首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
Flow splitting in the Rhine valley has been observed with a transportable wind lidar (TWL) during a shallow-foehn event in the framework of the Mesoscale Alpine Programme (MAP). The Doppler lidar recorded in detail flow splitting, foehn wind gusts, and flow reversal. Such structures have not previously been observed with comparable detail by conventional in-situinstruments. In addition to the TWL, boundary-layer processes have been documented by means of rawinsondes and surface stations. This paper presents an analysis of the processes giving birth to flow splitting between the Seez and Rhine valleys during Intensive Observation Period (IOP) 5 (1–3 October 1999) by combining the collected data with hydraulic theory. The study shows thatthe splitting of the channelled flow is associated with (1) the existence of a stagnation point at the intersection of the Seez and Rhine valleys, and (2) the deflection of the flow by the lateral sidewalls of the valleys.  相似文献   

2.
Summary This paper investigates the characteristics of channelled airflow in the vicinity of a junction of three idealized valleys (one valley carrying the incoming flow and two tributaries carrying the outflow), using a two-dimensional single-layer shallow water model. Particular attention is given to the flow splitting occurring at the junction. Nondimensionalized, the model depends on the valley geometry, the Reynolds number, which is related to the eddy viscosity, and on the difference of the hydrostatic pressure imposed at the exit of the tributaries. At the spatial scale considered in this study, the Rossby number relating the inertial and Coriolis forces is always larger than 1, implying that the effect of earth rotation can be neglected to a first approximation. The analysis of the flow structure within the three valleys as well as the calculation of the split ratio (fraction of the air flow diverted into one of the two downstream valleys with respect to the total mass flux in the upstream valley) show that (i) the flow pattern depends strongly on the Reynolds number while the split ratio is comparatively insensitive; (ii) the valley geometry and the difference between the upstream and downstream hydrostatic pressures affect the flow pattern, the location of the split point and the split ratio; (iii) the relative contribution of flow deflection by the sidewalls and the blocking/splitting mechanism differs between the settings of a “Y-shape” valley and a “T-shape” valley. Quantitative comparison of the present results with numerical simulations of realistic cases and with observations collected in the region of the Rhine and Seez valleys (Switzerland) (“Y-shape” valley) and in the region of the Inn and Wipp valleys (Austria) (“T-shape” valley) during the Mesoscale Alpine Programme (MAP) field experiment shows good agreement provided that the normalized valley depth NΔH/Uu significantly exceeds 1, i.e., when “flow around” is expected. A structural disagreement between the idealized simulations and the observed wind field is found only when NΔH/Uu ≃ 1, that is, in the “flow over” regime. This shows that the dimensionless valley depth is indeed a good indicator for flow splitting, implying that the stratification is a key player in reality.  相似文献   

3.
Summary The wind regime of the Lake Tekapo Basin is examined with reference to the interaction of multi-scale local, regional and synoptic circulations. Analysis of the historical wind direction record from Mt John identifies airflow from three principal directions to most frequently affect the study area. Both seasonal and diurnal trends in the frequency of each directional category are identified, which reflect the influence of local thermal forcings and seasonal changes in synoptic circulation on the Lake Tekapo windfield. Meteorological observations from a network of automatic weather stations and anemographs within the study area identified Lake Tekapo to generate its own circulation system, a lake/land breeze. This combines with the larger scale valley wind, which during ideal conditions continues after sunset in the upper reaches of the lake catchment. During light to moderate foehn northwesterly conditions, the combined lake breeze/valley wind system remains decoupled from the prevailing synoptic airstream. Towards evening when local thermal circulations weakened, a channelised foehn airstream often becomes dominant over the entire field area. Observations made during this investigation have a number of applied implications with respect to air pollution dispersion modelling and forecasting within alpine lake basins.With 7 Figures  相似文献   

