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1.
Summary This study examines the exceptional Alpine south foehn event of 14–16 November 2002 using routine observations and high-resolution
numerical simulations. Besides its long duration and an extremely high temperature level related to warm-air advection from
the northern Sahara, this foehn event exhibited an unusual spatial structure of the low-level wind and temperature field.
Whereas the foehn was largely restricted to the first half of 14 November in the western part of the Alps (Switzerland), it
extended over the full period in the inner-Alpine valleys in the eastern Alps. The duration and intensity of the foehn also
tended to decrease from the Alpine crest towards the northern rim of the Alps. Most surprisingly, continuous foehn even occurred
on the windward side of the Alpine crest, namely in a basin located in the southeastern Alps. The distribution of the orographic
precipitation associated with the foehn case was unusual as well. In Switzerland, intense precipitation was not restricted
to the windward (southern) side of the Alps but extended to the northern side of the Alpine crest, particularly on 16 November.
The results indicate that the spatio-temporal distribution of the foehn in the northern Alps was related to the fact that
the western Alps were within a synoptic-scale transition zone between extremely warm air advected from the south and colder
air lying over western Europe. The colder air was advected around the western Alps whereas extremely warm air descended from
the Alpine crest farther east. Moreover, a small cyclone formed on 14 November north of the Alps and generated a shallow cold
front propagating eastward along the northern Alps. Thus, the tendency towards foehn decreased from west to east and from
the Alpine crest towards the north. The occurrence of foehn on the windward side of the Alpine crest was made possible by
the extreme strength of the large-scale southerly flow, combined with the fact that the upstream precipitation field did not
reach the southeastern edge of the Alps. Finally, the pronounced spillover of precipitation to the northern side in the Swiss
part of the Alps appears to be related to the colder air present north of the crest. This prevented the formation of orographic
gravity waves and downslope air motion, which usually leads to a rapid evaporation of the precipitation on the lee side of
the Alpine crest. 相似文献
2.
Klaus P. Hoinka 《Boundary-Layer Meteorology》1991,57(3):289-294
The note deals with looming during foehn north of the Alps. The results show that there is a weak effect in stretching optically the orography. However, the increase in view angle is probably not detectable by human observers looking south from Munich towards the Alps. The impression that the orography seems to be stretched during a foehn is also due to psychological effects which might be a factor of greater importance. 相似文献
3.
K. P. Hoinka 《Boundary-Layer Meteorology》1987,40(1-2):199-203
The present note discusses physical mechanisms which may contribute to cold air channelling close to the Alps. This involves the modification of the prefrontal air by the warm foehn air and of the postfrontal air by blocking effects resulting in an increase in precipitation. Additionally the influence of a sloping surface in the vicinity of the orography is considered. The problems are discussed in term of a north-south-oriented cold front behaving as an atmospheric gravity current propagating along the east-west oriented Alps. 相似文献
4.
Xia LI Keming ZHAO Shiyuan ZHONG Xiaojing YU Zhimin FENG Yuting ZHONG Ayitken MAULEN Shuting LI 《大气科学进展》2023,40(1):29-43
The air pollution in Urumqi which is located on the northern slope of the Tianshan Mountains in northwestern China,is very serious in winter. Of particular importance is the influence of terrain-induced shallow foehn, known locally as elevated southeasterly gale(ESEG). It usually modulates atmospheric boundary layer structure and wind field patterns and produces favorable meteorological conditions conducive to hazardous air pollution. During 2013–17, Urumqi had an average of 50 d yr–1... 相似文献
5.
Summary Regional and local scale windfield and air mass characteristics during two distinct synoptic foehn wind events over southern New Zealand are examined. The Southern Alps were observed to effectively block low level onshore gradient northwesterly airflow and to channel it through both Cook and Foveaux Straits. Blocking of the onshore synoptic northwesterly airstream also resulted in barrier jet formation along the western slopes of the Southern Alps. This feature of the regional windfield has not previously been documented and develops during favourable conditions to a height of between 1500 to 1800 m above sea level. In the immediate lee of the Southern Alps at Lake Tekapo, classic foehn conditions such as warm ambient air temperatures, low relative humidities and gusty winds were monitored throughout both foehn events examined. Differences in the local windfield were however observed, which reflect the importance of local topography on lee side windfield dynamics during foehn events. Spillover of precipitation to the lee of the mountains was monitored in the latter stages of each case study and appeared to be associated with the passage of the cold front over the Southern Alps. Observations made by this investigation have a number of applied and theoretical implications with respect to meso-scale modelling, orographic rainfall distribution and forecasting.With 12 Figures 相似文献
6.
