The spatial structure of turbulence at production wavenumbers using orthonormal wavelets

Orthonormal wavelet expansions are applied to surface-layer measurements of vertical wind speed under various atmospheric, stability conditions. The orthonormal wavelet transform allows for the unfolding of these measurements into space and scale simultaneously to reveal the large intermittent behavior in space for the turbulent production wavenumbers. Both Fourier and wavelet power spectra indicated the existence of a –1 power law for the vertical velocity measurements at the production wavenumbers. The –1 power law in the turbulent production range was derived from surface-layer similarity theory. A dimensionless skewness, structure function is applied to the wavelet decomposed vertical velocity field to trace the destruction of the shear-or buoyancy-induced anisotropy under various stability conditions. The structure skewness function revealed shear- or buoyancy-induced eddy asymmetry dependence on stability at each scale within the –1 power-law wavenumber range with more isotropy during propagation from smaller to larger wavenumbers. The asymmetry of these events at the turbulent production wavenumbers appeared very localized in space, as well as in scale, and could be described with a simple eddy-overturning model. It is demonstrated that the wavelet transform is suitable for such analysis.     

新疆克拉玛依强下坡风暴的机理研究

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。     

A field study of orographic precipitation anomalies during high velocity winds using an aerofoil rain gauge

Summary This study was prompted by a need to explain an anomaly in the apparent precipitation distribution over a prominent mountain subject to a high velocity westerly airstream. Historical data reveal that the lee receives 58% more precipitation than the summit. A novel rain gauge with a vertically orientated collecting orifice, 3 standard rain gauges and complementary wind data were procured from 3 sites on the mountain to assess the effect of the airstream on the precipitation distribution. The catch efficiency of standard rain gauges decrease after the wind velocity exceeds 2.5 ms^–1 Results indicate that the summit actually receives 59% more precipitation than the lee, where a vortex directly affects the precipitation distribution over, and east, of the mountain.With 11 Figures     

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     

地形影响的飞机颠簸及其数值仿真实验

首先利用中尺度数值模式ARPS模拟气流过山生成飞行数值仿真所需要的风场，然后利用飞机载荷因数变量方程进行飞机飞行的数值仿真试验。研究结果表明，气流过山产生的山脉重力波由于风切变临界层破碎，一方面能在对流层产生较强的湍流引起晴空飞机颠簸，另一方面也能在山脉背风面产生强烈下坡风，背风转子环流及低空湍流，影响飞机的起飞和着陆。揭示了飞机在过山脉地形背风面所产生的大气湍流中飞行时引起飞机颠簸的物理机制，有助于增强飞机颠簸的预测能力和飞行气象保障能力。     

Numerical Simulation of the Novaya Zemlya Bora

The development of the bora in case of strong southeastern wind in the area of Novaya Zemlya in the winter-spring of 2016 is simulated using the WRF-ARW numerical atmosphere circulation model with high spatial resolution. The features of wind speed and air temperature fields are considered which define the formation of the intensive near-surface flow, the bora, over the lee western slope of the mountain range. It is demonstrated that the bora development leads to the air temperature rise over the eastern part of the Barents Sea, to the increased surface heat fluxes, and to the formation of the cloudless zone over the sea westward of Novaya Zemlya. It was found that the main reason for the bora development is the high stability of the atmospheric boundary layer over the Kara Sea. It is shown that in case of western wind the Novaya Zemlya archipelago does not exert considerable influence on the air exchange in the Kara Sea area.     

对称和非对称地形对冷锋锋生过程的影响

本文利用半地转模式研究了对称和非对称地形以及双地形对冷锋锋生过程的影响。计算结果显示，地形坡度愈大，越容易产生背风气旋，并且其强度也愈大；背风气旋的强度主要由背风坡坡度决定。山对锋面垂直速度的增幅作用随地形坡度增大而增大，地形坡度越大．出现多次极大上升速度的可能性也越大。冷锋过双地形时，其锋面强度有两次减弱和加强的过程，上升速度得到明显加强，并有可能在锋前产生多条中尺度上升运动带，在合适的水汽条件下，冷锋过双地形有可能产生多重中尺度雨带，其间隔与锋面本身的尺度相当。     

复杂山区上空垂直速度场和热力对流活动的理想数值模拟

利用英国气象局高分辨率的边界层数值模式BLASIUS,针对中国西北一个复杂山区进行了一系列的理想数值模拟,分析了在不同天气条件下山区上空的垂直速度场分布和对流特征以及地形对热力对流活动的影响,同时讨论了与地形有关的对流触发机制。模式结果表明,复杂山区的垂直运动在稳定层结和风速较大的情况下较易预测,而在中性层结下,山区上空的垂直运动分布随机性强。在Froude数小于0.5的条件下,气流往往被山峰阻塞而在迎风坡造成地形强迫和辐合性抬升,从而易在迎风坡触发深对流活动;在背风坡则由于迎风坡的绕流重新辐合而造成垂直运动。绕流的辐合是触发深对流活动的另一重要因子。在大风或Froude数较大的条件下,地形重力波容易在山地下游被激发。地形重力波与对流活动的相互影响在模式中清楚可见。在适当的条件下,重力波除了可以与对流活动相耦合从而使气团上升到更高的高度外,重力波的走向很可能会影响到深对流系统的传播路径。研究还发现稳定度对相邻两条对流线之间的距离长短也有影响。稳定度较小时,相邻两条对流线之间的平均宽度趋向变大而单个对流线的强度也相应变大。定量化的结论和理论升华值得进一步的数值模拟研究。     