4.
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   

5.
Summary Trans-scale relationships are established between fluctuations in the direction of the geostrophic flow over the British Isles and spatial variations in rainfall over Devon and Cornwall, South West England. The rationale for using such an approach is to provide the basis for assessing changes to the region’s rainfall climatology that may result from possible future enhanced greenhouse effect forced alterations to large-scale wind flow patterns. A new method, the concentration factor (CF), that relates rainfall totals to the frequency of the flow, is applied to investigate spatial variations in rainfall totals at twelve stations in the two counties under eight wind direction groups (N, NE, E, SE, S, SW, W, NW). S and SW flow types are found to produce the highest daily rainfall totals at all locations, with the three easterly groups (NE, E, SE) yielding higher daily precipitation intensities than the maritime NW group. Inter-annual and seasonal variations in daily wind direction – rainfall (WD/R) relationships are then assessed at two contrasting sites in Cornwall (St. Mawgan, Culdrose) using 40 years of data (1957–96). In general, there is a trend over this period toward maritime airflows (S, SW, W, NW) producing higher daily rainfall totals, with the continental groups (N, NE, E, SE) yielding lower totals relative to their frequency of occurrence. The trend toward maritime airflows producing higher rainfall totals is in line with recent trends in the North Atlantic Oscillation (NAO). Notable seasonal variations in WD/R relationships over Cornwall are interpreted in terms of the location’s exposure to the prevailing wind, sea temperature variations and the orography of the South West Peninsula. Received October 23, 2000/Revised February 2, 2001  相似文献   

6.
Sea-breeze dynamics in the Marseille area, in the south of France, is investigated in the framework of the ESCOMPTE experiment conducted during summer 2001 in order to evaluate the role of thermal circulations on pollutant transport and ventilation. Under particular attention in this paper is the sea-breeze channelling by the broad Rhône valley and the narrow Durance valley, both oriented nearly-north–south, i.e., perpendicular to the coastline, and its possible impact on the sea-breeze penetration, intensity and depth, which are key information for air pollution issues. One situation of slight synoptic pressure gradient leading to a northerly flow in the Rhône valley (25 June 2001) and one situation of a weak onshore prevailing synoptic wind (26 June 2001) are compared. The impact of the Rhône and Durance valleys on the sea-breeze dynamics on these two typical days is generalized to the whole ESCOMPTE observing period.The present study shows by combining simple scaling analysis with wind data from meteorological surface stations and Doppler lidars that (i) the Durance valley always affects the sea breeze by accelerating the flow. A consequence is that the Durance valley contributes to weaken the temperature gradient along the valley and thus the sea-breeze circulation. In some cases, the acceleration of the channelled flow in the Durance valley suppresses the sea-breeze flow by temperature gradient inhibition; (ii) the Rhône valley does not generally affect the sea breeze significantly. However, if the sea breeze is combined with an onshore flow, it leads to further penetration inland and intensification of the low-level southerly flow. In this situation, lateral constriction may accelerate the sea breeze. Simple scaling analysis suggests that Saint Paul (44.35°N, about 100 km from the coastline) is the lower limit where sea breeze can be affected by the Rhône valley. These conclusions have implications in air quality topics as channelled sea breeze may advect far inland pollutants which may be incorporated into long-range transport, particularly in the Durance valley.  相似文献   

