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。     

Mini-Sodar Observations of Drainage Flows in the Rocky Mountains

Summary  Vertical profiles of drainage winds were monitored continuously by a Doppler-Mini-Sodar during case studies in two valleys, on both sides of the U. S. Continental Divide. A tethered balloon provided additional information on the vertical temperature and wind structure up to the Divide level. Ambient wind data were collected by a radar wind profiler on the west side, and a tower on the crest of the Divide. The onset, evolution and breakup of the drainage flow were studied on two nights, when the ridge-top winds were westerly and skies were clear. To study the influence of the ambient flow on drainage winds, changes in drainage wind speed, direction and depth, along with the volume flux were examined. It was found that, on the leeward side, the drainage was strongly influenced by the ambient winds (King, 1995b), which led to interruption and erosion of the locally generated valley flow. The drainage on the windward side of the Divide was almost undisturbed. A comparison of balloon and sodar wind profiles showed very good agreement during steady drainage conditions. Received October 21, 1996 Revised November 30, 1998     

The feasibility of estimating large-scale surface wind fields for the summer MONEX using cloud motion and ship data

Cloud motion data were compared to ship observations over the Indian Ocean during the summer monsoon, 1 May to 31 July 1979, with the objective of using the cloud data for estimating surface wind and ultimately the wind stress on the ocean. The cloud-ship comparison indicated that the cloud motions could be used to estimate surface winds within reasonable accuracy bounds, 2.6 m s^-1 r.m.s. speeds and 22° to 44° r.m.s. directions (22° r.m.s. for winds < 10 m s^-1). A body of statistics is presented which can be used to construct an empirical boundary layer with the eventual goal of producing a stress analysis for the summer MONEX from cloud motion data.     

Observations of flow structure in a small forested valley system

Summary Features of the mean flow structure in a small valley system in the Rosalian mountain range are discussed using data from a wind measurement network. Tethered balloon measurements during periods of clear sky form the basic dataset for the analysis of drainage winds and temperature inversions. During periods of weak ambient winds the existence of a pure thermally driven nocturnal valley wind system is shown. With strong ambient winds opposing the drainage flow, a reduced drainage height but the same jet maximum as with weak ambient winds is found. On the other hand with aiding flow the drainage winds are suppressed and flow reversal can occur. This strong valley flow interaction with the ambient wind indicates considerable dynamic influence on the evolution of drainage winds and on the breakup of temperature inversion structure for small valleys.With 15 Figures     

The development and structure of nocturnal slope winds in a simple valley

The results of an observational and modeling study of the nocturnal slope winds in a simple valley are presented. The valley was approximately 225 m deep in the region of the measurements, and featured a uniform slope angle of approximately 23 ° on one of its sidewalls. The wind and temperature structure of the katabatic flows on the valley sidewalls were measured with tower-mounted instruments, and a Doppler sodar and instruments on a tethered balloon and a 61-m tower were used to determine the atmospheric conditions near the center of the valley. The temperature structure of the slope flows was summarized by characteristic scale parameters h and T for the inversion depth and strength, respectively. On the sidewalls 50 m above the valley floor, the inversion depths were generally smaller and the inversion strengths were weaker than they were on the sidewalls 100 m higher. These results differ significantly from those obtained over a simple slope of an isolated mountain or ridge. The down-valley winds are shown to be important in limiting the strength of the sidewall inversions. The formation of an inversion in the valley also has a pronounced effect on the structure of the slope flows. Numerical simulations suggest that the presence of adiabatic layers in the valley atmosphere is associated with decreases in the slope-flow inversion depth with increasing downslope distance. The simulations also indicate that the length scales that characterize the momentum and inversion depths behave similarly in flows down simple slopes but not in flows down the sidewalls of a valley.Work supported by the U.S. Army Research Office under Contract DA-AG29-K-0231 and the U.S. Department of Energy under Contract DE-AC06-76RLO 1830.     

Air flow analysis in the upper Río Negro Valley (Argentina)

The so called Upper Río Negro Valley in Argentina is one of the most important fruit and vegetable production regions of the country. It comprises the lower valleys of the Limay and Neuquén rivers and the upper Negro river valley. Out of the 41,671 cultivated hectares, 84.6% are cultivated with fruit trees, especially apple, pear and stone fruit trees. Late frosts occurring when trees are sensitive to low temperatures have a significant impact on the regional production. This study presents an analysis of air flow characteristics in the Upper Río Negro Valley and its relationship with ambient air flow. To such effect, observations made when synoptic-scale weather patterns were favorable for radiative frosts (light wind and clear sky) or nocturnal temperature inversion in the lower layer were used. In the Negro river valley, both wind channeling and downward horizontal momentum transport from ambient wind were observed; in nighttime, very light wind events occurred, possibly associated with drainage winds from the nearby higher levels of the barda. In the Neuquén river valley, the prevailing effect appeared to be forced channeling, consistent with the results obtained in valleys where the synoptic scale wind crossed the axis of the valley. In the Limay river valley, the flow was observed to blow parallel to the longitudinal valley axis, possibly influenced by pressure gradient and forced channeling.     