Spectra of surface wind speed and air temperature over the ocean in the mesoscale frequency range in JASIN-1978

Based on the data from an array of buoys during the JASIN-1978 field experiment made in an area northwest of Scotland, power spectra of surface wind speed and air temperature over the ocean in the mesoscale frequency range were studied. The averaged composite spectrum of wind speed for the whole period shows the existence of a spectral gap in the frequency range from 10^–4 to 5 × 10^–3 Hz. However, significant peaks in this range are often seen in particular spectra under certain weather conditions. Mesoscale spectral peaks of wind speed occur in 14 segments of the data record, approximately 10% of the total duration of the observations. In 4 of these segments, the mesoscale spectral peaks of both wind speed and air temperature occurred simultaneously. Several wave patterns of mesoscale atmospheric disturbances when mesoscale spectral peaks were seen are derived from phase differences between buoys. Significant mesoscale peaks in spectra appear in relatively strong winds and unstable or near-neutral atmospheric conditions, and none in stable atmospheric conditions. A criterion of wind speed and atmospheric stability is found for the mesoscale spectral peak appearance.     

Mountain effect on the motion in the atmosphere's boundary layer

An analysis of 3 years' (1967–70) radiosonde wind data on the windward (Salt Lake City, Utah) and lee (Denver, Colorado) sides of mountains indicates that at these two stations: (1) the distributions of the kinetic energy of the mean and turbulent motions are similar above the mountain top; (2) below the mountain top, on the windward side, mountains tend to divert the component of the mean motion normal to the mountains to that parallel to the mountains; (3) the meridional eddy transport of westerly momentum is affected by the presence of the mountains to a higher level to the lee of the mountains than upwind of them; (4) the production of turbulent energy is higher below the mountain top in the vicinity of mountains than it is for the zonal average; (5) high frequencies of the motion show a more pronounced contribution in the meridional motion in the windward side, but in the zonal motion in the lee of the mountains; (6) disturbances of 1–2 day periods can be maintained deep into the valley, whereas disturbances of longer periods reduce their amplitudes rapidly with decreasing height from the mountain top; (7) the cospectra of the wind velocities show that the southward/northward transport of westerly momentum results from a southward/northward contribution from most frequencies. The main contributions come from eddies with periods longer than two days.     

斜压大气中的地形扰动

本文主要讨论地形对天气系统的影响,研究了斜压大气中地形背风气旋的问题。用小参数法简化基本运动方程,利用青藏高原和落基山脉的实际地形,采用FFT的数值方法,考虑了不同地形、风切变、大气层结等诸因素对地形扰动的影响。结果表明:青藏高原和落基山脉东侧的地形扰动与统计结果较一致;层结对地形扰动的影响较敏感。     

Interaction of Orographic Disturbance with Front

1.IntroductionTheimportanceoforographiceffectsonfrontwasrecognizedintheearly20thcentury.Butforthecomplexityofthisproblem,theinvestigationoforographiceffectsonfrontfromdynamicalviewpointisnottakenuntilthe1980s.Bannon(1983)derivedanalyticalsolutionsforthequasi--geostrophicfrontforcedbyahorizontalwinddeformationfieldthatmovesoveratwo--dimensionalmountainridge.Thesolutionsshowthatasacoldfrontapproachestheridge,itweakens,relativetotheflat--bottomsolution,andthefrontstrengthensasitmovesdowntothelees…     

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.     

三层模式中风场对背风波的影响

为了研究风场对背风波的影响,针对边界层附近为弱稳定层结的背风波,建立了一个三维三层的理论模型和线性计算模式,分析了各层中风速和风向的变化对背风波特征的影响,揭示了气流过孤立山脉产生背风波的有利风场条件。结果表明：背风波的波长、振幅等特征对各层风速和风向的变化具有相当的敏感性,波长随着低、高层风速的增大而增大,随着中层风速的增大先减小后增大;振幅随着低、中层风速的增大先增大后减小,随着高层风速的增大而增大。此外,风速和上下层风向切变的增大均使背风波的形态逐渐由横波型转为辐散型,但是上下层风向的切变对背风波形态的影响比风速更为显著。     

