压脊、高压底部或是倒槽前部,地面有弱冷空气南下时不一定能改变污染状况。极厚、极强的逆温层和极小风速的持续存在是造成污染物高浓度最重要的气象条件。此外,地形也是影响南昌市空气质量水平的因素之一。     

“0806”华南持续性暴雨诊断分析与数值模拟

用NCEP再分析资料、常规气象观测等资料,对2008年6月11—13日发生在华南地区的持续性暴雨的大尺度背景进行了分析,用MM5模式模拟出了暴雨的大尺度背景,利用模拟结果对这次暴雨过程的系统演变做了进一步的分析。结果表明:(1)西南涡是暴雨的直接影响系统,随着西南涡的东移,暴雨从广西北部一直推进到广东东南部。(2)中高纬稳定的两槽一脊的环流形势和偏南的西太平洋副高,及不断南下的小股冷空气,为这次暴雨提供了有利的中高纬环流背景冷空气条件;东亚夏季风偏南,越赤道气流偏强,给暴雨的维持提供了丰富的水汽条件。(3)模拟的结果验证了暴雨发生的大尺度环流背景,并在700hPa上模拟出了3个正涡度中心,是华南暴雨发生的动力因子。     

Convective initiation by topographically induced convergence forcing over the Dabie Mountains on 24 June 2010

The initiation of convective cells in the late morning of 24 June 2010 along the eastward extending ridge of the Dabie Mountains in the Anhui region,China,is studied through numerical simulations that include local data assimilation.A primary convergence line is found over the ridge of the Dabie Mountains,and along the ridge line several locally enhanced convergence centers preferentially initiate convection.Three processes responsible for creating the overall convergence pattern are identified.First,thermally-driven upslope winds induce convergence zones over the main mountain peaks along the ridge,which are shifted slightly downwind in location by the moderate low-level easterly flow found on the north side of a Mei-yu front.Second,flows around the main mountain peaks along the ridge create further convergence on the lee side of the peaks.Third,upslope winds develop along the roughly north–south oriented valleys on both sides of the ridge due to thermal and dynamic channeling effects,and create additional convergence between the peaks along the ridge.The superposition of the above convergence features creates the primary convergence line along the ridge line of the Dabie Mountains.Locally enhanced convergence centers on the primary line cause the initiation of the first convection cells along the ridge.These conclusions are supported by two sensitivity experiments in which the environmental wind(dynamic forcing) or radiative and land surface thermal forcing are removed,respectively.Overall,the thermal forcing effects are stronger than dynamic forcing given the relatively weak environmental flow.     

2015年广西区北部超高压输电线路一次连续覆冰事件的天气学特征

广西区是我国南方电网西电东送输电的重要通道, 2015年1月26日—2月8日期间广西区北部桂林地区发生了一次大范围超高压输电线路的连续覆冰事件, 其最大覆冰厚度可达24.83 mm。利用ERA5再分析资料和气象观测资料结合南方电网超高压输电线路覆冰观测资料, 从天气形势、温湿垂直层结、局地气象要素以及大气环流指数等方面综合分析了此次电线覆冰的天气学成因。结果表明, 东亚大槽偏强, 阻塞高压引导脊前偏北气流南下, 冷空气入境与西太平洋副热带高压带来的暖湿气流汇合, 在北方寒潮与南方水汽的共同作用下, 地处高海拔的输电线塔杆易出现覆冰。冷暖气团在桂林北部山区上空相互对峙形成准静止锋时出现两种覆冰变化特征: 当冷空气强盛且水汽充沛时, 过冷却雨滴冻结或者雾滴凝华形成电线积冰; 而在暖气团主导下电线覆冰则自然融化。准静止锋的锋区移动在很大程度上影响着电线的覆冰增长过程, 特别地, 冷暖空气的交替主导是电线反复积冰的主要原因。      

Coastally trapped stratus events in British Columbia

Abstract

Coastally trapped stratus events involve the alongshore propagation of a low‐level mesoscale ridge of high pressure, and generally occur only during summer in British Columbia. During these events, a body of cool dense air overlain by stratus cloud propagates northwards in the marine layer. This situation results because Coriolis effects trap the dense air against the coastal mountains and an inversion layer prevents vertical leakage of energy. Detailed analysis of a late summer event and comparison of the observations with available theory are presented. It is shown that the theoretical speeds for the event agree reasonably with observations except near major topographic variability. The theoretical trapping scale (internal Rossby radius) computed from the sounding data matches the observed width of the coastal stratus seen on satellite imagery. It is found that the properties of the event may be accounted for by the propagation in the marine layer of a coastally trapped intrusion with steep leading edge or shock. Attention is drawn to the role that the complex topography of Vancouver Island plays in influencing the propagation of the event, and some suggestions for forecasting these phenomena are made.     

2014年“5.8”广东大暴雨中尺度特征分析

利用NCEP/NCAR的1°×1°再分析资料、地面常规观测、FY-2E卫星TBB资料,对2014年5月8—9日发生在华南南部的一次暖区暴雨过程进行了研究。得出以下结论:1)第1阶段暴雨发生变性高压脊后部,未受冷空气影响,属于华南典型的回流暖区暴雨过程,第2阶段在东路弱冷空气的触发下再次产生暴雨,属于非典型回流暖区暴雨。2)1个中α尺度MCC和1个中β尺度MCS是该次广东大暴雨直接制造者,其中MCS-D在广西境内生成并逐渐东移,多个对流系统的并入延长了MCS-D的生命史,最后形成中α尺度MCC。3)东南气流是该次暴雨的主要水汽来源,中层小股干冷空气侵入,高层强烈辐散、深厚的上升运动的配置条件有利于中尺度对流系统发展和维持。4)区域自动站风场资料分析表明,夜间陆风(偏北风)与加强的东南风在112.5°E附近的汇合,可能触发了中尺度对流系统MβCS-H的生成。     

西北地区东部一次连阴雨过程等熵位涡分析

应用等熵位涡原理,对2005年5月中旬西北地区东部的一次连阴雨过程进行了诊断分析。结果表明:影响这次连阴雨过程的大环流背景是乌拉尔山附近存在一高压脊,同时在巴尔喀什湖附近存在一不断加深的槽;这次连阴雨过程是由高原不断生成的低涡与北边分裂下来的冷空气所共同造成的;由于青藏高原大地形的作用,等熵面上高位涡沿高原北侧向下游输送;等熵位涡高值区与降水区有较好的对应关系,IPV正值区能够反应冷空气的移动,具有较好的预报指示意义。