7.
Summary ?Numerical simulations of the south foehn in the region of Innsbruck are presented. They are semi-idealized in the sense that realistic orography but idealized initial and boundary conditions are used. The focus of this study is on typical features of the fully developed foehn, the breakthrough phase of the foehn and the diurnal cycle of the foehn. In addition, the impact of the large-scale wind direction is examined, including conditions leading to shallow foehn. The simulated flow fields have been found to be in very good agreement with observations except for a few minor details. In the lower part of the Sill Valley (the valley going from the Brenner pass down to Innsbruck), the wind speed is significantly higher than in the upper part. The acceleration can be traced back to the three-dimensional propagation of gravity waves excited over the adjacent mountain ridges. The amplitude of the gravity waves over the various mountain ridges depends sensitively on the wind direction, large wave amplitudes occurring only when the angle between the wind direction and the ridge line is not too small. For southwesterly or south–southwesterly large-scale flow, wave amplitudes are significantly larger to the east of Innsbruck than to the west. Foehn breakthrough at Innsbruck is usually preceded by a moderate westerly (downvalley) wind that is restricted to a rather small area around Innsbruck. The simulations reveal that this so-called pre-foehn is mainly a consequence of the gravity wave asymmetry, producing an asymmetric pressure perturbation with lower pressure to the east of Innsbruck. Shallow foehn, defined as a foehn occurring when the large-scale flow at crest height (700 hPa) is approximately westerly, is associated with relatively weak wave activity along the Sill Valley. It is found that at least a weak southerly wind component below crest height is necessary to maintain a significant shallow foehn over a longer time. Received October 10, 2001; accepted June 20, 2002 Published online: February 20, 2003  相似文献   

8.
Summary The local wind system in the upper Isar Valley (Bavarian Alps) near Mittenwald has the peculiarity that regularly strong foehn-like nocturnal flows occur, mainly during clear nights in autumn and winter. We will refer to this phenomenon as “Minifoehn”, as its properties are similar to the classical deep foehn in the sense that its breakthrough into the Isar Valley usually brings a striking increase in temperature and a concomitant decrease in relative humidity. Numerical simulations with the MM5 model reveal that this phenomenon is related to a nocturnal drainage flow originating from a plateau south of Mittenwald. This flow is driven by the temperature difference between this plateau (1180 m) and the free atmosphere above Mittenwald (920 m, 15 km north of the plateau) at the same level. The air masses flow through two different valleys that merge again further downstream. The upper part of one of the two drainage currents goes over a small mountain ridge (1180 m) south-west of Mittenwald and then descends into the Isar Valley, leading to an advection of potentially warm air towards Mittenwald. This branch of the drainage current constitutes the Minifoehn. The remaining part of the drainage current flows through a narrow gap towards the Isar Valley and then joins the drainage flow of this valley. As these air masses are significantly cooler than the Minifoehn branch, large horizontal temperature gradients can be found around Mittenwald. The dynamical behaviour of the cold air flow turns out to be qualitatively consistent with shallow-water theory only in the absence of a forcing by large-scale winds. Otherwise, gravity-wave induced pressure perturbations interact with the drainage flow and modify the low-level flow field. The simulations show that the gravity waves are excited by the mountain range that separates the two valleys mentioned above. Moreover, the simulations indicate that the structure of this nocturnal wind system is not very sensitive to the direction of synoptic-scale winds as long as they come from the southern sector. On the other hand, ambient northerly winds are able to prevent the drainage flow and therefore the local foehn effects in the Isar Valley provided that synoptic winds are strong enough. The results of the MM5 simulations are in good agreement with the measurements and observations described in part 1 of this study.  相似文献   

9.
Summary The present paper is the continuation of two recent studies investigating the foehn-like valley wind system around Mittenwald (Bavarian Isar Valley). We deal with the synoptic/mesoscale conditions causing the local foehn (“Minifoehn”), considering field campaigns from both the mesoscale and the climatological point of view. Furthermore, we describe the structure and further features of the local foehn at smaller scales, using both the results of the VERTIKATOR field campaign and numerical simulations. We obtain as a new result that the foehn-caused local warm air pool around Mittenwald induces slight nocturnal upvalley winds between an adjacent valley basin located some 8 km north of Mittenwald and the basin of Mittenwald. Furthermore, a weak northerly flow may also occur at Mittenwald prior to the onset of the Minifoehn. Numerical simulations indicate that the local pressure gradient responsible for this phenomenon is related to a gravity wave forming over the hill range southwest of Mittenwald. Observations within a five-year period indicate that Minifoehn frequently occurs when ambient winds coming from the southern sector are predominant, but, contrary to deep foehn, weather conditions with northerly synoptic-scale flows do not necessarily exclude the development of the local foehn which comes from the southwest. We also present further evidence that in the presence of southerly synoptic-scale winds, orographic gravity waves interact with the drainage flow. Another new result is that strong synoptic-scale westerly winds are able to suppress the occurrence of Minifoehn. In addition, the possible influence of the Inn Valley wind system as well as dynamical differences between the thermally driven up- and downvalley winds are briefly discussed.  相似文献   

