大地形上空地转基流中波动的传播和稳定性

研究表明，地形北坡上空东风基本气流中的重力惯性波最易出现不稳定发展，地形坡度大于１‰的地形南坡上空强西风气流中重力惯性波和地形罗斯贝波都是不稳定的。只有在西风气流中重力惯性波才会出现明显的相对基本气流的东、西传播现象。     

地形影响下冷锋的变形及锢囚

根据传统理论，将锋面看成密度的零级不连续面，设计了一个二层锋面模式，分别研究了东西走向与东北－西南走向的地形影响下冷锋的变形及锢囚现象。分析表明：不同高度的地形对于冷锋变形及锢囚的影响不同，同一高度的地形对于不同强度的冷锋运动也有不同的影响。通过分析研究，形象、直观地揭示了地形影响下冷锋变形及锢囚的一些规律，并与实际天气现象做了一些对比。此外，还分析了山区冷锋的垂直结构，指出冷锋在爬山时锋面中层会出现“隆起”现象，这反映了地形对于冷锋的非线性相互作用；锋面的这种“隆起”现象非常类似于气流过山时形成的“涌浪”（ｂｏｒｅ）。     

二层模式中小地形对于气压跳跃形成的初步研究

本文应用一个两层密度不同的流体所组成的模式,讨论了小地形对于气流的影响。由一维常定问题的解指出,对于气流一定的上游条件,山脉存在一临界高度;山区气流的被扰状态,决定于上游条件和山的高度。当上游状态中的弗罗德(Froude)数小于1,而山脉的高度等于临界高度时,背风面将出现常定的“气压跳跃”。也由一维非常定的解,研究了“气压跳跃”的形成过程。     

过山气流二维定常模式的实际应用

本文对气流流经三种不同水平宽度的地形所产生的动力学效应进行了研究。应用定常的二维模式作数值模拟,结果表明:大尺度地形可影响到山脊上游的大气运动,山脊上游出现下沉气流,造成干燥气候;中尺度山脉促使下游大气产生波状扰动,可影响飞机飞行;小尺度山丘(或建筑群)对气流中的气溶胶扩散会产生影响。充分孓解小尺度地形气流的特点,可给山区城镇工厂烟囱的合理布局以及作物育苗阶段的秧田选地提供依据。     

山东特大暴雨雷达回波及地形作用分析

利用常规、非常规资料对2009年8月17—18日山东省出现的特大暴雨过程进行分析。结果表明:“817”特大暴雨主要出现在西风槽东移、副高南退时的副高西北侧边缘,对流有效位能(CAPE)等热力环境指数对鲁南强降水有较强的反应能力。降水分布具有典型的地形影响特征,复杂山地地形对降水有显著增幅作用和对对流系统传播的阻碍作用。存在低空急流和列车效应,强回波维持时间较长是费县出现强降水的重要原因,较强的偏南气流、湿度层深厚是费县出现特大暴雨的根本原因。地形影响气流理论很好地解释了山东“817”特大暴雨降水分布,费县特殊地形对特大暴雨有很大贡献。雷达资料可以弥补常规观测的不足;地形对强降雨及其分布有重要影响。     

层结大气中Froude数对过山气流的影响

本文建立了一个层结大气中二维过山气流模式，对层结大气的Ｆｒｏｕｄｅ数对过山气气流的影响进行了数值研究，通过地形上空流场，位、场以平行于山 风场的分析，探讨了Ｆｒｏｄｕｄｅ数影响地形上空大气运动的一些规律。     

用遮蔽图法确定不规则地形的日照时数

地形对小气候的影响,主要表现在两个方面。一是周围地形的遮蔽作用,造成太阳照射时间和辐射强度与平坦地形不一样;二是地形对气流的影响,使不同地形的热量、水分收支有差异。结果,地形差异形成了山地、丘陵地带多种的、特殊的“立体气候”。     

