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1.
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
2.
Summary Numerical simulations of the 24 October 1999 south foehn (MAP-IOP 10) are performed with the Penn State/NCAR mesoscale model MM5 for the Wipp Valley and the adjacent parts of the Inn Valley. The model is run in a multiple-nest configuration, the area of interest being resolved at a mesh size of 800m in most experiments. The study serves to complement an earlier work in which typical flow features of the foehn in the Wipp Valley region were investigated by means of idealized simulations, assessing whether it is possible to reproduce the temporal evolution and the spatial structure of a particular foehn case. A further objective of the paper is to examine the dependence of the model performance on the horizontal resolution, giving some information which resolution will probably be needed for future high-resolution forecasts.An encouragingly large part of the observed flow features could be well reproduced in the simulations. Except for a small region to the east of Innsbruck, the foehn breakthrough is predicted correctly to within an hour. The spatial structure of the so-called pre-foehn, an enhanced westerly wind occurring at Innsbruck prior to the breakthrough of the foehn, also agrees very well with the observations. Moreover, the maximum extent of the foehn in the Inn Valley, the structure of the gravity wave field above the Wipp Valley and the upvalley progression of a shallow cold front in the evening are consistent with the observations. Except for a few places where the airmass boundary between the warm foehn air and the adjacent colder air is not captured correctly throughout the time, the simulated surface temperatures range within 2K of the observed values. Discrepancies between the model results and the observations are found in the vicinity of Innsbruck where a flow-splitting phenomenon induces a very complex flow pattern at low levels. Another source of problems is the surface potential temperature along the Wipp Valley. The observed potential-temperature increase between the Brenner Pass and Innsbruck, which appears to be related to turbulent vertical mixing of stably stratified air, is underestimated by the model. Reducing the horizontal resolution from 800m to 1.4km deteriorates the model performance in marginally resolved side valleys, but the results obtained for the Wipp Valley and the Inn Valley are still of high quality. 相似文献
3.
Summary In this paper, very-high-resolution numerical simulations are presented to analyze the small-scale dynamics of the foehn in
the lower Wipp Valley and the adjacent parts of the Inn Valley. This region was one of the target areas for foehn observations
during the Mesoscale Alpine Programme (MAP). Our simulations consider two MAP cases that markedly differed in the depth of
the foehn flow. To isolate the dynamical effect of the key orographic features in the Wipp Valley region, we performed sensitivity
experiments with different topography modifications. These involve lowering or even removing the Nordkette range, which constitutes
the northern side wall of the east–west-oriented Inn Valley, and closing the Stubai Valley, which is the northernmost and
largest tributary of the Wipp Valley. A comparison with surface and lidar observations indicates that our present model resolution
of 467 m significantly improves the realism of the simulations compared to a resolution of 800 m, as used in a previous study.
The Nordkette is found to have a twofold impact on the dynamics of foehn breakthrough into the Inn Valley. In reality, this
mountain chain deflects part of the southerly foehn current coming from the Wipp Valley into the perpendicularly oriented
Inn Valley. Our sensitivity tests indicate that this flow deflection tends to accelerate the foehn breakthrough into the Inn
Valley, while upstream blocking effects induced by the Nordkette act to slow down the process of foehn breakthrough. The flow
pattern in the Wipp Valley reveals that the upstream effects of the Nordkette are not quite far-reaching. The amplitude of
the gravity waves over the lower Wipp Valley gets somewhat reduced by these upstream effects, but the overall flow pattern
remains largely unaffected. Closing the Stubai Valley also has a minor effect of the wave structure and tends to reduce the
cross-valley variability of the foehn flow in the lower Wipp Valley. 相似文献
4.
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
5.
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
6.
太行山东麓焚风天气的统计特征和机理分析Ⅱ:背风波对焚风产生和传播影响的个例分析 总被引：1，自引：0，他引：1
利用中尺度模式WRF3.3对太行山东麓焚风典型个例进行了数值模拟。结果表明,太行山东麓焚风的发生和移动与山脉背风波密切相关。由此建立了太行山东麓焚风的概念模型:西北或偏西气流途经山西盆地、山西境内的山脉或高原,再越过太行山,在其东麓形成背风波。背风波的下沉气流气温按干绝热方式上升,同时下沉气流也会对低层大气产生压缩增温效应,使得太行山东麓产生焚风。背风波即为重力波,可以伴随着下沉气流向下游移动,正变温区同时也向东移动。变温区移动的速度和重力波的传播速度相同。背风波的产生,需要Scorer数向上足够的减小,而且不连续,即要求大气是稳定的且存在明显的风速切变。 相似文献
7.
