Numerical experiments investigating the orographic effects on a heavy rainfall event over the northwestern coast of Taiwan during TAMEX IOP 13

An unusual heavy coastal rainfall event (>231?mm?day^?1) occurred during the period of 24?C25 June 1987 over the lowland (elevation less than 200?m) and coastal areas in northwest and central Taiwan. The Weather Research and Forecasting (WRF) model is used to investigate the role of synoptic forcing, orographic effects and the diurnal heating cycle on the generation of a prefrontal localized low-level convergence zone offshore leading to the observed coastal rainfall maximum. This case is well simulated by the control experiment initialized at 0000 UTC (0800 LST) 24 June 1987 using the European Centre for Medium-Range Weather Forecasts data. A model sensitivity test without Taiwan??s terrain fails to reproduce the observed coastal rainfall maximum. It is apparent that for this case, synoptic forcing by the Mei-Yu jet/front system is inadequate to initiate deep convection leading to the development of coastal heavy precipitation. The generation of the localized low-level convergence zone is closely related to the simulated strong winds with a large southwesterly wind component (or the barrier jet) along the northwestern coast as the surface front approaches. The development of the simulated barrier jet is due to a 50?C60% increase in the meridional pressure gradient as a result of orographic blocking. The diurnal heating cycle also impacts the strength of the simulated barrier jet over the northwestern Taiwan coast. The simulated barrier jet is stronger (~3?m?s^?1) in the early morning than in the afternoon as orographic blocking is most significant when the surface air is the coldest. The representation of the terrain in the model impacts the simulated barrier jet and rainfall. With a coarse horizontal resolution (45?km), orographic blocking is less significant than the control run with a much weaker meridional wind component over the northwestern coast of Taiwan. The coarse resolution model fails to reproduce the observed rainband off the northwestern coast. Thus, to successfully simulate this type of event, high-resolution mesoscale models adequately depicting Taiwan??s terrain are required.     

Diurnal and seasonal variation in surface wind at Sita Eliya, Sri Lanka

Summary Hourly measurements of surface winds at Sita Eliya (6° 58′ N, 80° 46′ E, 1860 m a.s.l.) located atop the North-South mountain ridge in Sri Lanka were analyzed to investigate the diurnal and seasonal variation in the wind climate. Surface winds are dominated by the monsoon regimes, with Northeasterlies from November to January, Southwesterlies from February to May and Westerlies and Northwesterlies from June to October. Through most of the year, the average wind speed is around 6–8 m/s. However from June to August, it is around 10–14 m/s. Wind in June is gusty due the location of the low-level Easterly jet over Sri Lanka. The wind undergoes a reversal in both zonal and meridional directions in March and November coincident with the migration of the Inter-Tropical Convergence Zone. Notwithstanding the period from May to September being designated as the Southwest monsoon, the wind is from West, South-West-West and North-North-West. During the Southwest monsoon, wind speed during the night is nearly as high as that during the day. This anomalous diurnal variation in wind speeds may be related to orographic influences. The high wind speeds at Sita Eliya, in conjunction with the moderate diurnal and seasonal variability in wind speed, is suitable for wind-energy generation. Received January 2, 2001 Revised May 26, 2001     

Aspects of the Fine-Scale Climatology Over Lake Tanganyika as Resolved by a Mesoscale Model

Summary  An hourly averaged climatology at 0.05 ° horizontal resolution over the Lake Tanganyika region was created by making simulations with a mesoscale model (HIRLAM) using a high resolution physiography to represent the surface. Initial and boundary values were interpolated from ECMWF analyses. Climatologies for a typical dry season month (July 1994) and wet season month (March 1994) were created by 7-day segmenting. Model results were validated by utilizing a special coastal observation network. A number of experiments were made with changes to the physiography (mountains/no mountains, lake/no lake). The results reveal local channelling and blocking effects of the near-surface southeasterly trade winds by the high mountain chains in the region of the East African rift. Furthermore, surface winds display regular diurnal cycles in many places, due to slope winds over hills and lake-land-type breezes near the coast. The diurnal coastal winds (defined by the observation network) are reasonably well simulated. Precipitation patterns display the semi-annual march of the ITCZ across the area, plus considerable topographic effects. There is high evaporation from lakes and wetlands during the windy dry season, while evaporation from the moist land surface dominates the rainy season. Received October 15, 1998/Revised September 2, 1999     

Regional and local scale characteristics of foehn wind events over the South Island of New Zealand

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     

Local foehn effects in the Upper Isar Valley,Part 3: Additional investigations

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.     

