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1.
Mesoscale diagnostics of prefrontal and frontal precipitation in the Southeast Alps during MAP IOP 5
Summary In terms of heavy precipitation, the MAP IOP 5 was a two-phase event. During the first phase – on 3 October 1999 – there was strong precipitation in the Lago Maggiore MAP target area, while the prefrontal precipitation was mainly limited to the mountain ranges of the MAP mission area in the Julian and the Karnic Alps involving a series of thunderstorms developing continuously for about 15 hours and contributing most to precipitation levels. During the second phase – on 4 October – the main precipitation was limited to the Julian and the Karnic Alps where a frontal passage was noted by a squall line moving from Veneto region towards the east, accompanied by a strong SW upper-level jet. At the same time, a strong low-level cold flow invaded the region to the north of Adriatic Sea from the east as a significant amount of cold air moving ageostrophically around the eastern edge of the Alps was arriving in the area. To study MAP IOP 5 in detail, we describe the development for mesoscale features of the events radar images, time-height cross-sections and estimates of Convective Available Poteintial Energy (CAPE) based on radio-sounding data, and how surface-measured precipitation offers some smaller scale information. Surface potential temperature and winds are also studied. Very large precipitation accumulation gradients are diagnosed (150mm per day/25km in S–N direction) and time distributions of hourly precipitation shows completely diverse regimes in the Friuli plain and in the Alps with peak intensities in the Julian Alps. The mesometeorological mechanisms for high precipitation rate in the SE Alps are diagnosed and some characteristics of the squall line are discussed. 相似文献
2.
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
3.
1.IntroductionTheimportanceoforographiceffectsonfrontwasrecognizedintheearly20thcentury.Butforthecomplexityofthisproblem,theinvestigationoforographiceffectsonfrontfromdynamicalviewpointisnottakenuntilthe1980s.Bannon(1983)derivedanalyticalsolutionsforthequasi--geostrophicfrontforcedbyahorizontalwinddeformationfieldthatmovesoveratwo--dimensionalmountainridge.Thesolutionsshowthatasacoldfrontapproachestheridge,itweakens,relativetotheflat--bottomsolution,andthefrontstrengthensasitmovesdowntothelees… 相似文献
4.
Summary An isentropic limited area model is used to simulate and investigate the frontal passages of 3 May and 8 October 1987. It is demonstrated that a southward outbreak of air with high potential vorticity on 3 May 1987 affected the propagation of the front to the north of the Alps and the related formation of an orographic jet. Moreover, the outbreak plays a crucial role in the genesis of a lee cyclone. On the other hand, no such outbreak occurred on 8 October 1987 and it is shown that the propagation of the front near the ground was hardly affected by the flow at upper levels.With 10 Figures 相似文献
5.
利用实况观测资料和NCEP再分析格点资料,对2011年10月12—14日广东一场全省范围的暴雨过程及水汽特征进行了分析。结果表明：（1）此次暴雨过程分为2个阶段,分别是12日夜间的暖区暴雨和13日夜间的锋面暴雨。（2）第1阶段降水主要由500 hPa西风短波槽配合850 hPa东南风辐合导致的,第2阶段降水是高层西风槽过境,配合地面冷空气前锋南下和低层的切变线共同作用所致。（3）降水发生前广东水汽充沛,湿层从低层往上伸展的厚度大,估计约有50%的降水量可由雨区本地的水汽所提供。（4）低层850 hPa或者925 hPa的东南风是暴雨发生的重要水汽输送通道,暴雨发生前南海热带系统的活跃是维持850 hPa和925 hPa东南风输送的重要原因。 相似文献
6.
地形对于气流运动影响的数值研究 总被引：7，自引：3，他引：7
建立了二维、非静力平衡的数值模式，研究地形对上游气流的阻挡以及大振幅背风波谷与下坡风的形成。结果表明：地形的阻挡效应受地形高度、大气层结及地形非对称性等因子的影响。数值试验与理论分析都证明地形越高、层结越稳定时阻挡作用越强；同样条件下，迎风坡坡度大的地形容易对气流形成阻挡。此外，分析了地形高度、大气层结、地形非对称性以及基本入流大小对背风波谷及下坡风强度影响的规律，并通过一次实际观测对数值模拟结果进行了检验。 相似文献
7.
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
8.
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
9.
Dr. A. Tafferner 《Meteorology and Atmospheric Physics》1990,43(1-4):31-47
Summary A conceptual model of lee cyclogenesis is given which combines the commonly observed predominant features inherent in Alpine lee cyclogenesis, to be: blocking of cold air due to the Alps and progression of a potential vorticity maximum over the Alps. Numerical simulations of observed cases of Alpine lee cyclogenesis are carried out whose flow development turn out to bear close resemblance to this model. Therefore far-reaching conclusions are drawn.With 14 Figures 相似文献
10.
