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1.
The impact of weak synoptic forcing on the valley-wind circulation in the Alpine Inn Valley 总被引：1，自引：0，他引：1
Günther Zängl 《Meteorology and Atmospheric Physics》2009,105(1-2):37-53
This paper investigates the impact of weak synoptic-scale forcing on the thermally induced valley-wind circulation in the Alpine Inn Valley and one of its largest tributaries, the Wipp Valley. To this end, high-resolution numerical simulations with realistic topography but idealized large-scale atmospheric conditions are performed. The large-scale flow has a speed increasing linearly from 5 m s?1 at sea level to 12.5 m s?1 at tropopause level, but its direction is varied between each experiment. For reference, an experiment without large-scale winds is conducted as well. The results indicate that the sensitivity to ambient flow forcing differs substantially between the Inn Valley and the Wipp Valley. The valley-wind circulation of the Inn Valley is found to be fairly robust against weak ambient forcing, changing by a much smaller amount than the along-valley component of the imposed large-scale flow. The valley wind tends to be intensified (weakened) when the ambient flow is aligned with (opposite to) the local valley orientation. However, the flow response is complicated by larger-scale interactions of the ambient flow with the Alpine massif. Most notably, northerly and northwesterly flow is deflected around the Alps, leading to the formation of a low-level jet along the northern edge of the Alps which in turn affects the valley-wind circulation in the lower Inn Valley. For the Wipp Valley, which is oriented approximately normal to the Alpine crest line and constitutes a deep gap in the Alpine crest, two distinctly different flow regimes are found depending on whether the large-scale flow has a significant southerly component or not. In the absence of a southerly flow component, the valley-wind circulation is similarly robust against ambient forcing as in the Inn Valley, with a fairly weak response of the local wind speeds. However, southerly ambient flow tends to force continuous downvalley (southerly) wind in the Wipp Valley. The flow dynamics can then be described as a pressure-driven gap flow during the day and as a mixture between katabatic flow and gap flow during the night. The responsible pressure forcing arises from the larger-scale interaction of the ambient flow with the Alpine massif, with southerly flow causing lifting on the southern side of the Alps and subsidence in the north. 相似文献
2.
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
3.
E. Mursch-Radlgruber 《Theoretical and Applied Climatology》1995,52(1-2):3-17
Summary Features of the mean flow structure in a small valley system in the Rosalian mountain range are discussed using data from a wind measurement network. Tethered balloon measurements during periods of clear sky form the basic dataset for the analysis of drainage winds and temperature inversions. During periods of weak ambient winds the existence of a pure thermally driven nocturnal valley wind system is shown. With strong ambient winds opposing the drainage flow, a reduced drainage height but the same jet maximum as with weak ambient winds is found. On the other hand with aiding flow the drainage winds are suppressed and flow reversal can occur. This strong valley flow interaction with the ambient wind indicates considerable dynamic influence on the evolution of drainage winds and on the breakup of temperature inversion structure for small valleys.With 15 Figures 相似文献
4.
J. C. Doran 《Boundary-Layer Meteorology》1991,55(1-2):177-189
The interaction of katabatic winds with ambient winds has been investigated for an idealized valley using Clark's nonhydrostatic model. Ambient ridgetop wind speeds ranged from 0.5 to 6 m/s, and made angles with the valley axis ranging from 0 ° to 90 °: cooling of the valley was based on measured values of sensible heat fluxes taken from observations in Colorado's Brush Creek Valley. The depth and strength of the down-valley winds decreased with increasing ambient wind speeds but showed relatively little sensitivity to wind directions in the range of 10 ° to 60 ° from the valley axis. An observed inverse linear decrease of drainage depth with wind speed in a 100 m thick layer above the ridgetops was also found in the simulations for parts of the valley but not near the valley mouth. Vertical motions over the valley showed marked patchiness, and implications of this structure on valley flow dynamics are discussed.This work was supported by the U.S. Department of Energy (DOE) under Contract DE-AC06-76RLO 1830. 相似文献
5.