Dr. P. Seibert 《Meteorology and Atmospheric Physics》1990,43(1-4):91-103
Summary After the ALPEX Special Observing Period in 1982, further field measurement campaigns have been carried out to study the south foehn in the Eastern Alps. In addition, foehn situations were investigated statistically on the basis of a 4-years-record of upper-air and synoptic data combined with data from a special regional network. In this paper, distribution of air masses around the Alps, flow patterns and the typical evolution of a foehn situation are illustrated by the case 8 November 1982. Several possible definitions of shallow foehn are given, yielding a share of approximately 10% of shallow foehn observations. Mainly statistical data are used to falsify the wide-spread version of the thermodynamical foehn theory attributing the warmth of the foehn to release of latent heat and postulating foehn air rising from the ground of the Po Valley. Instead of this, the foehn air originates from 2000–2500 m asl over the Po Valley, while the air near ground remains blocked. The potential temperature difference between southern and northern valleys during foehn is to be explained by the stable stratification in the south, not by the effects of precipitation. These findings are in agreement with Hann's original theory.With 10 Figures 相似文献
7.
Summary
The interaction of a cold front with the Alps is studied by means of real-case numerical simulations for a case occurring
at the end of the Alpine Experiment (ALPEX) on 28 April–2 May, 1982. Simulations are performed with the numerical weather
prediction model chain Europa-Modell (EM) and its one-way nested high-resolution model (HM) of the German and Swiss Weather
Services. The outer EM simulation (56 km horizontal resolution) uses initial and lateral boundary fields taken from the ALPEX-IIIb
reanalysis data set. This reanalysis data set is based on the operational EM analysis scheme, but takes into consideration
a wide range of field phase data taken during the ALPEX field campaign. A comparison of simulations driven by the ALPEX and
ECMWF reanalysis (ERA) data is performed. The simulation driven by the former captures the intensity and vertical depth of
the developing lee cyclone substantially better than the corresponding run driven by the ERA.
The transient development of the impingement of a cold front on the Alps induces a wide range of mesoscale phenomenon such
as flow splitting, mistral, north foehn, cyclogenesis, anticyclonic vortex and bora. These flow evolutions are analyzed using
HM simulations with a horizontal resolution of 14 km and visualized performing comprehensive trajectory calculations.
Received September 9, 2000/Revised November 28, 2000 相似文献
8.
A. P. Sturman 《Meteorology and Atmospheric Physics》1992,47(2-4):229-236
Summary The Southerly Change Experiment (SOUCHEX) was conducted to examine the influence of the New Zealand Southern Alps on the structure and evolution of cold fronts, locally called southerly changes, as they travel up the east coast. The extensive data obtained by the augmented surface weather station network is used to examine in detail the mesoscale wind field associated with the events observed during the experiment. A comparison of the wind fields observed during the different events illustrates the influence of local dynamic and thermal factors. In particular, lee trough-induced northeasterlies and thermally developed diurnal wind systems are seen to interact with the wind field created by the passage of the front over the Southern Alps.It is apparent that the wind field associated with southerly changes responds to a variety of factors as the cold fronts propagate northwards. For example, there is a tendency for the flow to turn onshore producing a southeast wind during daytime over the Canterbury Plains south of Banks Peninsula probably due to diabatic heating of the mountains and plains. This onshore flow is in direct opposition to pre-frontal foehn northwesterly flow which often continues in the mountain regions and aloft after the front has moved up the coast. The interaction of these air masses over Canterbury creates difficulties for local forecasting. Also, the nocturnal passage of a southerly change is often difficult to detect in surface anemograph traces because of the decoupling of the boundary layer air from that above, producing low level drainage flow over the Canterbury Plains. The overall effect is to create a complex mesoscale wind field resulting from interaction of cold fronts with regional orographic and thermal influences.With 8 Figures 相似文献
9.
G. Zängl 《Meteorology and Atmospheric Physics》2006,91(1-4):85-105
Summary This study presents high-resolution numerical simulations of north foehn in the Austrian Inn Valley which have been performed
with the Penn State/NCAR mesoscale model MM5. As the Inn Valley is located north of the Alpine crest, north foehn occurs comparatively
rarely in this valley, and there are only sparse observations available for this phenomenon. Simulations of the 24 January
1993 case as well as idealized simulations are performed to get a deeper insight into the dynamics of the north foehn. Moreover,
the synoptic conditions leading to the occurrence of north foehn in the Inn Valley are investigated.