10.
Summary An unusually strong nocturnal downvalley wind can be regularly observed in the upper Isar Valley close to Mittenwald (Bavarian Alps) when a high-pressure system is located over Central Europe or when ambient southerly winds are present. Due to the structure of the local topography, this downvalley wind has foehn-like properties in the sense that the breakthrough of the flow into the valley is characterized by a strong increase in temperature and a decrease in relative humidity. Therefore the author called this flow Minifoehn. In fact, wind speeds are low in comparison to deep foehn, but gusts may reach values up to 20ms–1, even under the influence of high pressure systems with weak atmospheric pressure gradients. To investigate the Minifoehn, surface stations have been installed for collecting temperature, humidity, wind and pressure data. Measurements have shown that the Minifoehn represents the upper part of one of the drainage currents which flows over a mountain ridge into the valley at Mittenwald. Nocturnally cooled air drains from a plateau south of Mittenwald through different valleys which merge again near Mittenwald. It seems that the forcing of the nocturnal currents is dominated by the temperature difference between this plateau and the free atmosphere above Mittenwald at the same level. Strong temperature differences are found during clear winter nights and in case of subsidence inversions. Moreover, the appearance of the Minifoehn in autumn and winter is so frequent that we even may find a climatic effect: the upper Isar Valley is usually free of fog during these seasons and nocturnal temperatures are often considerably higher than in other Bavarian Alpine valleys at comparable altitude.  相似文献   

11.
Summary Local scale windfield and air mass characteristics during the onset of two foehn wind events in an alpine hydro-catchment are presented. Grounding of the topographically modified foehn was found to be dependent on daytime surface heating and topographic channelling of flow. The foehn front was observed to advance down-valley until the valley widened significantly. The foehn wind appeared to decouple from the surface downstream of the accelerated flow associated with the valley constriction, and to be lifted above local thermally generated circulations including a lake breeze. Towards evening, the foehn front retreated up valley in response to reduced surface heating and the intrusion into the study area of a deep and cool air mass associated with a regional scale mountain-plain circulation. Differences in the local windfield observed during both case study events reflect the importance of different thermal and dynamic forcings on airflow in complex terrain. These are the result of variation in surface energy exchanges, channelling and blocking of airflow. Observations presented here have both theoretical and applied implications with regard to forecasting foehn onset, wind hazard management, recreational activities and air quality management in alpine settings. Received January 23, 2001 Revised October 17, 2001  相似文献   

12.
The so called Upper Río Negro Valley in Argentina is one of the most important fruit and vegetable production regions of the country. It comprises the lower valleys of the Limay and Neuquén rivers and the upper Negro river valley. Out of the 41,671 cultivated hectares, 84.6% are cultivated with fruit trees, especially apple, pear and stone fruit trees. Late frosts occurring when trees are sensitive to low temperatures have a significant impact on the regional production. This study presents an analysis of air flow characteristics in the Upper Río Negro Valley and its relationship with ambient air flow. To such effect, observations made when synoptic-scale weather patterns were favorable for radiative frosts (light wind and clear sky) or nocturnal temperature inversion in the lower layer were used. In the Negro river valley, both wind channeling and downward horizontal momentum transport from ambient wind were observed; in nighttime, very light wind events occurred, possibly associated with drainage winds from the nearby higher levels of the barda. In the Neuquén river valley, the prevailing effect appeared to be forced channeling, consistent with the results obtained in valleys where the synoptic scale wind crossed the axis of the valley. In the Limay river valley, the flow was observed to blow parallel to the longitudinal valley axis, possibly influenced by pressure gradient and forced channeling.  相似文献   