Matthias Hornsteiner 《Meteorology and Atmospheric Physics》2005,88(3-4):175-192
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
6.
Philippe Philippe Alain M. Dabas Christian Haeberli Pierre H. Flamant 《Boundary-Layer Meteorology》2003,106(3):483-505
The flow structure at the intersection between the Rhine and the Seez valleys nearthe Swiss city of Bad Ragaz has been documented by means of wind and pressuremeasurements collected from 9 September to 10 November 1999 during the MesoscaleAlpine Programme (MAP) experiment. To understand better the dynamics of theageostrophic winds that develop in this part of the Rhine valley, some key questionsare answered in this paper including the following: (i) How does air blow at theintersection of the Rhine and Seez valleys? and (ii) what are the dynamical processes(mechanical or thermal) driving the flow circulations in the valleys? Statistical analysis of the wind and pressure patterns at synoptic scale and at the scaleof the valley shows that five main flow patterns, SE/S, NW/W, NW/N, NW/S, SE/N(wind direction in the Seez valley/wind direction in the Rhine valley) prevail. The SE/S regime is the flow splitting situation. It is mainly driven by a strong pressure gradient across the Alps leading to foehn, even though some nocturnal cases are generated bylocal thermal gradients. The NW/W and NW/N regimes are mechanically forced bythe synoptic pressure gradient (as the flow splitting case). The difference between thetwo regimes is due to the synoptic flow direction [westerly (northerly) synoptic flowfor the NW/W (NW/N) regime], showing that the Rhine valley (particularly from BadRagaz to Lake Constance) is less efficient in channelling the flow than the Seez valley.The NW/S (occurring mainly during nighttime) and SE/N (occurring mainly duringdaytime) regimes are mainly katabatic flows. However, the SE/N regime is also partlyforced at the synoptic scale during the foehn case that occurred between 18 October and 20 October 1999, with a complex layered vertical structure. This analysis also shows that, contrary to what was observed in a broad section of theupper Rhine valley near Mannheim, very few countercurrents were observed near BadRagaz where the valley width is much smaller. 相似文献
7.
山谷地形对盛行气流影响的数值模拟 总被引：4，自引：0，他引：4
本文考虑的地形为理想的南北向长谷。建立了地形坐标下的二维非静力模式，详细研究了谷地中的风速和风向随盛行气流和谷轴的交角β、谷地的宽度、谷地的深度、谷地的相对深度以及温度层结的变化规律。模式计算的结果与三峡实际观测的统计结果相当一致。 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.
Mini-Sodar Observations of Drainage Flows in the Rocky Mountains 总被引：1，自引：0，他引：1
F. A. Woülfelmaier C. W. King E. Mursch-Radlgruber G. Rengarajan 《Theoretical and Applied Climatology》1999,64(1-2):83-91
Summary Vertical profiles of drainage winds were monitored continuously by a Doppler-Mini-Sodar during case studies in two valleys,
on both sides of the U. S. Continental Divide. A tethered balloon provided additional information on the vertical temperature
and wind structure up to the Divide level. Ambient wind data were collected by a radar wind profiler on the west side, and
a tower on the crest of the Divide. The onset, evolution and breakup of the drainage flow were studied on two nights, when
the ridge-top winds were westerly and skies were clear. To study the influence of the ambient flow on drainage winds, changes
in drainage wind speed, direction and depth, along with the volume flux were examined. It was found that, on the leeward side,
the drainage was strongly influenced by the ambient winds (King, 1995b), which led to interruption and erosion of the locally
generated valley flow. The drainage on the windward side of the Divide was almost undisturbed. A comparison of balloon and
sodar wind profiles showed very good agreement during steady drainage conditions.
Received October 21, 1996 Revised November 30, 1998 相似文献
10.