The simulations indicate that there are at least four different paths for the foehn to penetrate into the valley. Two of them
are running along side valleys entering the upper Inn Valley from the west. These flow paths appear to be most important when
the large-scale flow has a significant westerly component. The other possible flow paths enter the Inn Valley from the northwest
or north and require a strong northerly component of the large-scale flow. From a dynamical point of view, north foehn appears
to be similar to the well researched south foehn in that vertically propagating gravity waves force the descent of the ambient
flow into the valleys. However, there are also indications that trapped lee waves have a significant impact on the surface
wind field, which has not been reported for south foehn so far. Moreover, the model results show that a precondition for the
formation of north foehn in the Inn Valley is the absence of significant orographic precipitation. Evaporative cooling induced
by precipitation falling into subsaturated air not only reduces the surface temperatures but also inhibits the formation of
large-amplitude gravity waves, suppressing the development of stormy surface winds. 相似文献
10.
Dr. D. Heimann 《Meteorology and Atmospheric Physics》1992,48(1-4):165-171
Summary The separation of potential temperature and equivalent-potential temperature patterns at cold fronts with pre-frontal foehn is explained by means of conceptual considerations and discussed with reference to observations and numerical simulations. It is shown that foehn increases the potential temperature north of the Alps but does not necessarily affect the equivalent-potential temperature. As a consequence potential temperature and equivalent-potential temperature may form different structures and generate zones of large gradients at different locations even if no diabatic heat conversions are present.With 5 Figures 相似文献
11.
Summary The German Front Experiment 1987 was an experimental effort to determine the influence of the European Alps on cold fronts using data collected by aircraft, radar, surface and upper-air systems between October and December 1987. Measurements were concentrated in the Alpine foreland south of Munich and in the Inn valley in Austria. It was a cooperative effort by groups from Austria, Germany and Switzerland. This study briefly recapitulates the conduct of the experiment and the intense observation periods which were carried through. Then it is outlined to what extent the scientific objectives have been achieved. The findings of the experiments are grouped into the following topics: orographically induced frontogenesis, the interaction of foehn and front, orographically trapped gravity currents, flow into valleys and the modification of precipitation by the Alps. 相似文献
12.
利用全国空气质量指数(Air Quality Index,AQI)、PM_(2.5)地面观测数据、全球数据同化系统GDAS数据和FNL再分析气象资料,研究了2015/2016年冬季南京北郊空气质量变化特征以及环境输送条件和污染物源区。结果表明:以AQI为代表的冬季江淮地区污染程度存在3种典型的污染物跨区域输送路径—西北路径、北方路径和西南路径。西北路径通常发生在蒙古高压较强,且处于平均位置时刻,南京北郊上空有冷平流,不利于污染物扩散;北方路径对应蒙古高压弱,东北附近为弱高压控制,偏北气流将污染物带至南京北郊,如跨海洋,则污染减弱;西南路径对应南京北郊为边界层内反气旋式环流中心,下沉气流十分不利于污染物扩散。影响南京北郊污染的潜在源区主要分布在河北南部、山东西部、河南南部、安徽东部和湖北西部。河北省是重要的污染源区,河北南部和山东西部污染物通过北方路径输送至南京北郊,因此北方路径虽发生污染概率少于其他两种,却是形成南京北郊严重污染的重要路径。河南南部污染物通过西北路径输送。安徽和湖北污染物通过西南路径输送。定量分析表明,平流输送是南京北郊重度污染的重要原因,近地层风速对AQI的平流输送占AQI变化的贡献率超过70%,甚至可达85%。 相似文献
13.
H. Hoinkes 《Meteorology and Atmospheric Physics》1949,2(1):82-96
Zusammenfassung Der zwischen dem 2. und 4. Oktober 1941 in den n?rdlichen Alpent?lern beobachtete Südf?hn ist aus den in der Bodenwetterkarte
ersichtlichen Luftdruckgradienten nicht zu verstehen. Er verdankt seine Entstehung einem kleinen abgeschlossenen H?hentief,
das auf die mittleren Schichten der Troposph?re beschr?nkt ist, in dessen Str?mungsbereich sich der Alpenkamm in den genannten
Tagen befindet. Die w?hrend dieser Zeit zwischen Süd- und Nordseite der Alpen entstehenden Druckunterschiede sind durch Temperaturunterschiede
v?llig zu verstehen, die auf die H?henschicht zwischen 900 und 3700 m beschr?nkt sind und die durch das unter Kondensation
erfolgende Aufsteigen der Luftmassen auf der Südseite der Alpen und das Absinken auf deren Nordseite entstanden sind. Die
obere Südstr?mung kann in diesem Fall erst dann als F?hn bis zum Boden der n?rdlichen Alpent?ler durchbrechen, wenn durch
die Einstrahlung die Luft l?ngs der Talh?nge auf die potentielle Temperatur der oberen Südstr?mung erw?rmt worden ist. Die
Bedeutung dieses Vorganges auch bei normal verlaufenden F?hnf?llen ist aus dem t?glichen und j?hrlichen Gang der F?hnh?ufigkeit
in Innsbruck zu erkennen.