13.
A two week observation program was carried out in the summer of 1981 in the Lake Kinneret (Northern Israel) area. The main purpose was to study the mesoscale flow patterns in and around the lake valley and compare them with the results of mesoscale model simulations in the same area. The main effort of the program was directed to the determination of three dimensional trajectories from various points around the valley. For that purpose a new method for the deployment of relatively long term no-lift balloons was used. In addition, surface observations as well as upper air wind observations using pibals were taken at four fixed locations along a straight line across the lake valley. Based on previous studies using surface observations and model results it appeared that the flow regime was determined by the combination of three main mechanisms: the Mediterranean sea breeze, the lake breeze and the mountain-valley wind. This combination results in a daily cycle divided into three distinct flow regimes. The results of the present experiment confirm this basic classification as well as the general structure of the flow for each of the three regimes. The experiment also confirmed the assumption that the large scale synoptic flow has only a minor influence in the valley, and contributes only to the general direction of the winds. In spite of the overall agreement several deficiencies of the model simulations came to light as a result of the experiment. These have to do with the horizontal and vertical resolutions employed in the models, with the fact that all of them were two dimensional (even though several of the models are capable of three dimensional simulations), and with the fact that most of them use the hydrostatic approximation. Due to the lack of appropriate equipment no vertical soundings were performed in order to determine the thermal and humidity stratification. These will have to be completed in subsequent experiments in order to provide the missing data.  相似文献   

14.
Summary Flow in long and deep main valleys with tributaries is studied for constant surface heating switched on att=0. The valley flows are obtained from a numerical model which combines slope wind layer equations with equations for the valley flow off the slopes. Much simpler linear models are used for the intepretation of the model results. If there are no sidevalleys an up-valley wind regime evolves in the main valley after the switch-on of the heating which protrudes towards the head. It is shown that the topographic amplification factor which captures the geometry of the valley and stratification are important factors in determining the intensity of the along-valley flow. However the up-valley winds are also quite sensitive to the specification of the boundary conditions at the upper end of the slope wind layers. If sidevalleys are added strong inflow to these tributaries is found only if their topographic amplification factors are larger than that of the main valley. This flow into the tributaries is mainly balanced by downward motion on top of the main valley but flow entering through the mounth of the main valley can contribute as well. Tributaries can induce flow in the main valley long before the main valley's own up-valley wind regime has reached the location of the tributary.With 10 Figures  相似文献   

15.
In order to characterise the local low-level circulation in the Tyrrhenian Sea coastal area near Rome, the wind field observed by conventional anemometers, Doppler sodar, and rawinsonde has been analysed. The prevailing diurnal behaviour of wind speed and direction as a function of season was highlighted, and the existence of two different patterns of the local circulation, mainly due to land and sea breezes and to the drainage flow from the mouth of the Tiber valley, revealed. The comparison between the low-level circulation and synoptic flow allowed us to determine the influence of the large-scale flow on nocturnal currents that are observed at the Pratica di Mare site and the way that wind direction evolves during the day. Numerical simulations are consistent with experimental data and depict the main features of the low-level wind field in the area.  相似文献   

16.
太行山东麓焚风天气的统计特征和机理分析Ⅰ:统计特征   总被引：2，自引：0，他引：2  
利用2007-2008年京津冀区自动站资料,根据小时变温,对太行山东麓焚风天气进行了详细的统计和分析。结果表明,焚风具有明显的季节变化和日变化,其分布特征在夜间表现明显,强焚风一般出现在太行山东侧50km内,而弱焚风则可到达太行山以东100km范围内;太行山北段的东南侧和南段的东侧呈现出两个焚风中心,而中段附近焚风出现的相对较少;焚风出现的位置与风向的关系密切,西北风造成的焚风主要出现在太行山北段东侧,偏西风主要影响太行山南段,而西南风主要影响中段;焚风强度与风速大小有一定的对应关系,具有明显的阵性特征;在红外云图中焚风常表现为一条明显的暗带;太行山东麓焚风造成的加热区可向东或东南方向移动,影响河北平原。  相似文献   