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
8.
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
9.
两层正压准平衡海洋模型的中纬度定常风场强迫解 总被引：2，自引：2，他引：0
建立了风场强迫下考虑瑞利摩擦的水平二维两层正压准平衡海洋模型,并在中纬度大尺度理想风场强迫的情况下,进行了解析求解。结果表明,在西风急流强迫下,在理想海洋的西海岸以东的上层流场上,在西风急流处会出现较强的东向流;在接近β通道侧壁处则有西向逆流出现;在近西海岸处,在上述东向流的两侧有气旋性曲率与反气旋性曲率的流动;而下层流场的强度与上层流场大致相同,但流向大体相反。由理想西风急流异常强迫出的流场异常的分布形式与以上结果类似。与实际风场异常强迫下的北太平洋上层流场异常进行比较后可知,模型得到上层海洋的结果与实际情况有相像之处。最终,该海洋模型的解趋于风场强迫特解。 相似文献
10.
图3.2.1—3.2.5表示了地形对长波和超长波移动和发展影响的一般规律。如以地形脊对东移西风冷槽的影响为例,当地形相当涡度和扰动涡度的振辐比a_m/a_(20)较小时,在它越过地形脊时,首先流场槽减弱而温度槽加强,减弱的流场槽以较快的速度移动,而温度槽则相对减慢,越过地形脊后,流场槽又重新增强。 我们根据地形的影响,初步讨论了一锢囚气旋在向风坡填塞,在背风坡又重新产生的物理过程。超长波的斜压不稳定的不能发展,是由于温度场东进和流场西退造成的。根据地形脊对超长波脊移动的影响,可以推测,在两个地形超长波脊之间的地区,最有利于超长波脊的不稳定发展。阻塞脊出现频数的地理分布的观测结果和理论推测有很好的一致性。 相似文献
11.
12.
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
13.
新疆克拉玛依强下坡风暴的机理研究 总被引：1，自引：0，他引：1
利用美国中尺度数值模式 WRF 对2013年3月7—8日克拉玛依强风进行了模拟,对下坡风发生、发展和结束3个阶段的三维结构特征进行了分析,并由此提出克拉玛依强下坡风的形成机制模型：上游地区出现中高层西南风、低层西北风并伴有强冷平流的配置,当风速不断增大时,气流能够翻越加依尔山在背风坡侧形成重力波,重力波相位向气流上游方向倾斜产生非线性效应,促进了波不稳定区域的形成并导致波破碎,形成湍流活跃层,不断把上层的能量向下传播;克拉玛依中低层形成三层夹心的大气层结稳定度分布,出现明显的过渡气流带从而导致强下坡风的形成;南北风分量在低层和中层符号相反,形成了临界层,不断吸收上层波能量并向地面传送,强下坡风暴不断维持发展。最后利用2006—2012年克拉玛依33个强下坡风过程中的探空观测资料对提出的形成机制进行了验证。 相似文献
14.
正压准平衡海洋模型及其解（Ⅰ）：中纬度大尺度风场强迫情况 总被引：1，自引：1，他引：0
本文建立了风场强迫下考虑瑞利摩擦的水平二维正压准平衡海洋模型,并对中纬度大尺度风场强迫的情况做了解析求解。结果表明,在近地面西风急流的强迫下,在理想海洋的西海岸以东会出现由气旋涡与反气旋涡构成的涡旋偶;当西风强迫越大,粘性越小时其强度越大,而β越大,β通道的半宽越小,则其沿纬向的变化越剧烈;该涡旋偶的性质属准平衡的涡旋波。将该结果与北太平洋上层流场异常的复EOF分析的第一模态作比较后可见,两者有类似之处。最终,该海洋模型的解则趋于该风场强迫特解。 相似文献
15.