Mit 8 Textabbildungen. 相似文献
Summary A case of foehn from the south observed in the northern valleys of the Alps betweeen October 2 and 4, 1941 cannot be accounted for by the pressure gradient taken from the surface weather map. It has its origin in a small, limited high-level depression confined to the medium layers of the troposphere in the range of which the Alpine ridge lays on the above mentioned days. The differences of pressure between the southern and the northern side of the Alps occurring during this period can be perfectly explained by the differences of temperature which were confined to the atmospheric layer between 900 m and 3700 m and caused by the effect of air masses ascending with condensation on the south side and descending on the north side of the Alps. In this case the upper south wind can make its way as foehn to the bottom of the northern Alpine valleys until the air along the valley slopes is heated by insolation to the potential temperature of the upper southerly current. Also in cases of regular foehn the importance of this process is evident if the daily and yearly variation of foehn frequency at Innsbruck is considered.
Résumé Le foehn du Sud observé entre le 2 et le 4 octobre 1941 dans les vallées du Nord des Alpes ne peut trouver son explication dans le gradient de pression au sol des cartes isobariques correspondantes. Ce foehn doit son origine à une petite dépression limitée aux couches moyennes de la troposphère et au niveau de laquelle le fa?te alpin se trouvait ces jours là. Il est possible d'expliquer entièrement les différences de pression existant alors entre les versants sud et nord des Alpes par les différences de température localisées dans la couche de 900 à 3700 m., et qui ont pour origine l'ascension (avec condensation) des masses d'air au Sud des Alpes et l'affaissement de ces masses au Nord. Le courant supérieur du Sud ne peut dans ce cas atteindre comme foehn les vallées du Nord des Alpes que lorsque l'air des versants s'est réchauffé par irradiation et a atteint de cette manière la température potentielle du courant supérieur méridional. On peut reconna?tre l'importance de ce processus, même dans les cas normaux de foehn, dans la variation diurne et annuelle de la fréquence du foehn à Innsbruck.
Mit 8 Textabbildungen. 相似文献
14.
Dipl.-Met. M. Kurz 《Meteorology and Atmospheric Physics》1990,43(1-4):61-68
Summary In the paper results of investigations of the first two front events of the German Front Experiment 1987/88 are presented. In both cases a frontogenesis took place in the lower troposphere north of the Alps—in the first case directly at the cold front, in the second case ahead of it. The main reason for that was the incorporation of warm and dry air produced by foehn or advected along the Alps into the frontal zone of the approaching front. In the second case the cooling through evaporation of falling rain was an additional important factor for the increase of the temperature gradient. The computation of kinematic parameters yields as an interesting result the proof of areas with absolute vorticity near or below zero within the southwesterly current at the northwestern flank of the Alps.With 6 Figures 相似文献
15.
使用NCEP FNL资料对2013年12月浙北北部两次重度霾过程进行分析,结果发现,两次过程均与冷空气影响有关,发生前存在大范围污染物的持续积累,都是本地排放积累和周边污染传输的综合影响结果。两次过程发生的气象背景不同： 4日过程因静稳天气影响所致,污染物主要来自本地和周边（江苏中东部）的传输积累;26日过程与冷空气影响关系密切,为冷空气前锋携带大量污染物快速南下影响所致,污染物主要来源于黄淮平原远距离输入。边界层逆温有利于增强并导致重度霾发生。风廓线资料显示,边界层偏北风影响至地面时,重度霾发生。1500 m高度以下风力持续低于4级,重度霾和雾发展并持续;边界层风力增大至6级,重度霾消散。静稳天气下,污染物浓度变化与本地活动关系密切,过程中PM颗粒物共出现3个浓度高峰,而冷空气影响下,仅出现单个浓度高峰。 相似文献
16.