Characteristics of Spatiotemporal Distribution of Sea Surface Wind along the East Coast of Guangdong Province

We analyzed the frequency distribution characteristics of wind speeds occurring at different offshore sites within a range of 0–200 km based on the sea surface wind data captured via buoys and oil platforms located along the east coast of Guangdong Province. The results of the analysis showed that average wind speed measured for each station reached a maximum in winter while minima occurred in summer, corresponding to obvious seasonal variation, and average wind speed increased with offshore distance. The prevailing wind direction at the nearshore site is the easterly wind, and the frequency of winds within 6–10 m s^–1 is considerable with that of winds at > 10 m s^–1. With the increase of the offshore distance, the winds were less affected by the land, and the prevailing wind direction gradually became northerly winds, predominately those at > 10 m s^–1. For areas of shorter offshore distance (< 100 km), surface wind speeds fundamentally conformed to a two-parameter Weibull distribution, but there was a significant difference between wind speed probability distributions and the Weibull distribution in areas more than 100 km offshore. The mean wind speeds and wind speed standard deviations increased with the offshore distance, indicating that with the increase of the wind speed, the pulsation of the winds increased obviously, resulting in an increase in the ratio of the mean wind speed to the standard deviation of wind speed. When the ratio was large, the skewness became negative. When a relatively great degree of dispersion was noted between the observed skewness and the skewness corresponding to the theoretical Weibull curve, the wind speed probability distribution could not be adequately described by a Weibull distribution. This study provides a basis for the verification of the adaptability of Weibull distribution in different sea areas.     

The vertical structure of summertime local winds in the Wright Valley,Antarctica

Intensive observations of summertime up- and down-valley winds in a dry valley utilising airsondes, pilot balloons and a monostatic acoustic sounder are described. Both circulations show a distinctive layered vertical wind and temperature structure. Westerly down-valley flow is typically neutral and is characterised by strong surface winds overlain by light variable winds extending to an inversion between 2000 and 4000m in depth. Above this inversion, gradient winds prevail. This structure is similar to that of downslope winds observed elsewhere. The thermally-induced up-valley easterly flow is shown to be extremely well-developed in terms of its strength, depth and persistence. The strong surface easterly may reach 800 m in depth and usually undercuts the warmer westerly. The boundary between the two regimes is marked by an inversion. During easterly flow a surface-based, super-adiabatic layer of 100–200 m in depth is evident and is associated with weak convective activity. An intriguing aspect of the wind regime is the interaction between the easterly and westerly circulations in the valley. These are separated at the surface by a frontal zone which migrates up and down the valley. Further observational and modelling studies are recommended.     

Doppler sounder observations of trade winds and sea breezes along the African west coast near 34 ° S, 19 ° E

Summer weather conditions along the west coast of Africa near 34 ° S, 18 ° E are investigated using doppler acoustic sounder profiles. Case studies were selected from a two-year record to form composite analyses over the diurnal cycle. The SE trade wind exhibited a low level jet at the level of the temperature inversion due to a sharp reversal in the thermal wind vector aloft. Mean wind speeds reached 14 m s^–1 just before midnight as the surface and upper inversions strengthened. Seabreezes were categorised by the supporting gradient wind and found to have mean depths of 400 m, speeds of over 6 m s^–1 at the 200 m level, and advance/retreat times of 09 hr and 16–20 hr. During seabreezes and weak on-shore gradient flow conditions, the thermal internal boundary layer (TIBL) was monitored with sounder transects in the first 12 km of the coastal zone. The growth height was observed to be 1:20 in the first 5 km and 1:50 farther inland. The sounder climatology, together with surface network and aerial survey results, illustrate the four-dimensional characteristics of trade winds and seabreezes near Cape Town.     

Modelling the Interaction Between a River Surface and the Atmosphere at the Bottom of a Valley

The role of a river of small dimensions in driving the surface exchange of sensible and latent heat fluxes at the bottom of a valley is investigated using large-eddy simulation (LES). Simulations were performed using different valley topographies, river widths and large-scale wind speed and direction. In all cases, the river acted as a sink of both sensible and latent heat during daytime. Despite the general agreement concerning the flux direction above the river surface, specific differences exist between the simulations. The topography enhances the wind divergence caused by the river, and the larger negative surface fluxes above the river occur when there are no slopes, a consequence of larger wind speeds above the river. For large-scale winds aligned with the valley axis, the surface fluxes depend on the large-scale wind speed, but this dependence is reduced if the large-scale wind is perpendicular to the valley axis. There is a minimum of temperature and a maximum of specific humidity above the river surface. The scalar budgets show that sensible heat flux converges above the river, being balanced by the warm air subsidence at the centre of the valley. Latent heat fluxes, on the other hand, converge above the river surface, and they are balanced by the horizontal advection of humidity towards the river margins.     