17.
Summary This study presents a flow climatology for Greece based on a survey of climate and synoptic weather types, and a five-year trajectory climatology. Isobaric back trajectories at the 850 and 700 hPa levels were calculated daily at 0 and 12 Z for the years 1983–1987 for the Aliartos, Greece EMEP station, using the Air Resources Laboratory (ARL) trajectory model. Trajectories were classified by direction and speed to examine monthly, seasonal and annual variability in flow characteristics. The main features of the flow are similar from year to year. Dominant transport sectors are NW and W, followed by N, SW and NE. Four-day back trajectories from NW and W usually originate from beyond 2,400 km, while those from other directions tend to originate closer to Greece. Cases of most rapid transport occur in January and December. Potential pollutant source regions are identified corresponding to preferred trajectory directions and distance ranges.With 6 Figures  相似文献   

18.
Zusammenfassung Ein enges Netz von mit Thermohydgrographen ausgerüsteten Beobachtungsstationen im Nordabhang der Hohen Tauern im Rauriser Tal und Gasteiner Tal ergab die Möglichkeit, im Vergleich mit den Beobachtungen auf dem Sonnblick-Obervatorium (3106 m) und auf der nordwärts vorgelagerten Schmittenhöhe (1964 m) die Entwicklung des Föhns vom Alpenkamm bis in die unteren Talböden hin statistisch zu untersuchen. Diei Statistik bezieht sich auf die Häufigkeit der Föhntage und der Föhnstunden, die Föhndauer pro Föhntag, den Jahresgang und den Tagesgang der Föhnhäufigkeit, die Zahl der Tage mit zyklonalem und antizyklonalem (freiem) Föhn, die Häufigkeiten der Föhndauer nach Stunden, die Häufigkeiten von Beginn und Ende des Föhns zu den verschiedenen Zeiten des Tages, die Abhängigkeit der Föhnfälle vom Höhenwind und die Häufigkeiten der Windrichtungen und Windstärken bei Föhn im Tal. Die Unterschiede im Föhnvorkommen in den verschiedenen Höhenlagen werden eingehend diskutiert und erklärt. Besonders unterhalb von Talstufen finden sich tagsüber auch Föhnfällen mit talaufwärts gerichteter Bodenwindströmung, die als maskierter Föhn bezeichnet werden können und offenbar durch eine durch die Talstufen und durch die Talwindentwicklung begünstigte Ausbildung einer Zirkulation um eine quer zum Tal gelegene horizontale Achse erklärt werden können.
Summary A close network of observation stations outfitted with thermohygrographs on the northern side of the Hohe Tauern in the Rauris and the Gastein valley offered the possibility of investigationg statistically the development of foehn from the ridge of the Alps down to the bottom of the valleys by comparing observations with those of Sonnblick Observatory (3106 m) and the Schnittenhöhe station (1964 m) lying to the north of the Sonnblick. The statistical evaluation refers to frequency of foehn days and foehn hours, duration of foehn on a foehn day, annual and daily march of foehn frequency, number of days with cyclonic and anticyclonic (free) foehn, frequencies of foehn duration by hours, frequencies of beginning and ending of foehn at different times of the day, dependence of foehn on the uppe wind, frequencies of wind direction and wind velocity for foehn in the valleys. Differences in the occurrence of foehn at different levels are thoroughly discussed and explained. During day hours cases of foehn were also found, particularly at the lower end of valley steps with the surface wind blowing valleyupwards, which can be characterized as masked foehn and may be due to the formation of a circulation around a horizontal axis across the valley favoured by the valley steps and the development of the valley breeze.
Résumé Un réseau dense de stations équipées de thermohygrographes sur le versant Nord des Hohe Tauern, dans les vallées de Rauris et de Gastein, ainsi que les stations de l'Observatoire du Sonnblick (3106 m) et de Schmittenhöhe (1964 m), ont permis d'étudier par voie statistique le développment du foehn depuis de crête des Alpes jusqu'au fond des vallées. Cette étude porte sur la fréquence des jours et heures de foehn, la durée du vent par jour de foehn, les variations annuelle et diurne de la fréquence, le nombre de jours avec foehn cyclonique et anticyclonique, les fréquences de durée par heures, les fréquences du début et de la fin aux différents moments du jour, la dépendance du foehn à l'égard du vent en altitude et les fréquences d'azimuts et de force du vent par foehn dans les vallées. On examine les cas de foehn à différents niveaux et les différences de son comportement. Au-dessous de certains gradins de vallées apparaissent parfois par foehn des courants remontants que l'on peut désigner par foehn masqué et qui s'expliquent par une circulation autour d'un axe horizontal transverse due au gradin liu-même et à la brise d'aval.