Numerical simulations of flow over two-dimensional valleys are conducted in order to study the occurrence of pools of cold
air that form at the bottom of valleys during stable nighttime conditions. The results show that during strong surface radiative
cooling and light-wind events, the near-surface potential temperatures that occur at the bottom of valleys can be several
kelvin below the environmental mean. This is true for quite shallow valleys with depths and widths of 50 m and 1 km, respectively,
and is a result of in situ sheltering at the valley bottom. For windier conditions or less rapid cooling, the cold-pool temperature
contrasts are reduced. For shallow valleys the magnitude of the difference between the potential temperature at the bottom
of the valley and the mean value increases with increasing valley depth. However there is a critical valley depth, beyond
which the valley flow becomes decoupled from that aloft and there are no further increases in the potential temperature difference.
This critical valley depth depends on the wind speed and radiative cooling rate and the results indicate it is a function
of a non-dimensional valley depth (or inverse Froude number), which is itself a property of the undisturbed profiles of wind
and stability. 相似文献
11.
Water-tank experiment on the thermal circulation induced by the bottom heating in an asymmetric valley 总被引：1，自引：0，他引：1
Water tank experiments were carried out to investigate the thermal convection due to the bottom heating in an asymmetrical valley under neutral and stably stratified approach flows with the Particle Image Velometry (PIV) visualization technique. In the neutral stratification approach flow, the ascending draft induced by bottom heating is mainly located in the center of the valley in calm ambient wind. However~with ambient wind flow, the thermal convection is shifted leeward, and the descending draft is located on the leeward side of the valley, while the ascending draft is located on the windward side. The descending draft is minorly turbulent and organized, while the ascending draft is highly turbulent. With the increase of the towing speed, the descending and ascending drafts induced by the mechanical elevation begin to play a more dominant role in the valley flow, while the role of the thermal convection in the valley airflow becomes limited. In the stable stratification approach flow, the thermal convection is limited by the stable stratification and no distinct circulation is formed in calm ambient wind. With ambient wind, agravity wave appears in the upper layer in the valley. With the increase of the ambient wind speed, a gravity wave plays an important role in the valley flow, and the location and intensity of the thermal convection are also modulated by the gravity internal waves. The thermal convection has difficulty penetrating the upper stable layer. Its exchange is limited between the air in the upper layer and that in the lower layer in the valley, and it is adverse to the diffusion of pollutants in the valley. 相似文献
12.
A reexamination of the valley wind system in the Alpine Inn Valley with numerical simulations 总被引：2，自引：0，他引：2
G. Zängl 《Meteorology and Atmospheric Physics》2004,87(4):241-256
Summary This paper presents idealized numerical simulations of the valley wind circulation in the Alpine Inn Valley, which are compared with existing data and are used to improve our dynamical understanding of the valley wind. The simulations have been performed with the Penn State/NCAR mesoscale model MM5. They use a high-resolution realistic topography but idealized large-scale conditions without any synoptic forcing to focus on the thermally induced valley wind system. The comparison with the available observations shows that this simplified set-up is sufficient to reproduce the essential features of the valley wind.The results show that the tributaries of the Inn Valley have a considerable impact on the along-valley mass fluxes associated with the valley wind circulation. The upvalley mass flux is found to increase where tributaries enter the Inn Valley from the north, that is, from the direction where the Alpine foreland is located. On the other hand, the upvalley mass flux is reduced at the junctions with southern tributaries because part of the upvalley flow is deflected into these tributaries. For the downvalley flow, the situation is essentially reversed, but the influence of the valley geometry on the flow structure is larger than for the upvalley flow. The most important feature is a lateral valley contraction near the valley exit into the Alpine foreland. It reduces the downvalley mass flux at low levels, so that the wind maximum in the interior of the valley is shifted to a fairly large distance from the ground. North of the valley contraction, however, the downvalley flow strongly accelerates and forms a pronounced low-level jet. A dynamical analysis indicates that this acceleration can be interpreted as a transition from subcritical to supercritical hydraulic flow. Another interesting feature is that the low-level jet maintains its structure for several tenths of kilometres into the Alpine foreland. This appears to be related to the fact that the lateral wind shear on the flanks of the jet is associated with a strong dipole of potential vorticity (PV). Due to the conservation properties of the PV, the downstream advection of the PV dipole leads to the formation of a band-like feature that decays fairly slowly. 相似文献
13.