三维非静力E—ε闭合模式对山体流场及浓度分布的模拟

本文采用三维非静力E-ε闭合模式,在对流层边界层方程组中,加入湍能和耗散率方程描述边界层中的湍流运动。求解孤立三维山地地形上的流场、湍流场和浓度场,并分析地形对气流的影响及对污染物浓度分布的影响。同时做了不同坡度和不同风速下的孤立三维山体的流场、湍流场和浓度场比较。数值计算结果表明,地形能改变气流运动,气流过山时将出现分支现象,山后背坡有反向回流出现,并且回流区大小随着山体坡度的增大而增大。山体对气流的阻挡作用随山体高度的增加而更明显,随风速的增加而减弱。湍能及耗散率随高度衰减较快。随着山体坡度增加,山前气流的垂直速度也增加,湍流运动更强。风速大时湍流运动越显著。污染物在山前出现辐散,迎风坡是浓度高值区,山后背风坡也是相对高值区。山坡度越大,山前迎风坡污物浓度越高。     

台湾岛地形对台风“海棠”(0505)移动路径影响的数值试验研究

利用非静力模式MM5模拟台风“海棠”（0505）穿过台湾岛再次登陆的移动路径，分析了“海棠”登陆台湾岛前后结构特征变化。结果表明：台风自身的非对称结构与台风异常移动路径密切相关。另外，就台湾岛地形对台风“海棠”登陆台湾前打转和在台湾海峡出现“V”型移动异常路径影响进行数值试验表明：台湾岛地形不但可以直接影响台风移动路径，而且通过影响台风非对称结构来改变台风移动路径，因此，登陆台湾前逆时针打转异常路径是在弱引导气流中台风自身非对称结构和台湾岛地形共同作用的结果；台湾岛地形有使台风东北-西南向非对称增大趋势，而在台风进入台湾海峡前后对东南。西北向非对称有明显不同影响。     

切变气流中地形强迫激发的非线性长波

经推导得到了包括地形和耗散的ＦＫｄＶ－Ｂｕｒｇｅｒｓ方程,利用数值解讨论了地表强迫激发的弧波演变以及移动性孤波与地形的相互作用,结果显示,对于α〉０或α〈０,不论是在气旋式切变气流还是反气旋式切变气流中,地形都能在强迫激发出定常孤波,在其下游产生调制椭圆余弦波列的背风波,当α＝０时,在反气旋式切变气流中强迫区产生的是大振幅定常孤波,气旋式切变气流中强迫区产生的是复杂的非定常孤波,地形强迫产生的ｍ＝     

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

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.     

Blocking in periodic valleys

Results are presented from a study of blocked flow (practically stagnant or recirculating light winds) in periodic valleys in thermally stably stratified ambient conditions. Inviscid and turbulent diffusion cases were modelled numerically to clarify the effects of turbulence on the blocking. The reflection of gravity waves from the top boundary of the hydrostatic model atmosphere was avoided by employing the radiation condition given by Klemp and Durran (1983). The dissipative numerical results are compared with new laboratory experiments which utilized the technique of Baines and Hoinka (1985) to simulate a semi-infinitely deep region.A criterion for the occurrence of blocked flow cannot be defined for the inviscid case except when the Froude number, Fr, based on the peak-to-trough ridge amplitude is less than about 0.4: then blocking is clearly identifiable before wave-breaking occurs. Breaking of waves is evident for Fr as large as 0.75, in agreement with analytical results given by Lilly and Klemp (1979).At small Froude number (Fr 0.5) in the dissipative flow simulations, blocked flow (stagnation) is present in the valleys, but a lee rotor (complete stagnation) is not evident. For order unity Froude numbers, blocking is a wave phenomenon, resulting from wave steepening and overturning or turbulent mixing. A finite thickness is brought to rest or participates in a recirculating flow when it first appears. A strong upward flow appears ahead of the rotor in the valleys, and the downslope wind over the windward side of the valleys is strengthened. Thus the present study shows that conditions for the onset of a rotor, and of stagnant flow, in periodic valleys are different.When blocked flow exists, the amplitudes of gravity waves in the upper layer are only 15% (Fr = 0.3) to 80% (Fr = 1.5) of those given by linear theory; this is supported by observations.     