Summary Numerical experiments are performed for inviscid flow past an idealized topography to investigate the formation and development of lee mesolows, mesovortices and mesocyclones. For a nonrotating, low-Froude number flow over a bell-shaped moutain, a pair of mesovortices form on the lee slope move downstream and weaken at later times. The advection speed of the lee vortices is found to be about two-thirds of the basic wind velocity, which is due to the existence of a reversed pressure gradient just upstream of the vortices. The lee vortices do not concur with the upstream stagnation point in time, but rather form at a later time. It is found that a pair of lee vortices form for a flow withFr=0.66, but take a longer time to form than in lower-Froude number flows. Since the lee vortices are formed rather progressively, their formation may be explained by the baroclinically-induced vorticity tilting as the mountain waves become more and more nonlinear.A stationary mesohigh and mesolow pressure couplet forms across the mountain and is produced in both high and low-Froude number flows. The results of the high Froude number simulations agree well with the classical results predicted by linear, hydrostatic mountain wave theory. It is found that the lee mesolow is not necessarily colocated with the lee vortices. The mesolow is formed by the downslope wind associated with the orographically forced gravity waves through adiabatic warming. The earth's rotation acts to strengthen (weaken) the cyclonic (anticyclonic) vortex and shifts the lee mesolow to the right for an observer facing downstream. The cyclonic vortex then develops into a mesocyclone with the addition of planetary vorticity at later times. For a flow over a steeper mountain, the disturbance is stronger even though the Froude number is kept the same.For a southwesterly flow past the real topography of Taiwan, there is no stagnation point or lee vortices formed because the impinging angle of the flow is small. A major mesoscale low forms to the southeast of the Central Mountain Range (CMR), while a mesohigh forms upstream. For a westerly flow past Taiwan, a stagnation point forms upstream of the mountain and a pair of vortices form on the lee and move downstream at later times. The cyclonic vortex then develops into a mesocyclone. A mesolow also forms to the southeast of Taiwan. For a northeasterly flow past Taiwan, the mesolow forms to the northwest of the mountain. Similar to flows over idealized topographies, the Taiwan mesolow is formed by the downslope wind associated with mountain waves through adiabatic warming. A conceptual model of the Taiwan southeast mesolow and mesocyclone is proposed.With 16 Figures 相似文献
16.
D. Heimann 《Meteorology and Atmospheric Physics》1997,62(1-2):49-70
Summary Mesoscale flow characteristics in the Alpine region are deduced from a set of daily large-scale analyses (1981–1990) by means of statistical-dynamical downscaling. This method utilizes the results of a large number of mesoscale numerical simulations in combination with known statistics of the forcing large-scale conditions. The investigation is restricted to cross-Alpine large-scale flow from 165 to 265 degrees at 500 hPa. Such types of flow are favourable to south foehn.The results provide model-based climatological estimates of surface wind direction and upper-level gravity-wave formation at a horizontal resolution of 20 and 10 km. Simulated surface wind roses agree well with observations and show a dominance of low-level flow around the Alps with bimodal frequency distributions of wind direction north and south of the mountains. The areas where splitted flows preferably merge are identified. Gravity waves are most likely to occur above the western parts of the Alps. A secondary maximum of likelihood was found above Tyrol and Trentino. Surface wind roses and gravity-wave formation are both checked with respect to their sensitivity to season (spring vs. autumn) and large-scale flow direction (south to southwest vs. southwest to west).With 13 Figures 相似文献
17.
