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1.
The Little Ice Age climate of New Zealand reconstructed from Southern Alps cirque glaciers: a synoptic type approach 总被引：2，自引：1，他引：1
Andrew Lorrey Nicolas Fauchereau Craig Stanton Petra Chappell Steven Phipps Andrew Mackintosh James Renwick Ian Goodwin Anthony Fowler 《Climate Dynamics》2014,42(11-12):3039-3060
Little Ice Age (LIA) austral summer temperature anomalies were derived from palaeoequilibrium line altitudes at 22 cirque glacier sites across the Southern Alps of New Zealand. Modern analog seasons with temperature anomalies akin to the LIA reconstructions were selected, and then applied in a sampling of high-resolution gridded New Zealand climate data and global reanalysis data to generate LIA climate composites at local, regional and hemispheric scales. The composite anomaly patterns assist in improving our understanding of atmospheric circulation contributions to the LIA climate state, allow an interrogation of synoptic type frequency changes for the LIA relative to present, and provide a hemispheric context of the past conditions in New Zealand. An LIA summer temperature anomaly of ?0.56 °C (±0.29 °C) for the Southern Alps based on palaeo-equilibrium lines compares well with local tree-ring reconstructions of austral summer temperature. Reconstructed geopotential height at 1,000 hPa (z1000) suggests enhanced southwesterly flow across New Zealand occurred during the LIA to generate the terrestrial temperature anomalies. The mean atmospheric circulation pattern for summer resulted from a crucial reduction of the ‘HSE’-blocking synoptic type (highs over and to the west of NZ; largely settled conditions) and increases in both the ‘T’- and ‘SW’-trough synoptic types (lows passing over NZ; enhanced southerly and southwesterly flow) relative to normal. Associated land-based temperature and precipitation anomalies suggest both colder- and wetter-than-normal conditions were a pervasive component of the base climate state across New Zealand during the LIA, as were colder-than-normal Tasman Sea surface temperatures. Proxy temperature and circulation evidence were used to corroborate the spatially heterogeneous Southern Hemisphere composite z1000 and sea surface temperature patterns generated in this study. A comparison of the composites to climate mode archetypes suggests LIA summer climate and atmospheric circulation over New Zealand was driven by increased frequency of weak El Niño-Modoki in the tropical Pacific and negative Southern Annular Mode activity. 相似文献
2.
A. P. Sturman 《Meteorology and Atmospheric Physics》1992,47(2-4):229-236
Summary The Southerly Change Experiment (SOUCHEX) was conducted to examine the influence of the New Zealand Southern Alps on the structure and evolution of cold fronts, locally called southerly changes, as they travel up the east coast. The extensive data obtained by the augmented surface weather station network is used to examine in detail the mesoscale wind field associated with the events observed during the experiment. A comparison of the wind fields observed during the different events illustrates the influence of local dynamic and thermal factors. In particular, lee trough-induced northeasterlies and thermally developed diurnal wind systems are seen to interact with the wind field created by the passage of the front over the Southern Alps.It is apparent that the wind field associated with southerly changes responds to a variety of factors as the cold fronts propagate northwards. For example, there is a tendency for the flow to turn onshore producing a southeast wind during daytime over the Canterbury Plains south of Banks Peninsula probably due to diabatic heating of the mountains and plains. This onshore flow is in direct opposition to pre-frontal foehn northwesterly flow which often continues in the mountain regions and aloft after the front has moved up the coast. The interaction of these air masses over Canterbury creates difficulties for local forecasting. Also, the nocturnal passage of a southerly change is often difficult to detect in surface anemograph traces because of the decoupling of the boundary layer air from that above, producing low level drainage flow over the Canterbury Plains. The overall effect is to create a complex mesoscale wind field resulting from interaction of cold fronts with regional orographic and thermal influences.With 8 Figures 相似文献
3.
Summary A fine mesh analysis scheme (grid distance approximately 47 km) has been developed which takes care of mountain ridges of different height, to be able to study cyclogenesis induced by the Alps from the standpoint of observational quantitative vorticity dynamics.The method represents a 2-D univariate statistical interpolation scheme with isotropic correlation function over flat terrain but anisotropic across major mountain ridges like the Alps and Pyrrences.The checked and corrected ALPEX-IIb data set-which is the best data set for the Alpine region concerning time and space resolution as well as quality up to now-is used to produce analyses of basic and derived atmospheric variables. It is shown that structures in the upper meso-scale are resolved with pronounced gradients across mountains.With 9 Figures 相似文献
4.