Waves to the lee of a meso-scale elliptic orographic barrier

Summary ?The paper considers a meso-scale, adiabatic, inviscid and Boussisnesq flow of a stably stratified fluid over a three-dimensional (3-D) meso-scale orographic barrier with elliptic contour, with special reference to a part of the Western ghats mountain along west coast of India and on the Khasi-Jayantia hill in the northeast India. The airstream characteristics are simplified by assuming that the upstream wind velocity (U) and buoyancy frequency (N) are constant with height. Solutions for perturbation vertical velocity (w′) and streamline displacement (η′) are expressed in terms of double integrals. These integrals cannot be evaluated exactly, hence they have been approximated by asymptotic expansion method. Side by side solutions using numerical method have also been obtained. The results of the study indicate that the updraft regions in the asymptotic solution are crescent shaped, symmetrical about the axis y = 0, tilting upwind and spreading laterally with height. The study also shows that in both asymptotic solution and numerical solution w′ and η′ fall off down wind of the barrier in the central plane (y = 0), further more in the asymptotic solution w′ and η′ fall off as x ⁻¹. The study also indicates that the discrete updraft regions obtained in the numerical solution, when joined, take a crescent shape. Received November 26, 2001; accepted February 27, 2002     

A multi-scale simulation of an extreme downslope windstorm over complex topography

Summary A severe localized windstorm, with near-surface winds > 60 ms⁻¹, occurred in an isolated valley within the Alpine mountains (> 1800 m) of central Norway on 31 January 1995. A multi-scale numerical simulation of the event was performed with the Naval Research Laboratory (NRL)’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS), configured with four nested grids telescoping down to 1-km horizontal resolution. The windstorm occurred in response to topographic blocking and deformation of a lower-tropospheric warm front and attendant jet (> 35 ms⁻¹ at 2 km). The key findings are: i) mountain wave resonance and amplification arising from the interaction of the surface-based front and jet with complex orography, ii) sensitivity of the wave response to differential diabatic heating (vertical) gradients above the front, and iii) trapped response within the layer of large frontal stratification in the lower troposphere and subsequent amplification consistent with the theoretically-established two-layer windstorm analogue of Durran (1986). Received September 29, 1999 Revised December 30, 1999     

Relationship of Late-Winter Temperatures in Europeto North Atlantic Surface Winds: A Correlation Analysis

Summary  The relationship between European surface temperature and winds over the eastern North Atlantic are investigated for the years 1988 to 1997. Daily Special Sensor Microwave Imager SSM/I observations are used to evaluate a monthly surface wind index that quantifies the influence of southwesterly flow. Our wind index and the monthly-mean surface-air temperatures in late winter and early spring over France and northern-latitude Europe are highly correlated. In February, the year-to-year increases/decreases match every year for France (correlation of 0.82 with the Index); and every year with just one exception for Europe (correlation with the Index of 0.76 for a longitudinal strip through Europe 45–50° N, and 0.73 for the 50–60° N strip). In March, the increases/decreases of the wind Index and of the temperatures for France also match, but the correlation with the Index is lower, 0.65. The high correlation between our Index and the large interannual fluctuations in the monthly temperature in late winter and early spring indicate that the onset of the spring conditions in Europe is significantly influenced by the wind patterns over the eastern North Atlantic. Coinciding with the fluctuations from warm-Europe/high-Index winter to the opposite conditions, we observe “seesaw” effects, fluctuations over the North Atlantic, in opposite directions in the east (25–5° W), and the west (65–45° W). In the low-Index years we find that: (a) the surface-air temperatures in the west are appreciably higher than in the east (but slightly lower in the high-Index year), and (b) the difference between the 500 mb meridional wind in the west and that in the east is positive and large, exceeding 10 m s⁻¹ (but it becomes negative and small in the high-Index years). The “seesaw” effects suggest that a positive feedback exits between these cross-Atlantic temperature differences and the surface winds. Received August 7, 1998 Revised April 23, 1999     