Estimation of the Exchange Coefficient of Heat During Low Wind Convective Conditions

For the first time, the exchange coefficient of heat C_H has been estimated from eddy correlation of velocity and virtual temperature fluctuations using sonic anemometer measurements made at low wind speeds over the monsoon land atJodhpur (26°18' N, 73°04' E), a semi arid station. It shows strong dependence on wind speed, increasing rapidly with decreasing wind speed, and scales according to a power law C_H = 0.025U₁₀ ^-0.7 (where U₁₀ is the mean wind speed at 10-m height). A similar but more rapid increase in the drag coefficient C_Dhas already been reported in an earlier study. Low winds (<4 m s^-1) are associated with both near neutral and strong unstable situations. It is noted that C_H increases with increasing instability. The present observations best describe a low wind convective regime as revealed in the scaling behaviour of drag, sensible heat flux and the non-dimensional temperature gradient. Neutral drag and heat cofficients,corrected using Monin–Obukhov (M–O) theory, show a more uniform behaviour at low wind speeds in convective conditions, when compared with the observed coefficients discussed in a coming paper.At low wind convective conditions, M-O theory is unable to capture the observed linear dependence of drag on wind speed, unlike during forced convections. The non-dimensional shear inferred from the present data shows noticeable deviations from Businger's formulation, a forced convection similarity. Heat flux is insensitive to drag associated with weak winds superposed on true free convection. With heat flux as the primary variable, definition of new velocity scales leads to a new drag parameterization scheme at low wind speeds during convective conditionsdiscussed in a coming paper.     

A numerical study of boundary-layer dynamics in a mountain valley

The characteristics of dynamics and thermodynamics of the atmospheric boundary layer in a part of the Colorado River Valley, centered around Lake Mohave, have been investigated by analysis of measurements conducted during a field program in late spring and early summer of 1986 and a series of numerical simulations by a three-dimensional second-moment turbulence-closure model. The model was validated against measurements described in a companion article (Engeret al., 1993). According to airsonde measurements performed on eight nights, the depth of the surface inversion was around 200 m with an average temperature gradient of about 30 K km^–1. Analysis of acoustic sounder data collected during one month revealed significant diurnal variations ofU andV wind-speed components related to slope and valley flows, respectively. Some of the dynamics properties have been explained by the simulation results. It has been shown that the appearance of supergeostrophic southerly valley flow is associated with the westerly component of the geostrophic flow. Since a westerly component of the geostrophic wind is quite common for this area in summer, this effect also explains the frequently observed southerly valley flow in summer. Elevated minima of the measured wind speed around valley ridges appear to be related to the interaction of conservation of momentum in theX andY directions. The critical direction of the geostrophic wind relevant for reversal of up-valley flow to down-valley flow has also been studied. The critical direction is about 300° for one of the measurement sites and, depending on the angle between valley axis and south-north direction, the critical direction is expected to vary by about 15–20°. The scale analysis of the simulated equations of motion and turbulence kinetic energy emphasizes the strong impact of meandering of the flow due to actual topographic complexity.     

Daytime Turbulence Statistics above a Steep Forested Slope

Six levels of simultaneously sampled ultrasonic data are used to analyse the turbulence structure within a mixed forest of 13 m height on a steep slope (35°) in an alpine valley. The data set is compared to other studies carried out over forests in more ideal, flat terrain. The analysis is carried out for 30-min mean data, joint probability distributions, length scales and spectral characteristics.Thermally induced upslope winds and cold air drainage lead to a wind speed maximum within the trunk space. Slope winds are superimposed on valley winds and the valley-wind component becomes stronger with increasing height. Slope and valley winds are thus interacting on different spatial and time scales leading to a quite complex pattern in momentum transport that differs significantly from surface-layer characteristics. Directional shear causes lateral momentum transports that are in the same order or even larger than the longitudinal ones. In the canopy, however, a sharp attenuation of turbulence is observed. Skewed distributions of velocity components indicate that intermittent turbulent transport plays an important role in the energy distribution.Even though large-scale pressure fields lead to characteristic features in the turbulent structure that are superimposed on the canopy flow, it is found that many statistical properties typical of both mixing layers and canopy flow are observed in the data set.     