选取北疆50个测站1971-2015年的逐日平均气温和最低气温资料,运用小波分析、M-K突变检验等统计方法分析了北疆地区不同等级冷空气活动过程的时空变化特征。分析表明：近45年北疆地区冷空气活动持续日数在1～15d,平均持续日数为1.79 d。其中弱冷空气过程持续日数以1～2 d为主;中等强度冷空气、较强冷空气和寒潮过程以1～3 d为主;而强冷空气过程以2～4 d为主。近45年北疆单站冷空气发生频次为95.3次/a,且具有盆地多、山地少的特点;其中中等强度冷空气为11.8次/a,自东向西逐渐减少;弱冷空气、较强冷空气发生频次为70.9次/a、1.8次/a,呈自西南向东北逐渐减少的趋势;强冷空气和寒潮发生频次为4.2次/a 、7.3次/a,呈现由西南向东北增加趋势。近45年北疆地区冷空气活动频次总体呈减少趋势,年代际表现为先减少后增加的特点, 70年代最多,90年代最少,减少速率为0.33/10a;弱冷空气和较强冷空气呈增加趋势,增加速率为0.17/10a和0.03/10a;中等强度冷空气、强冷空气和寒潮活动频次呈下降趋势,其中寒潮的下降速率达-0.37/10a;不同等级冷空气活动频次均存在一个15-20a的显著周期,突变点出现在1980、2003年左右。 相似文献
17.
利用全天空成像仪、地面、高空气象观测资料、数值模式资料和NCEP/NCAR再分析资料,对2010年4月24日甘肃河西走廊的特强沙尘暴过程进行了分析。结果表明,从观测到沙尘至天空完全被沙尘遮蔽,仅仅只有2.5min;中西伯利亚—新疆北部的偏北大风携带着极地强冷空气入侵,造成了甘肃河西走廊的特强沙尘暴。进一步的分析还表明,沙尘区域上空存在一个西风急流中心,其高度在200-250hPa之间,沙尘暴爆发时,风速增大到40m.s-1,高度降低,范围扩大。由急流中心向地面伸展的最大风速带将高空动量向下传播,引发了河西走廊的沙尘暴;南北风的变化主要发生在250hPa以下,最大中心高度均位于400hPa附近,北风前锋到达之处,沙尘暴爆发;冷空气爆发时,首先造成地面气温的急剧下降,其次是700hPa和500hPa气温下降;地面热低压的强烈发展,一方面使气压梯度加大,另一方面导致边界层对流不稳定,二者的作用都增强了沙尘暴的强度。 相似文献
18.
利用常规气象观测资料、以及国家气候中心的海温监测资料等,对2008年初呼市地区出现低温降雪天气的强度、范围、持续时间和灾害影响作了初步分析,并与历史同期进行了比较。分析表明：此次低温降雪天气是在近年来最严重的一次“拉尼娜”事件背景下发生的,与欧亚地区持续大气环流异常密切相关,在“北脊南槽”和西太平洋副高偏北偏强的形势下,北方冷空气不断南下,同时印度洋和盂加拉湾暖湿气流源源不断地往北到东北方向输送,冷暖气流在我国中东部地区频繁交汇,造成了我市持续低温、降雪异常偏多。 相似文献
19.
本文利用气象台站资料和NCEP再分析资料对2017年12月18日～2018年1月2日成都出现的一次持续性污染天气过程的气象条件进行了分析。结果表明,此次污染过程是一次中到重度污染,局部时段出现了严重污染。500hPa环流形势及其调整与此次污染状况的发展有着密切的联系,其中500hPa高压脊前西北气流下沉区所产生的下沉逆温层和等温层是导致此次污染过程的主要因素。随着环流调整,低槽东移,受槽前西南气流影响,污染状况得到了改善,并最终结束了此次污染过程。此次污染天气过程中,贴地逆温及静风的稳定大气抑制了污染物的扩散,但对污染天气的生成及演变并未起主导作用。此外,有四次冷空气进入盆地的过程出现。冷空气主要来自四川盆地北部和东部,使污染状况得到缓解,但并未对污染过程的结束起到决定性的作用。 相似文献
20.
青藏高原下垫面对中国夏季环流影响的研究 总被引：14，自引：4，他引：14
发展了一个CCM3－RegCM2单向嵌套模式，用以研究青藏高原中西部地区下垫面特征对我国夏季环流和降水的影响。结果表明：若青藏高原中西部植被破坏，变为沙漠，则该地区地面返照率增加，热容量减少，气温升高。从而导致高原北侧的温度梯度增大，西风槽则被削弱；西风急流被推至更西更北的地区，使得北方冷空气难以到达我国长江、黄河流域；由于高原上空气温增高，导致该地区上空的反气旋环流增强，使原来位于槽前西南气流的长江中下游地区处在平直西风气流当中，不利于降水的产生。与此同时，副热带高压西伸北抬，使得长江、黄河中下游地区处在副热同压控制之下，造成我国大部分地区夏季降水减少。 相似文献