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
5.
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
6.
Analysis of the wind field and heat budget in an alpine lake basin during summertime fair weather conditions 总被引：2，自引：0，他引：2
M. Kossmann A. P. Sturman P. Zawar-Reza H. A. McGowan A. J. Oliphant I. F. Owens R. A. Spronken-Smith 《Meteorology and Atmospheric Physics》2002,81(1-2):27-52
Summary ?Observational data collected in the Lake Tekapo hydro catchment of the Southern Alps in New Zealand are used to analyse the
wind and temperature fields in the alpine lake basin during summertime fair weather conditions. Measurements from surface
stations, pilot balloon and tethersonde soundings, Doppler sodar and an instrumented light aircraft provide evidence of multi-scale
interacting wind systems, ranging from microscale slope winds to mesoscale coast-to-basin flows. Thermal forcing of the winds
occurred due to differential heating as a consequence of orography and heterogeneous surface features, which is quantified
by heat budget and pressure field analysis.
The daytime vertical temperature structure was characterised by distinct layering. Features of particular interest are the
formation of thermal internal boundary layers due to the lake-land discontinuity and the development of elevated mixed layers.
The latter were generated by advective heating from the basin and valley sidewalls by slope winds and by a superimposed valley
wind blowing from the basin over Lake Tekapo and up the tributary Godley Valley.
Daytime heating in the basin and its tributary valleys caused the development of a strong horizontal temperature gradient
between the basin atmosphere and that over the surrounding landscape, and hence the development of a mesoscale heat low over
the basin. After noon, air from outside the basin started flowing over mountain saddles into the basin causing cooling in
the lowest layers, whereas at ridge top height the horizontal air temperature gradient between inside and outside the basin
continued to increase.
In the early evening, a more massive intrusion of cold air caused rapid cooling and a transition to a rather uniform slightly
stable stratification up to about 2000 m agl. The onset time of this rapid cooling varied about 1–2 h between observation
sites and was probably triggered by the decay of up-slope winds inside the basin, which previously countered the intrusion
of air over the surrounding ridges. The intrusion of air from outside the basin continued until about mid-night, when a northerly
mountain wind from the Godley Valley became dominant. The results illustrate the extreme complexity that can be caused by
the operation of thermal forcing processes at a wide range of spatial scales.
Received June 25, 2001; Revised December 21, 2001 相似文献
7.
8.
条件不稳定大气中二维小尺度双脊地形上空对流及降水特征 总被引：3，自引：1，他引：2
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
9.
Based on observational meteorological data since A.D. 1864 and tree-ring records of debris-flow activity, this paper assesses changes in rainfall characteristics and their impact on the triggering of geomorphic events in a high-elevation watershed of the Swiss Alps since the end of the Little Ice Age. No trends are visible in the frequency of heavy rainfall events, but we observe a reduced number of heavy, short-lived rainfalls in summer and a concentration of advective storms is recorded in late summer and early fall since the late 1980s. These changes in triggering meteorological conditions resulted in a cluster of debris flows in the early decades of the twentieth century and a lowering of debris-flow activity since the mid 1990s, and may be mirroring the observed changes in persistent high-pressure systems over the Alps. We also observe intra-seasonal differences in debris-flow system response reflecting the state of the permafrost body in the source area of debris flows, allowing for very small debris flows to be released by limited rainfall inputs (<20 mm) in June and July. The same quantities of rain will not trigger debris flows in August or September, when a thick active layer of the permafrost body is capable of absorbing water. With the projected amplitude of climatic change, seasonality, return intervals and volumes of debris flows are likely to be altered. RCM projections based on the IPCC A2 scenario suggest a decrease in heavy summer rainfalls which will most likely result in a (further) reduction of the overall frequency of debris flows, leaving more time for sediment to accumulate in the channel. Such an increase of channel accumulation rates along with the projected destabilization of the steep rock-glacier body is likely, in turn, to exert control ultimately on sediment volumes released from the source areas during future events. Observations from adjacent catchments suggest that extremely large debris flows, beyond historical experience, could occur at the study site and in similar debris-flow systems of the Valais Alps originating from periglacial environments. 相似文献
10.