Summary Heavy precipitation events to the south of the Alps are usually associated with a southerly pre-frontal low-level jet advecting
moisture toward the southern slopes of the Alps. Here we use idealised numerical simulations to assess the nature of the associated
flow regimes and the mechanisms leading to vertical lifting and precipitation. The idealisations comprise: a simplified arc-shaped
barrier-like orographic obstacle of Alpine scale; neglection of the tropopause; a stationary two-dimensional upstream flow
configuration that includes a frontal structure and a low-level jet; hydrostatic dynamics with free-slip lower boundary conditions;
and a simplified set of parameterizations to address dry, moist absolutely stable, and moist conditionally unstable upstream
flow configurations.
Within the dry dynamics, typical settings lead to Alpine-scale flow splitting with pronounced left/right asymmetries with
respect to the incident southerly flow. Strong vertical lifting occurs over the western portion of the upstream slopes, within
the stream of air that tries to circum go the elongated obstacle on the western flank. Thus, despite belonging to the “flow-around”
regime, these flow configurations can exhibit vertical lifting over the whole height of the obstacle. The responsible asymmetry
is primarily induced by the Coriolis effect in the presence of an elongated mountain, but it can further be intensified by
the impinging low-level jet and the arc-shape of the Alpine topography. With a conditionally unstable moist upstream profile,
the flow is able to surmount the obstacle without pronounced horizontal deflections. Maximum precipitation rates of are obtained. When moist convection is suppressed by using a moist absolutely stable upstream profile, the flow is again
substantially deflected and shows the typical characteristics of the dry flow regime discussed above, with somewhat reduced
precipitation rates as compared to the convective case. Overall there is evidence that the asymmetry introduced by the Coriolis
effect, a pronounced low-level jet, and a moist upstream profile, all facilitate vertical lifting and thereby provide a suitable
environment for heavy condensation and precipitation.
Received March 22, 1999/Revised August 18, 1999 相似文献
11.
Summary This study analyzes the mechanisms of the development of a heavy rainfall event (17 June 1987) over the lee side of the Central
Mountain Range (CMR) in northeastern Taiwan during the southwesterly monsoon. This heavy rainfall event was examined using
gridded data from the European Centre for Medium-Range Weather Forecasts, surface rainfall data and numerical model results,
employing a non-hydrostatic fifth-generation mesoscale model (MM5) developed by the National Center for Atmospheric Research
and Pennsylvania State University. A tropical depression was simulated over the northern South China Sea on 16 June. Convergence,
resulting from the southeasterly winds associated with the circulation from the tropical depression, and northeasterly winds
over the Taiwan Strait, occurred over the northern Bashi Channel at 850 hPa. The convergence amplified planetary vorticity
and the vorticity associated with the intensifying tropical depression. Consequently, a mesovortex with low pressure over
the northeastern edge of the tropical depression near southern Taiwan was produced. Additional convergence over the ocean
adjacent to southern Taiwan caused by the interaction between the northeasterly flow, which was deflected over the southeastern
slope of the CMR, and the southeasterly flow of the tropical depression, also affected the intensity of the mesovortex. When
the mesovortex moved northward and reached southern Taiwan, the southeasterly flow associated with it interacted with an east-southeasterly
flow, which was related to the tropical depression, to form a mesoscale convective system (MCS) over the ocean adjacent to
southeastern Taiwan. As the mesovortex moved northward, the MCS, which was embedded in the southeasterly flow, also drifted
inland toward northeastern Taiwan. The orographic lifting and the ascending motion associated with the deceleration of the
easterly flow near the CMR enhanced the MCS over northeastern Taiwan and produced heavy rainfall.
To examine the role of Taiwan’s orography on the modelled rainfall, two simulations were conducted; one which included Taiwan’s
orography and one which excluded it. In both simulations, the mesovortex in the northern Bashi Channel and the MCS near southeastern
Taiwan were reproduced. However, in the simulation excluding the orography, the mesovortex was slightly less intense. In addition,
without the extra orographic lifting and the ascending motion caused by flow deceleration, rainfall over northeastern Taiwan
was weaker than in the simulation with the orography. 相似文献
12.