This study examines wave disturbances on submonthly (6–30-day) timescales over the tropical Indian Ocean during Southern Hemisphere summer using Japanese Reanalysis (JRA25-JCDAS) products and National Oceanic and Atmospheric Administration outgoing longwave radiation data. The analysis period is December–February for the 29 years from 1979/1980 through 2007/2008. An extended empirical orthogonal function (EEOF) analysis of daily 850-hPa meridional wind anomalies reveals a well-organized wave-train pattern as a dominant mode of variability over the tropical Indian Ocean. Daily lagged composite analyses for various atmospheric variables based on the EEOF result show the structure and evolution of a wave train consisting of meridionally elongated troughs and ridges along the Indian Ocean Intertropical Convergence Zone (ITCZ). The wave train is oriented in a northeast–southwest direction from Sumatra toward Madagascar. The waves have zonal wavelengths of about 3,000–5,000 km and exhibit westward and southwestward phase propagation. Individual troughs and ridges as part of the wave train sequentially travel westward and southwestward from the west of Sumatra into Madagascar. Meanwhile, eastward and northeastward amplification of the wave train occurs associated with the successive growth of new troughs and ridges over the equatorial eastern Indian Ocean. This could be induced by eastward and northeastward wave energy dispersion from the southwestern to eastern Indian Ocean along the mean monsoon westerly flow. In addition, the waves modulate the ITCZ convection. Correlation statistics show the average behavior of the wave disturbances over the tropical Indian Ocean. These statistics and other diagnostic measures are used to characterize the waves obtained from the composite analysis. The waves appear to be connected to the monsoon westerly flow. The waves tend to propagate through a band of the large meridional gradient of absolute vorticity produced by the mean monsoon westerly flow. This suggests that the monsoon westerly flow provides favorable background conditions for the propagation and maintenance of the waves and acts as a waveguide over the tropical Indian Ocean. The horizontal structure of the wave train may be interpreted as that of a mixture of equatorial Rossby waves and mixed Rossby-gravity wavelike gyres. 相似文献
18.
黄山连续性云海过程的天气学分析 总被引：2，自引：0，他引：2
用美国NCEP／NCAR的全球日平均分析场资料,分析了黄山1984年1月19～24日连续6天云海天气过程的大气环流和物理量时间演变特征。结果表明,500hPa平均高度场鄂霍次克海为阻高,乌拉尔山以东为高压脊,我国东北存在一低压,黄山受偏西气流影响;1000hPa平均高度场黄山处在南伸的高压底部,受NE气流影响。物理量时间演变图上,18～19日降温、降湿,垂直速度由上升气流转变为弱下沉气流;云海出现阶段,气温、比湿、垂直速度稳定少变,风速较小,风向较乱。 相似文献
19.
The characteristics of dynamics and thermodynamics of the atmospheric boundary layer in a part of the Colorado River Valley, centered around Lake Mohave, have been investigated by analysis of measurements conducted during a field program in late spring and early summer of 1986 and a series of numerical simulations by a three-dimensional second-moment turbulence-closure model. The model was validated against measurements described in a companion article (Engeret al., 1993). According to airsonde measurements performed on eight nights, the depth of the surface inversion was around 200 m with an average temperature gradient of about 30 K km–1. Analysis of acoustic sounder data collected during one month revealed significant diurnal variations ofU andV wind-speed components related to slope and valley flows, respectively. Some of the dynamics properties have been explained by the simulation results. It has been shown that the appearance of supergeostrophic southerly valley flow is associated with the westerly component of the geostrophic flow. Since a westerly component of the geostrophic wind is quite common for this area in summer, this effect also explains the frequently observed southerly valley flow in summer. Elevated minima of the measured wind speed around valley ridges appear to be related to the interaction of conservation of momentum in theX andY directions. The critical direction of the geostrophic wind relevant for reversal of up-valley flow to down-valley flow has also been studied. The critical direction is about 300° for one of the measurement sites and, depending on the angle between valley axis and south-north direction, the critical direction is expected to vary by about 15–20°. The scale analysis of the simulated equations of motion and turbulence kinetic energy emphasizes the strong impact of meandering of the flow due to actual topographic complexity. 相似文献
20.
西太平洋副热带高压东西位置变动特征分析 总被引：9，自引：0，他引：9
采用NCEP／NCAR再分析逐日资料，根据特定区域涡度值，定义描述西太平洋副高东西位置的指标，并利用该指标研究了6月副高东西位置变动特征。结果表明：该指标不仅避免了由于高度的逐年升高而引起的副高逐年西进的年代际变化，而且能很好地反映大尺度环流场的特征；副高偏西年，赤道西风减弱，越赤道气流减弱，南海夏季风偏弱，江南和华南地区南风增强，其东侧负涡度发展，引导副高西伸，长江流域被副高北侧异常西南风控制，有利于雨带在此维持，降水偏多易涝。西太平洋副高东西位置变化的周期分析表明，6月副高西伸指数最强的周期信号为2、4、8a。 相似文献