黄山连续性云海过程的天气学分析

用美国NCEP／NCAR的全球日平均分析场资料,分析了黄山1984年1月19～24日连续6天云海天气过程的大气环流和物理量时间演变特征。结果表明,500hPa平均高度场鄂霍次克海为阻高,乌拉尔山以东为高压脊,我国东北存在一低压,黄山受偏西气流影响;1000hPa平均高度场黄山处在南伸的高压底部,受NE气流影响。物理量时间演变图上,18～19日降温、降湿,垂直速度由上升气流转变为弱下沉气流;云海出现阶段,气温、比湿、垂直速度稳定少变,风速较小,风向较乱。     

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。     

A study of a precipitation system in northeastern Taiwan during TAMEX IOP#10

Summary Two-thirds of the land mass of Taiwan island is covered by mountains that affect precipitation systems over the island. To understand the influence of such terrain on a precipitation system was one of the objectives of TAMEX (Taiwan Area Mesoscale Experiment, Kuo and Chen, 1990). During the passage of these precipitation systems, Doppler radar readings as well as conventional data were collected. On 17 June, 1987 a precipitation system moving toward northeastern Taiwan dumped over 100 mm of rainfall per day near the mountain foothills, not far from the ocean. Over the lee side, the precipitation amount was less. The radar data results indicate that a series of cells formed about 10 km upstream of the coastal area and moved toward the mountains under the influence of an easterly wind. The zonal speed was about 4 to 8 ms^–1. The time interval for the formation of these convective cells was about 40 minutes. They intensified near the coastal area, the foot hills and the mountain slope, but their intensity decreased on the lee-side. A two-dimensional, nonhydrostatic model with a terrain-following coordinate system was employed to study the influence of environmental wind patterns and terrain on the characteristics of a precipitation system. Simulation results indicate that a series of clouds associated with an updraft formed at the middle level, about 10 to 20 km east of the mountain foothills (near the coast line), under the influence of easterly winds in a very moist environment. Then, updrafts associated with cloud water travelled westward from the cloudy region, intensifying near the bottom of the mountains and in the coastal areas due to orographic lifting. Then, convective cells formed. As these cells continued moving westward and upward near the foothills as well as the upslope area near the mountain top, their intensity increased. But once they passed over the mountain top to the lee side, their intensity decreased. The time interval for the formation of cells was about 35 minutes and the size of the cells was about 5 to 8 km horizontally. The numerical results are qualitatively consistent with the observations. Sensitivity studies indicate that the magnitude of the wind speed influenced the formation of the cells. The low level wind profiles affected the movement of cells on the lee-side of the mountain, and the height of mountain also had an impact on the characteristics of the precipitation cells.With 18 Figures     

小尺度地形引起的切变重力波

在大气边界层中,特别是山区的边界层中,经常可以观测到重力波的活动。例如Bull和Neisser分析了1974年4月至7月的2300时的微气压计记录。他们发现约38％的观测时间里都存在振幅至少为10—20 dPa的重力波活动,其中大多数重力波的振幅为25—70 dPa。由于风速切变和温度层结的不同,重力波引起的垂直位移的大小可能为百米,也可能达到整个夜间边界层厚度的量级。 很多大气活动都可能引起重力波,例如锋面、低空急流、局地风切变、逆温及强对流等等。地形特别是在夜间也是产生重力波的重要来源。当稳定度随高度急剧减少或风速随高度急剧增加时,在山脊的背风坡经常可以发现一系列的波动活动。     

Simulation of bora in Novorossiysk

The bora event observed in Novorossiysk on February 6–8, 2012 was analyzed using the WRF numerical model of regional atmospheric circulation. The main meteorological parameters are reproduced. It is demonstrated that the maximum wind speed is reached in the area of the lee slope directly over the bay. It is obtained that the bora development is accompanied by intensive waves associated with the flowing around coastal mountains whose regime changes in time. Computed and measured changes in the surface wind speed are in a good agreement.     

青藏高原东北边坡复杂地形重力波的数值模拟

基于2007年7月青海祁连站的野外加密探空资料,结合高分辨率的三维边界层模式,模拟研究了青藏高原东北边坡复杂地形条件下,边界层对流引起的干动力过程对该地区地形重力波产生及传播的影响机理。结果表明:在不同的背景场强迫下,高原东北边坡复杂地形上空对流和重力波的空间结构存在较大差异。当背景风向与山体垂直时,随着风速增加,山脊背风坡混合层顶附近大气不稳定能量加强,激发了下游区域较强的重力波信号,此时对流线组织性增强、重力波波列较长,高水汽含量的空气被波峰传输到较高的高度,为对流云发展提供了有利条件;当背景风向与山脊走向平行时,山顶上空对流发展旺盛,山脊背风坡混合层顶大气状态较稳定,激发的地形重力波信号较弱且波列较短,整个混合层顶附近水汽较少,对流云形成条件减弱;当背景大气浮力频率减小时,整个区域上空对流发展更加旺盛但组织性减弱,背风坡下游重力波向上传输的距离减小,信号不显著,混合层顶附近水汽分布均匀且变化幅度较小,有利于层状云发展。