This paper investigates the dependence on environmental conditions of altitudinal precipitation differences in the northern Alps, based on high-resolution numerical simulations with the MM5 model for a selected region in the Bavarian Alps (Zugspitze mountain and surrounding valley stations). Three exemplary precipitation events representing climatological regimes with different orographic enhancement characteristics are selected. After validating the MM5 precipitation fields against the available surface observations, the model results are used to analyse the interactions of atmospheric dynamics and cloud microphysics with the local orography. The first two cases (19–22 March 1997, 05–09 February 1999) are characterized by a strong northwesterly or northerly flow, associated with large precipitation differences between the mountain and the surrounding valley stations. For these cases, the model results indicate a dominance of the classical seeder–feeder mechanism, with strong orographic lifting generating dense orographic clouds over each individual mountain ridge, which in turn intensify precipitation. The related surface precipitation maxima can be found near the mountain peaks or somewhat in the lee due to hydrometeor drifting. The third case (05–07 December 1992) represents conditions with relatively small (i.e. below climatological average) precipitation differences between the Zugspitze and the surrounding valley stations. For this event, the model results indicate that relatively weak ambient winds at and below Alpine crest level (700 hPa) were primarily responsible for the lack of substantial precipitation enhancement. Precipitation was nevertheless moderately intense because of strong frontal lifting at higher levels. In all three cases, the agreement between simulated and observed precipitation patterns is so high that there is good reason to expect that mountain–valley precipitation differences will be quantitatively predictable for nonconvective events once a sufficiently high model resolution is computationally affordable. 相似文献
13.
Global climate change and variability and its influence on Alpine climate — concepts and observations 总被引：1，自引：0，他引：1
H. Wanner R. Rickli E. Salvisberg C. Schmutz M. Schüepp 《Theoretical and Applied Climatology》1997,58(3-4):221-243
Summary The paper discusses annual to decadal climate variability and change in the European Alps by utilizing the procedure of synoptic downscaling, i.e. it investigates the influence of global to continental scale synoptic structures and processes on the regional climate of the Alps. The European Alps lie to the southeast and under the right exit zone of the southwest-northeast oriented axis of the polar front jet over the North Atlantic ocean, in a transition zone between the Azores high and Icelandic low, between oceanic and continental and between Mediterranean and North Atlantic climates. Together with complex topographically induced phenomena like lee cyclogenesis, orographic precipitation, strong downslope winds and thermotopographical circulation systems, this transitional position makes climate studies in the Alps even more interesting. Only a minor correlation can be observed between global climate variability and Alpine climate. In contrast, the Alpine climate is strongly related to processes over the North Atlantic ocean and its sea ice system (e.g. it has a high correlation with the North Atlantic Oscillation and the dynamics and position of the Icelandic low), an area with a rather low climate prediction potential.Since the early 1970's (or just after the Great Salinity Anomaly in the North Atlantic Ocean) the intensification of the wintertime westerly jet over the North Atlantic area led to a noticeable northwest-southeast mass transport in the exit area of the jet over Central Europe, leading to pressure and temperature rises and an increase in the amount of precipitation. There is a question over whether this phenomenon is a consequence of natural climate variability or the beginning of an anthropogenic climate change.With 16 Figures 相似文献
14.
地形降水试验和背风回流降水机制 总被引：7，自引：2，他引：7
利用中尺度数值模式（ARPS模式）研究了湿气流过山脉地形和地形降水的产生机制。研究结果表明，地形降水是水汽、气流和地形相互作用而形成的。小山脉地形降水主要发生在山脉的迎风坡，表现出典型的迎风降水和背风雨影特征。而回流降水天气是湿气流过大的山脉地形的产物，大的山脉地形有利于风切变临界层的产生，地形降水并不只是简单的上坡降水，还有背风回流和背风波降水机制。 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.
The quasi-stationary feature of nocturnal precipitation in the Sichuan Basin and the role of the Tibetan Plateau 总被引：3，自引：0，他引：3
The nocturnal precipitation in the Sichuan Basin in summer has been studied in many previous works. This paper expands the study on the diurnal cycle of precipitation in the Sichuan Basin to the whole year. Results show that the nocturnal precipitation has a specific quasi-stationary feature in the basin. It occurs not only in summer but also in other three seasons, even more remarkable in spring and autumn than in summer. There is a prominent eastward timing delay in the nocturnal precipitation, that is, the diurnal peak of precipitation occurs at early-night in the western basin whereas at late-night in the center and east of the basin. The Tibetan Plateau plays an essential role in the formation of this quasi-stationary nocturnal precipitation. The early-night peak of precipitation in the western basin is largely due to strong ascending over the plateau and its eastern lee side. In the central and eastern basin, three coexisting factors contribute to the late-night peak of precipitation. One is the lower-tropospheric southwesterly flow around the southeastern edge of the Tibetan Plateau, which creates a strong cyclonic rotation and ascendance in the basin at late-night, as well as brings abundant water vapor. The second is the descending motion downslope along the eastern lee side of the plateau, together with an air mass accumulation caused by the warmer air mass transport from the southeast of the Yunnan-Guizhou Plateau, creating a diabatic warming at low level of the troposphere in the central basin. The third is a cold advection from the plateau to the basin at late-night, which leads to a cooling in the middle troposphere over the central basin. All these factors are responsible for precipitation to occur at late-night in the central to eastern basin. 相似文献
17.