天山北坡一次特大暴雨过程诊断分析

利用常规资料、NCEP1°×1°再分析资料对2007年7月16—17日天山北坡出现的特大暴雨过程进行了诊断分析。结果表明：暴雨发生在100hPa南亚高压呈双体型、500 hPa中亚低值系统在天山北坡东段稳定维持的环流背景下;沿天山两侧的强东西风切变组织了系统性的强上升运动、低层向山的偏北气流与天山北坡向北开口的喇叭口地形产生的辐合上升运动促使暴雨强度增大;暴雨区与低层辐合、高层辐散的气旋性涡柱区相对应;来自孟加拉湾沿高原东侧偏南气流北上的水汽,通过中高层的西南气流和低空的偏东气流两条路径接力输送到暴雨区;正位涡异常与暴雨的发生关系密切。     

太行山东麓一次强对流降雹过程中的地形强迫

利用WRF模式对2011年5月26日发生在太行山东麓的一次强对流降雹过程进行数值模拟,探讨了太行山及周边地形在本次强对流过程的作用。结果表明,控制华北平原的偏东暖湿气流受太行山阻挡并与切变线东南侧的西南暖湿气流汇合,在太行山东侧形成水汽高值区。太行山东坡下垫面向上热通量明显高于华北平原,午后850hPa高度山坡与平原的假相当位温梯度达到0.2 K·km~(-1),850~600 hPa假相当位温垂直梯度达4 K·km~(-1),对应上坡风的垂直速度大于1 m·s~(-1),热力环流为太行山东麓对流的发生提供了动力条件。太行山东侧暖湿气层之上为偏西干冷气流,由此形成的强热力不稳定与水汽高值区、上坡风共同造成太行山东麓强对流过程的发生。局地小尺度地形抬升与重力波共同促使太原盆地有对流单体生成,该单体移经太行山西侧迎风坡受阻挡抬升而增强,越过山顶后与维持在太行山东侧的对流单体发生合并,从而导致对流云的强烈发展。     

The Multi-Scale Structure of the Australian Cool Changes

Summary In Southern Australia summertime deep cold fronts are frequently preceded by a shallow cold frontal line connected to a prefrontal lower tropospheric trough. The advance of this line defines a “cool change” which in many cases causes severe weather events. The goal of this paper is to analyze the multi-scale structure of these cool changes using aircraft observations and synoptic-scale analyses. The aircraft measurements on cross-frontal tracks of horizontal lengths of up to 300 km are performed with an average resolution of 3 to 4 m along the track. Thus a multi-scale analysis from micro-scale events up to the synoptic-scale phenomena can be presented. All flights and thus all meso- and micro-scale analyses are performed over water only. The obviously very different characteristics of the cool change structure elements over land are not investigated. The synoptic analyses for one very typical case show a prefrontal trough as characterized by its position in relation to the main deep cold front, its source region in Western Australia and its extent to the southeast. Fields of strong wind shear, temperature gradients, vertical wind and Q-vectors are displayed. The meso-β-scale x, z-cross-sections derived from two aircraft missions (data of the second one in brackets) show: a shallow cold front with a 160 (60) km wide transition zone in which the near surface potential temperature drops rather steadily by 9 °C (20 °C); a shallow feeder flow topped by a strong inversion with a vertical gradient of potential temperature up to 5 °C/100 m between the top of the feeder flow at 400 (200) m and 1500 (700) m; a cross-frontal circulation expressed by the ageostrophic wind components u _ϕ,subscale and w with a center at 1200 m over the frontal edge of the feeder flow (for one mission only); a strong shear of the along-frontal wind component v _ϕ with a large increase of the negative v _ϕ-values with height, which very well fits to the synoptic-scale view of the wave structure of the geostrophic wind (well-known from the upper level synoptic charts) at different heights; a jet core of this along-frontal wind in the center of the cross-frontal circulation, again for one mission only. A very striking example of a micro-scale event is an approximately 1 km wide head of a frontal squall line. It shows dramatic changes of all meteorological parameters. The event is displayed in a horizontal domain of 4 km with full resolution (∼ 4 m). Derivatives of the measured parameters in the cross-frontal direction add information to the space series of the parameters themselves. Deformation frontogenesis of potential temperature and specific humidity show very large values on the scale resolved here. Fortunately the squall line could be sampled again at the same height, but in a somewhat degenerated state 1? h later. Received September 3, 1999 Revised December 14, 1999     