Orographic effects on a conditionally unstable flow over an idealized three-dimensional mesoscale mountain

Summary Idealized numerical simulations using the Weather and Research Forecast (WRF) model indicate that three flow regimes, based on the moist Froude number, can be identified for a conditionally unstable, rotational, horizontally homogeneous, uniformly stratified flow over an idealized, three-dimensional, mesoscale mountain stretched spanwise to the impinging flow: (I) a quasi-stationary upslope convective system and an upstream-propagating convective system, (II) a quasi-stationary upslope convective system, and (III) a stationary upslope convective system and a quasi-stationary downstream convective system. Several major differences from a similar type of flow with no rotation over a two-dimensional mountain range are found. One important finding is that relatively strong mean flow produces a quasi-stationary mesoscale convective system (MCS) and maximum rainfall on the windward slope (upslope rain), instead of on the mountain peak or over the lee side.We found that the Coriolis force helps produce heavy upslope rainfall by making transition from flow-around the eastern part of the upslope to flow-over the western part of the upslope (transits to a higher flow regime) by deflecting the incident southerly flow to become east–southeasterly barrier winds. We found that the addition of the western flank of the arc-shaped mountain helps slow down the barrier wind from east and causes the maximum rainfall to move east of the windward slope. A lower-Froude number flow tends to produce a rainfall maximum near the concave region.Several other important facts can also be found in this study. The ratio of the maximum grid scale rainfall to the sub-grid scale rainfall increases when the moist Froude number increases. When the CAPE decreases, it is found that the upstream moist flow tends to shift to a higher Froude-number regime. Therefore, the Froude number cannot solely be used to define a moist flow regime when different CAPEs are considered. In another word, other parameters, such as CAPE, might play an important role in determining moist flow regimes.     

Airflow patterns over and around a large three-dimensional hill

Summary Surface wind patterns and air flows within the planetary boundary layer over a large three-dimensional hill of moderate slope are grouped according to Froude number classes. An evolution of flow patterns is shown to occur as the Froude number increases.Separation of the surface flow begins at the base of the lee side of the mountain near the centerline, moving upward on the lee slope as the Froude number increases. Recirculating eddies follow the separation of the lee flow. Eventually the separation line moves forward to the windward side as the Froude number becomes very large. The recirculating eddy becomes unsteady, with indication of an intermittent counterrolating eddy near the lee surface in neutral flow. The lee-side turbulence is enhanced with respect to the windward side due to the large eddies in high Froude number regimes.The concept of a critical height for the approach flow is generally supported. The integral form of the Froude number does not appear to be superior to a bulk Froude calculation in representing a particular airflow pattern.With 6 FiguresDeceased.     

Numerical modeling studies of lee mesolows,mesovortices and mesocyclones with application to the formation of Taiwan mesolows

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     

2D Airflow over a Double Bell-Shaped Mountain

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     

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

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(F_w),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F_w 数从0.19 增加到1.81。当F_w≤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F_w 很小时,地形云不产生降水。当F_w>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。     

Gravity wave breaking in easterly flow over Greenland and associated low level barrier- and reverse tip-jets

A first evidence of severe turbulence in the lower stratosphere during easterly tropospheric flow over Greenland is presented. A numerical simulation shows the turbulence to be associated with gravity wave breaking and that simulating with a horizontal resolution of 3 km gives substantially greater and more realistic turbulence than at a 9 km horizontal resolution. It is concluded that real-time simulations at high resolutions would improve aviation forecasts. As the atmospheric flow impinges on South-Greenland a barrier jet, a reverse tip jet and amplified mountain waves with secondary wave breaking are generated at the same time.     

浙江天目山背风坡对流触发个例的对比分析

针对夏季副热带高压背景下浙北天目山附近的强对流天气个例,利用中尺度实况资料,分析了天目山对触发对流的作用。结果表明:浙江省夏季位于副热带高压边缘时,低层处于西南背景风时,在低Froude数条件下,气流经过黄山、天目山后在背风侧形成一段辐合线,在有利的热力条件配合下,容易触发对流。山地的热力强迫作用使地形上空新生了很多积云,积云分布基本与地形一致。同时,天目山背风侧出现一条积云线,其形成的原因是天目山背风侧辐合线的辐合抬升作用。背风侧辐合线尺度有几十千米,方向随环境风向转变。对流触发的位置位于这条辐合线上靠近山地的一端。这可能是由于山地热力强迫作用产生的积云移到辐合线上继续发展产生对流云,即山地的动力和热力作用共同触发了对流。