Summary In a recently published work it was shown that Alpine cyclone development depends on a short wave trough upstream of the Alps, the presence of an upper-level jet streak in this trough, and a low-level front interacting with the mountain barrier. Specifically, by modifying the strength of the upper baroclinic zone (potential vorticity) or by decreasing the strength of the low-level baroclinic zone impinging on the Alps in an initial field, the intensity of the resulting numerically predicted cyclogenesis could be modulated. The major finding of the work was that the ALPEX cyclones reacted differently to upper-level and lower-level modification, providing a basis for broadly classifying storms: that is, dependence on upper-level processes and on low-level processes.The present work extends this study by considering additional cyclones that occurred during the ALPEX Special Observing Period (SOP), and describes the influence of upper-level and lower-level processes on each. An index (I) discussed in the previous paper is examined in terms of its relative value from case to case, and in terms of its value in a time-dependent sense during the lifetime of the storm. The results show that the most powerful lee cyclones depended more on the strength of the upper-level jet or potential vorticity than on the strength of the low-level front of baroclinic zone. In most cases the time evolution ofI showed the influence of the upper-level jet to be important during the early phase of development. The front is increasingly important in midlife as intensification takes place, and the jet is increasingly important during late life. These conclusions were reached during the earlier study but are supported by the additional SOP cyclone cases.Cyclone steering flow and the impinging of the storm on the Alps had a great impact on defining its character: storms with NW-N trajectories were heavily dependent on low level frontal strength and WSW-WNW trajectories were dominated by effects of the upper level jet.With 16 Figures 相似文献
18.
Summary A semi-geostrophic model of frontal passage over topography was developed to examine the effects of the interaction of a
well developed front with an isolated mountain, and the subsequent orographically induced flow evolution. The analytic representation
of the primary wave and its frontal structure gives us the ability to control the environmental and geometrical constraints
and perform an exploration of parameter space. A number of problems appearing because of this approach are discussed. The
results shown relate the characteristics of the orographic perturbation to varying amplitude of the primary wave and varying
frontal intensity, but suggest that the representation of flow blocking by the mountain,which this model lacks, is crucial
for the representation of frontal intensification in the lee of orography.
Received March 2, 1999/Revised May 7, 1999 相似文献
19.
Summary A case study is presented of the meteorological situation of 26 July 1987, when a moderate lee cyclone formed south of the Alps associated with a cold front moving from the northwest. A convective line developed over the Po valley and the northern Adriatic sea, slightly in advance of the position of the cold front which, over that area, was associated with a north-easterly low-level flow. The mesoscale situation over the Po valley before and after the frontal passage was complex, with different flow configurations and weather phenomena affecting the eastern and western portions of the region.The analysis of observations has been complemented by the results of numerical experiments run with a limited area, high resolution model, initialized using interpolated ECMWF analyses. Sensitivity experiments show the important roles played by the orography and by the condensation-evaporation processes in determining the mesoscale field environment favourable for the development of the organized convection.With 26 Figures 相似文献
20.
Harindra J. S. Fernando Brett Verhoef Silvana Di Sabatino Laura S. Leo Seoyeon Park 《Boundary-Layer Meteorology》2013,147(3):443-468
Motivated by air quality and numerical modelling applications as well as recent theoretical advancements in the topic, a field experiment, dubbed transition flow experiment, was conducted in Phoenix, Arizona to study the evening transition in complex terrain (shift of winds from upslope to downslope). Two scenarios were considered: (i) the flow reversal due to a change of buoyancy of a cooled slab of air near the ground, and (ii) the formation of a transition front. A suite of in-situ flow, turbulence and particulate matter (PM) concentration sensors, vertically profiling tethered balloons and remote sensors were deployed, and a mesoscale numerical model provided guidance for interpreting observations. The results were consistent with the front formation mechanism, where it was also found that enhanced turbulence associated with the front increases the local PM concentration. During the transition period the flow adjustment was complex, involving the arrival of multiple fronts from different slopes, directional shear between fronts and episodic turbulent mixing events. The upward momentum diffusion from the incipient downslope flow was small because of stable stratification near the ground, and full establishment of downslope flow occurred over several hours following sunset. Episodic frontal events pose challenges to the modelling of the evening transition in complex terrain, requiring conditional parametrizations for subgrid scales. The observed increase of PM concentration during the evening transition has significant implications for the regulatory enforcement of PM standards for the area. 相似文献