The influence of shape and aspect ratio of a mountain valley on the wind field is studied with the use of a two-dimensional quasi-laminar model. A vortex with the axis directed along the valley appears in the simulations. In addition, an air flow along the valley is found. The speed of this flow at the vortex centre depends on the valley aspect ratio (the ratio of the valley width to its depth). This speed is less than the corresponding wind component at the same height in the undisturbed flow if the aspect ratio is smaller than a critical value, and it is greater than the undisturbed wind component if the aspect ratio is greater than the critical value. The latter is different for valleys having cross sections of different shape. 相似文献
14.
Prof. Dr. J. Egger 《Meteorology and Atmospheric Physics》1990,42(2):113-125
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.
A model of the drainage flow in a valley under calm conditions has been developed on the basis of the conservation laws of mass, momentum, and heat. The inflow of mass and heat from side-slopes is incorporated, and the momentum and sensible heat exchanges between valley drainage flow and valley floor are parameterized.The characteristic velocity of valley drainage flow is expressed in terms of the following parameters: three potential temperature differences representing the temperature field in the valey; topographic parameters of the valley; mean bulk coefficients representing the aerodynamic conditions of the valley floor; and the stability of the ambient atmosphere. The characteristic thickness includes additional parameters of side-slope flow.That the model satisfactorily predicts the characteristic thickness and velocity is shown from comparison with observations from valleys several hundred meters to a few hundred kilometers long. 相似文献
16.
17.
R. M. Banta L. D. Olivier W. D. Neff D. H. Levinson D. Ruffieux 《Theoretical and Applied Climatology》1995,52(1-2):27-42
Summary The study investigates two effects that a valley or canyon opening onto a plain can have on flow and contaminant dispersion over the downwind plain. The first effect is the channeling of strong ambient flow by the canyon when the wind is nearly aligned with the canyon axis. Two cases showed that these conditions produced a region of focused flow downwind of the canyon mouth. The second effect is the formation of canyon exit jets on nights with weaker ambient flow. In two case studies under these conditions strong exit jets formed that were several hundred meters deep. The jets remained narrow and strong at least 10 km onto the plains, and in one of the cases the jet extended more than 20 km over the plains. These deep jets only lasted 2–3 h, and they had a small but significant effect on surface-released tracer transport as indicated by surface sampling. We hypothesize that the near-surface advection of tracer was accomplished by a thin katabatic layer of flow, and that an elevated release or elevated sampling would have indicated a greater effect of the exit jet on tracer transport.With 18 Figures 相似文献
18.
Philippe Drobinski Alain M. Dabas Christian Haeberli Pierre H. Flamant 《Boundary-Layer Meteorology》2001,99(2):277-296
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
19.
2003年淮河流域致洪暴雨的环流背景及其与大气热源的关系 总被引：19，自引：14，他引：5
利用NCAR/NCEP的逐日再分析资料和降水资料,分析了2003年夏季淮河流域致洪暴雨的环流背景及其与大气热源的关系.结果表明,2002-2003年的El Nino事件是本次暴雨的前期背景;南海地区的视热源和视水汽汇异常可能是副高偏南维持的重要原因之一.与2003年夏季相比较,6月21日-7月22日淮河流域为正的异常视热源和视水汽汇,并且二者的高值中心与该时段雨量中心位置基本一致.孟加拉湾地区的异常加热源在其西北侧强迫出的高层反气旋性环流有利于南亚高压在青藏高原、江南、华南地区维持,从而使得淮河流域位于高压北侧高空西风急流入口区南侧的上升运动区,有利于淮河流域强降水发生和维持,形成该流域洪涝. 相似文献
20.
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