Mit 4 Textabbildungen.

Vorgetragen auf der dritten internationalen Tagung für alpine Meteorologie in Davos, 12. bis 14. April 1954.  相似文献   

19.
李唐棣  谈哲敏 《气象学报》2012,70(3):536-548
在条件不稳定大气条件下,二维小尺度双脊钟形地形上空对流触发、传播和降水分布特征主要决定于地形上游基流强度、双脊地形配置形式、地形高度及其山谷宽度。双脊地形在沿基流方向上有两种配置:高脊地形位于上游和低脊地形位于上游。对于高脊地形位于上游的双脊地形,上游高地形将起主导作用,山地上空对流及降水特征与单脊地形类似。对于低脊地形位于上游的双脊地形,上游低地形可明显地改变下游高地形的前方来流,同时,下游高地形也能够对上游低地形背风侧流动产生影响,从而导致出现地形上空复杂的对流传播、降水分布特征。对于低脊地形位于上游的双脊地形,其山谷宽度主要决定了双脊地形与单脊地形之间在对流、降水分布等的差异;当山谷宽度较小时,双脊地形可以近似为一个包络地形,此时地形上空的对流、降水特征与单脊地形类似;当山谷宽度较大时,双脊上空流动相互影响较小,此时双脊地形可以分成两个单脊地形;当山谷宽度在一定范围内,其上空的对流及其降水分布与单脊地形有明显差异。对于低脊地形位于上游、中等山谷宽度的双脊地形上空降水主要呈现4种类型:(1)山谷与低脊迎风坡降水;(2)高脊迎风坡降水;(3)低脊山峰与高脊迎风坡降水;(4)低脊背风侧、双脊山峰准静止降水。  相似文献   

20.
Regime-dependent evaluation is a relatively new approach to assess model performance. It consists of classifying the model biases according to a discrete number of regimes and evaluating model output within each regime. In this paper, the regimes are firstly defined by the large-scale atmospheric circulation, based on the objective Jenkinson-Collison classification technique which distinguishes synoptic patterns by strength, direction and vorticity of the geostrophic flow. Eight directional and two vorticity circulation regimes (circulation types) are specified. In this way, it is possible to quantify the model performance for cases with for example westerly winds only, or with cyclonic circulation only. A second regime classification is based on temperature, which allows for detection of temperature-dependent model performance. Modelled accumulated precipitation (mm/6?h) is evaluated with rain gauges for the years 2007 and 2008. Two variants of the COSMO model are evaluated: a fine-resolution version (2.8?km, COSMO-DE) and a coarse-resolution version (7?km, COSMO-EU). In COSMO-EU, a windward/leeward effect becomes visible since circulation is related to dominant wind direction, hence to windward and lee side of orography. In COSMO-DE, no circulation dependent but a height-related bias is identified and further explored, making use of temperature-dependent evaluation which unveils a positive model bias related to solid precipitation.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号