Analysis of the wind field and heat budget in an alpine lake basin during summertime fair weather conditions

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     

Modification of the local winds due to hypothetical urbanization of the Zagreb surroundings

Summary The aim of this study was to investigate possible effects of two hypothetical scenarios of the urbanization of Zagreb’s surroundings on the local winds, which are established under summertime anticyclonic conditions. For this purpose, the nonhydrostatic mesoscale meteorological model MEMO was applied to the greater Zagreb area. Three simulations were performed. One employed the current land-use distribution, while the other two corresponded to an increase of the densely urbanized area by 12.5% (test 1) and 37.5% (test 2), respectively. Apart from the hypothetically urbanized areas, where average surface wind speed reductions of 8% and 18% were obtained for test 1 and test 2, respectively, the rest of the domain was not significantly affected by hypothetical urbanization. The differences between the wind vectors for the predicted current state and the hypothetical state were more pronounced and found at higher altitudes during the night compared to daytime values. For all three simulations the same diurnal variation of the depth of anabatic/katabatic wind flow generated on south-facing slopes of 1 km high mountain Medvednica was obtained. During the night the depth of well-developed katabatic flow was about 370 m, while during the day the depth of anabatic flow grew from about 550 m in the late morning up to about 1140 m in the late afternoon. Received October 27, 2000 Revised August 4, 2001     

灾害性大风发生机理与飑线结构特征的个例分析模拟研究

2009年6月3日在我国河南发生了历史罕见的强飑线天气过程, 造成了严重的人员伤亡和灾害。为了解此次飑线天气的特征和产生的机理, 本文采用卫星、雷达及地面加密观测资料, 结合中尺度WRF(Weather Research and Forecasting)数值模式, 研究了此次飑线产生的天气背景、宏微观结构特征及造成灾害性大风的机理。结果表明, 此次飑线过程的主要影响系统是东北冷涡, 其后部横槽引导的南下冷空气与西南暖湿气流在河南新乡南部一带交汇促发强对流过程, 最后演变为飑线。但由于低层西南风偏弱, 水汽条件不足, 飑线发生的环境较为干冷。飑线产生区大气处于条件性不稳定状态, 对流有效位能(CAPE, Convective Available Potential Energy)在1300 J/kg左右, 并具有适平的垂直风切变。地面气象场显示飑线具有相对冷湿的雷暴高压和强冷池, 飑线过程产生的灾害性天气以大风而非强降水为主。数值模式结果显示飑线下沉气流的最大值仅为-13 m/s, 而地面风速最大值达到35 m/s, 是最大下沉气流的2.7倍。进一步的数值敏感试验表明, 降水粒子的蒸发和融化冷却过程对降低地面温度和产生地面强风速具有重要影响, 其中雨水蒸发过程产生的最大等效冷却率为-3 K/min, 远大于霰融化冷却率-0.7 K/min, 因此雨水蒸发过程是影响冷池强度的关键因素, 而地面强冷池在此次飑线灾害性大风的产生中具有重要作用。     

宜昌一次致灾极端短时强降水成因分析

利用常规气象观测资料、地面加密自动站资料、NCEP 1°×1°再分析资料、卫星及风廓线雷达和多普勒雷达资料,对2016年7月7日夜间湖北宜昌地区一次致灾极端短时强降水过程,从大尺度环流背景、中尺度特征以及地形等方面进行分析。结果表明:这次局地强降水产生于副热带高压边缘的西南暖湿气流中,表现出中低层中尺度动力抬升强、降水效率高、地形作用明显等特点。峡谷入口处地面中尺度涡旋与强垂直风切变相互作用造成强上升运动为强降水提供了充足的动力条件,较弱的引导气流和山体阻挡作用使得局地降水维持时间长,共同造成了此次极端短时强降水的发生。回波的低质心结构提高了降水效率,降水过程中单体的后向传播也使局地累计雨量增大。     