2019年冬季北京海陀山局地环流特征及机理分析

海陀山作为北京冬(残)奥会的主要室外赛场之一,其复杂的地形对风场的精细化预报提出了严峻的挑战,亟需开展加密的风场观测提高对复杂地形下局地环流特征及其影响机理的认识,并为提升赛区精细化预报与服务提供数据支撑.基于2019年度海陀山观测试验,利用加密自动气象站、激光测风雷达、涡动相关仪、云高仪等多源数据,对海陀山风场的水平...     

Studies of sodar surface-based shear echo structures

Shear echoes with various types of structures like surface layer with smooth top, short spiky top, tall spiky top and stratified layers have been seen on the monostatic system operating at the National Physical Laboratory, New Delhi. In this paper effects of surface wind speed on the formation of these structures have been considered. The data used for the purpose pertains to the period May 1977 to April 1982. It has been seen that most of the shear echo structures are formed under stable conditions with surface wind speed less than or equal to 2.5 m s⁻¹, however, the tall spiky surface based layer structures of height more than 150 m have been seen to occur significantly in the presence of higher surface wind speeds, suggesting that strong surface winds may be responsible for turbulence in the vertical plane to more depth.     

A numerical investigation of mechanisms affecting drainage flows in highly Complex Terrain

Summary Numerical simulations of increasing complexity are conducted to investigate topographic controls and ambient wind effects upon drainage flows along a portion of the Colorado Front Range in the central Rocky Mountains. A series of two-dimensional simulations show the effects upon the drainage flow of changing slope gradient at the mountain-plain interface. For a given mountain slope, a decrease in the slope of the plain decelerates the mountain drainage jet as it approaches the plain and causes the jet to elevate. The integrated effects of slope and valley drainage are presented with particle trajectories for a particular drainage basin along the Front Range. A nested grid simulation of drainage flow from multiple basins along the Front Range shows that basin area is an important factor in the strength of the drainage flow and that canyon topography variations greatly affect the behavior of the drainage jet as it flows through the canyon mouth onto the plain. Strong drainage winds developed on each of four case night simulations due to the presence of only weak ambient wind below mountaintop. The weak winds represent a decoupling of the near-surface from stronger winds above mountaintop. The canyon drainage exhibited substantial temporal variability in wind speed with the inclusion of ambient winds, due to interactions between ambient and drainage winds.With 11 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.     

An Observational Study of Wind-Induced Waving of Plants

The motions of individual plants and the turbulence statistics of surface winds measured near the top of a canopy are obtained over a wheat field and a rush field. Two typical cases of motions of individual plants are presented. The displacements of the ear of wheat (the plant height is 1.0 m) showed a natural oscillation in wind speeds of 1.6 m s⁻¹ measured at a height of 30 cm over a wheat canopy, while displacements of the stem of a rush plant were closely related to the fluctuations of surface winds in wind speeds of 1.7 m s⁻¹ measured at the top of the rush plant. The power spectra of displacements of a rush plant seem to support the negative seven-third power hypothesis proposed by Inoue. The frequency responses of displacements of plants to fluctuations of the instantaneous momentum flux are also presented.     

An observational study of wind-induced waving of plants

The motions of individual plants and the turbulence statistics of surface winds measured near the top of a canopy are obtained over a wheat field and a rush field. Two typical cases of motions of individual plants are presented. The displacements of the ear of wheat (the plant height is 1.0 m) showed a natural oscillation in wind speeds of 1.6 m s^–1 measured at a height of 30 cm over a wheat canopy, while displacements of the stem of a rush plant were closely related to the fluctuations of surface winds in wind speeds of 1.7 m s^–1 measured at the top of the rush plant. The power spectra of displacements of a rush plant seem to support the negative seven-third power hypothesis proposed by Inoue. The frequency responses of displacements of plants to fluctuations of the instantaneous momentum flux are also presented.     

基于自动气象站和风廓线雷达资料的大理机场风切变分析与应用

根据2016—2017年大理机场航空器报告的风切变事件,利用同时段的自动气象观测资料、风廓线雷达资料对大理机场风切变进行了统计和分析。结果表明:①风切变均发生在每年的11月至次年4月,1月、2月最多;主要发生于07:00—13:00,一半出现在晴天;发生在350m以下占83%。②100m以下的风切变,地面均有阵性风,最大最小风速差6m/s;发生在15～91m的6次风切变,5次报告风切变的一端风向变化超过180°,南北两端地面风出现对头风,风速差异明显。AWOS(Automated Weather Observation System)捕捉到风向风速的明显变化可为近地层风切变预警提供参考。③发生在高度较高的风切变,雷达资料在遭遇风切变高度的上下层存在≥8m/s风速差,能确定上下层风不连续的准确高度、开始时间和结束时间。④机场区域常出现地面风速大而上空风速小或地面风速小而上空风速大的情况,结合地面风和风廓线雷达资料可为今后低高度风切变的初步预警提供参考。