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
11.
条件不稳定湿大气中三维理想地形上空对流的动力学特征 总被引：5，自引：1，他引：4
条件不稳定湿大气情况下,气流经过三维地形可以形成不同性质的对流系统以及不同特征的地形流结构,其对流系统、地形流的性质主要取决于地形上空的对流触发、对流-地形流-重力波三者之间的相互作用,同样这些过程对于地形降水的性质、分布起重要的作用.根据不同湿Fr数(Froude number),湿条件不稳定大气经过三维小尺度山地上空时其对流和地形流动存在4种不同的流域(flow regirnes):(1)下游传播对流模态;(2)上游传播和下游传播共存对流模态;(3)山峰附近准静止和下游传播共存对流模态;(4)下坡稳定和下游传播对流共存模态.地形上空对流系统主要可以通过两种不同机制形成:(1)地形直接的抬升或减速作用;(2)在地形流形成后,由于地形流本身特性(如上游分离、背风涡旋和下坡重力波破碎)触发.在较大的Fr数情况下,地形上空对流生成后反过来可以破坏上、下游的地形流结构,但对背风坡的重力波破碎影响较小.不同初始对流有效位能(CAPE)不仅可以影响对流系统的传播、发展,而且可以影响整体地形流性质.较低的初始CAPE有利于地形流的形成,此时对流对地形流结构特征的影响相对较小,其流场性质与低Fr数流域性质相似. 相似文献
12.
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
13.
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
14.
During the movement of Typhoon Hato (2017) over land, heavy rainfall occurred when the spiral rainband which was about 100 km distance away from the center of the typhoon passed the Dayao Mountain (with an elevation of 1.2 km). In this study, the structures and forming mechanism of the heavy rainband along the mountain range are investigated by using high-resolution model simulations. The results show the importance of topography in causing the heavy rainband. Upslope of the steep terrain lifts the cyclonic flow to produce strong upward motion when the rainband passes across with high wind speed. At the same time, the warm and humid air is lifted to the steep slope, causing unstable energy to accumulate over the windward slope, which is conducive to the occurrence of rainfall. In particular, the convective cells generated upstream of rainband will further strengthen and develop due to the uplift when they move close to the mountain foot. Some precipitation particles in the convective cells fall to the ground while others move downstream with the intense updrafts, forming heavy rainfall near the summit. As a result, the largest accumulative rainfall coincides well with the orientation of the mountain ridge. 相似文献
15.
钟形地形动力抬升和重力波传播与地形云和降水形成关系研究 总被引：4，自引：0，他引：4
地形云和降水过程在区域水循环、水资源、生态环境及气候变化中具有十分重要的作用。本文利用中尺度数值模式WRF 数值模拟试验,以及通过引入表示大气层流速度、层结稳定度和地形特征的关系参数——湿Froude 数(Fw),研究了北京2009 年5 月1 日湿条件不稳定大气层结下,地形云和降水形成过程与地形动力抬升和地形重力波传播之间的关系及形成机理。研究表明,在地形最大高度2 km、半宽10 km 的条件下,层流速度从2.5 m/s 逐步增加到25 m/s 时,对应的湿Fw 数从0.19 增加到1.81。当Fw≤1 时,地形的阻挡起主要作用,由地形抬升形成的地形云主要产生在迎风坡一侧。地形重力波主要产生在迎风坡,并向上游传播,先形成层状云,最后演变为准稳定浅对流波状云。最大降水主要发生在紧靠山顶的迎风坡一侧,但当Fw 很小时,地形云不产生降水。当Fw>1 时,地形抬升形成的云主要发生在山顶附近,而地形重力波主要形成在背风坡,并向下游方向传播,形成准稳定波状云。最大降水主要产生在紧靠山顶的背风坡一侧。另外,在弱湿条件不稳定大气层流下,地形降水主要由地形动力抬升造成的暖云微物理过程产生,地形重力波形成的波状云几乎不产生降水。 相似文献
16.