Evolution of the atmospheric boundary-layer structure of an arid Andes Valley

Summary The boundary-layer structure of the Elqui Valley is investigated, which is situated in the arid north of Chile and extends from the Pacific Ocean in the west to the Andes in the east. The climate is dominated by the south-eastern Pacific subtropical anticyclone and the cold Humboldt Current. This combination leads to considerable temperature and moisture gradients between the coast and the valley and results in the evolution of sea and valley wind systems. The contribution of these mesoscale wind systems to the heat and moisture budget of the valley atmosphere is estimated, based on radiosoundings performed near the coast and in the valley. Near the coast, a well-mixed cloud-topped boundary layer exists. Both, the temperature and the specific humidity do not change considerably during the day. In the stratus layer the potential temperature increases, while the specific humidity decreases slightly with height. The top of the thin stratus layer, about 300 m in depth, is marked by an inversion. Moderate sea breeze winds of 3–4 m s⁻¹ prevail in the sub-cloud and cloud layer during daytime, but no land breeze develops during the night. The nocturnal valley atmosphere is characterized by a strong and 900 m deep stably stratified boundary layer. During the day, no pronounced well-mixed layer with a capping inversion develops in the valley. Above a super-adiabatic surface layer of about 150 m depth, a stably stratified layer prevails throughout the day. However, heating can be observed within a layer above the surface 800 m deep. Heat and moisture budget estimations show that sensible heat flux convergence exceeds cold air advection in the morning, while both processes compensate each other around noon, such that the temperature evolution stagnates. In the afternoon, cold air advection predominates and leads to net cooling of the boundary layer. Furthermore, the advection of moist air results in the accumulation of moisture during the noon and afternoon period, while latent heat flux convergence is of minor relevance to the moisture budget of the boundary layer. Correspondence: Norbert Kalthoff, Institut für Meteorologie und Klimaforschung, Universit?t Karlsruhe/Forschungszentrum Karlsruhe, Postfach 3640, 76021 Karlsruhe, Germany     

Small-scale dynamics of the south foehn in the lower Wipp Valley

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.     

乌鲁木齐机场东南大风数值预报及地形敏感性试验

利用WRF模式和GFS资料对乌鲁木齐机场一次东南大风天气进行了预报和地形敏感性试验。模式预报的结果表明:WRF模式对东南大风的起风时间、持续时间、风速大小等方面有较强的预报能力。地形敏感性试验表明:1乌鲁木齐市区与机场300m左右的高度差对机场风速的影响很大;2机场上空下沉运动的强弱与东南大风的强弱有很好的对应关系;3机场东南大风的风速变化并不总是与峡谷两端气压梯度力的变化同步。此次东南大风天气的产生是低空动量下传、狭管效应和下坡风共同作用的结果。动量下传主要出现在海拔2000m以下的高度,下坡风主要出现在海拔935m以下的高度。     

Local foehn effects in the upper Isar Valley, Part 2: numerical simulations

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.     

Wind Regimes Windward of a Shelterbelt Protecting Gravity Irrigated Crop Land from Moving Sand in the Gezira Scheme (Sudan)

Summary  The work reported on was part of a more comprehensive study dealing with threatening sand movement towards the northwestern part of the Gezira irrigation scheme, Sudan. An irrigated sand blocking shelterbelt Eucalyptus microtheca is used to prevent canals and gravity irrigated crop land to be further affected. The objective of this paper is to report on the wind data that were obtained, mainly windward of this belt, and to deal with their implications and those of the belt characteristics and deposited sand in interaction with the wind for combating local sand movements and further desertification. Outside the influence of the belt, wind speed over eroded land was increasingly higher than that over irrigated land with the growth of cotton. Both these values were considerably higher than those reported from the nearest meteorological stations. The two prevailing winds, of which the summer SW wind was perpendicular to the belt, had consistent annual patterns during three years of observation. Also the diurnal patterns were very reproducible. Wind speed within the belt, at 1 m height, showed fast reduction inwards above the deposited sand. Efforts to control moving sand in the source area should be joined with those made at the borders of agricultural land. Received August 20, 1997 Revised September 30, 1998