M. H. Memarian V. N. Kozhevnikov L. R. Dmitrieva-Arrago 《Russian Meteorology and Hydrology》2009,34(9):604-612
Main characteristics of the atmosphere disturbances over the mountains are calculated with a nonlinear two-dimensional stationary
model of the airstream over mountains depending on the properties of the unperturbed upstream. A spatial humidity distribution
over mountains is defined under assumption that water vapor is transported with airstreams like a passive pollutant. The derived
humidity fields and calculated temperature distribution are used for determining the spatial relative humidity distribution
over mountains. Critical values of the relative humidity in the interval of 70–100% are used for identifying the cloud zone
boundaries. The analysis is performed on the cloud zone dependence on vertical distribution of relative humidity in the upstream
and on the mountain form characteristics. The studies are conducted with a special reference to a complicated mountain relief
in Southern Zagros (Iran). 相似文献
17.
南海至西太平洋一带夏季低空越赤道气流和季风的初步研究 总被引：4，自引：3，他引：4
本文对南海至西太平洋一带夏季低空越赤道气流的情况和西南季风的来源,进行了初步研究。发现:(1)就气候平均而言,东非低空急流的影响范围,包括印度南部、孟加拉湾南部直到中南半岛南部和南海南部。在这一范围内,夏季月平均西南季风强度的年际变化十分一致;(2)夏季在中南半岛南部、南海南部,西南季风的主要来源是上游印度、孟加拉湾地区,直接来自南半球的气流比重不大。而热带西北太平洋的西南季风,则主要来自南半球;(3)在110-140°E 的赤道地区,并不存在一支主要的越赤道气流;(4)在150°E 附近的新几内亚东岸,有一条越赤道气流的通道。热带西北太平洋的西南季风,主要就是这支越赤道气流转向而成(但似乎要求这支气流的南风分量强度超过某一下限,即存在一阈值,才能对西北太平洋的西南季风变化有影响)。新几内亚岛上的山脉,对南半球东南信风的阻挡,是形成这支越赤道气流的重要原因之一;(5)大致在15°N 以南的南亚至西北太平洋地区,其西南季风主要由二支气流构成:一支在非洲东岸附近越过赤道,成为东非低空急流,经印度南部,往下游一直影响到南海南部;另一支在新几内亚东岸附近越过赤道,转向成西南气流影响西北太平洋。 相似文献
18.
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
19.
To assess the impacts of future climatic change, we need a better understanding of the relationships between debris flows dynamic and the climate. The subject of this paper is the variability in the response of debris flows to climatic change in recent decades in the Massif des Ecrins (French Alps). The climatic trend observed in the French Alps was characterized by analyzing data on extreme summer rainfall events recorded daily at nine stations located all over the Massif des Ecrins since 1960. According to the generalized Pareto law (GPD) our results showed that extreme summer rainfalls have increased significantly in the Massif des Ecrins since the 1980s. Statistical tests showed a significant increase in average annual air temperature. The response of 647 hill slope debris flows to this climatic change was investigated. Different types of hill slope debris flows were analyzed as a function of their lithology or the nature of the accumulated debris. A number of logistic regression based models were developed to characterize the relationship between climate and the occurrence of debris flows in a specific geomorphological context. Analysis of frequency and return period over the last two decades showed different patterns depending on the type of debris flow concerned. Hence, the response of hill slope debris flows to climatic change is not the same everywhere in the Massif des Ecrins. 相似文献
20.
Summary. ?The airflow over an idealized orography with two mountain peaks and a valley between is investigated using a non-linear numerical
model. The flow is assumed to be two-dimensional and nonrotational. Surface friction is neglected. This setup is a first step
in studying the modifications a finely structured “real” topography introduces to the well-studied flow over one isolated
obstacle. The sensitivity of the flow behavior to the valley width is examined for the case of specified mountain volume as
well as constant non-dimensional mountain height. Flow patterns for linear, weakly nonlinear, wave breaking and upstream blocking
cases are examined. Whereas the nondimensional mountain height is still the main measure of the nonlinearity of the flow,
the differing steepness of upslope and downslope caused by the separating valley, strengthens nonlinear effects. It also modifies
wave breaking and upstream blocking. For wide enough valleys wave breaking regions can form above both peaks.
Received January 20, 1999/Revised June 28, 1999 相似文献