Characteristics of nocturnal low-level jets over Ulsan airport

We analyzed wind profiler data collected over Ulsan airport during the period from 2008 to 2009 to examine the characteristics of low level jets (LLJs). The Ulsan airport is located within the narrow valley with north-south axis. The frequency analysis results indicates that the nearly 19% of the total nocturnal periods have the presence of jets and LLJ occurrence rate is high in winter (32%) and low in summer (10%). The mode in the wind speed histogram is 4?C6 m s^?1. A majority of jet occurs below 100 m (about 77.8 m) above ground. The predominant wind direction of jet is northerly. In order to examine the favorable conditions for LLJ formation of Ulsan airport, we investigated temperature difference between valley and plain at the surface and synoptic wind direction and speed at 850 hPa. Our results show that air temperature in the valley is lower than over the plain during the nighttime, indicating the existence of thermal forcing for along-valley wind. Under a significant temperature difference along the valley, westerly wind speed at 850 hPa is slightly weaker on LLJs event night than no event night, indicating weaker north-south large-scale pressure gradient on LLJ event night. The magnitude of northerly wind at 850 hPa is much stronger on event night than no event night, implying higher downward transfer of northerly wind on event night. Our findings suggest that jet formation over Ulsan airport is related to the strong northerly wind at 850 hPa in the presence of thermal forcing due to temperature contrast between valley and plain.     

洱海盆地一次冬季大风演变特征及其机制模拟分析

利用WRF模式耦合Noah陆面模式和CLM湖泊模式,对2015年1月23日大理地区洱海盆地的大风天气进行模拟,对大风的发展期、强盛期和减弱期的三维动力热力结构特征进行分析,并得出了洱海盆地大风形成机制:在洱海盆地大风发展期,高空以西风为主,盆地中部上空1km高度处出现局地小气旋,地面以偏东风为主,高空偏西气流翻越苍山形成波动扰动,在背风坡侧形成空腔区和二次涡,低层形成了波不稳定区域,波不稳定区域发生波破碎,波破碎区域湍流运动活跃,把上层的能量往下传播。大风强盛期,盆地南北侧高空为两支西风气流控制,中部变为弱的辐散场,造成高空扰动,苍山东侧近地面浅薄逆温层消失,低空逆温层之上温度廓线几乎垂直上升,大气层结处于不稳定状态,有利于高空动量向下输送。大风减弱期,高空西风减弱,扰动消失,湍流动能耗散,地面风速逐渐减小。     

Thermal and dynamic characteristics of alpine lake breezes,Lake Tekapo,New Zealand

Thermodynamic characteristics and temporal variation of alpine lake breezes in the eastern Southern Alps are examined. Research was conducted in a large glacially excavated basin dominated by an 87 square kilometre melt-water lake as part of a study of windblown dust dispersion. The surrounding mountain ranges were found to shelter the lake basin from most synoptic winds, thereby allowing local and regional thermally generated circulations to develop to ridge height, approximately 1300m above the surrounding landscape. During favourable synoptic conditions the local lake breeze becomes embedded within the regional valley wind forming an extended lake breeze. Tethersonde flights during these conditions made using a kite based sounding system identified both stable internal (SIBL) and thermal internal boundary layer (TIBL) conditions over the down wind shoreline. Two equations for estimating the height of both boundary-layer types were tested against observations and found to provide good first order predictive estimates of boundary-layer height.     

Evening and Morning Transition of Katabatic Flows

An experimental investigation of the evening and morning transition phases of katabatic slope flows has been conducted to identify the mechanisms for their development and destruction over an isolated slope. The momentum and energy equations of the flow have been used to describe these mechanisms for the particular topographic features of the studied slope, and to outline the differences from the dynamics of well-developed simple slope flows. In the lowest portion of the slope, frontal characteristics have been identified in early evening periods when the local pre-existing near-surface thermal structure does not impose a katabatic acceleration. The frontal shape is determined by the near-surface thermal stability and ambient wind. The flow initiation is distinctly different when it is linked to the local surface cooling, in which case it develops gradually and produces a slight local warming.The erosion of the katabatic layer at mid-slope precedes that at the foot and is closely linked to dilution of the local surface inversion. The flow erosion at the foot is often delayed, as the warming of air proceeds uniformly at all heights near the ground, so maintaining the inversion due to warming produced by mixing and advective processes linked to the upslope flow development. The latter initiates first at mid-slope and then at the foot, where for a non-negligible time period it flows over the persistent katabatic flow. The prerequisite for the development of this structure is the maintenance of a shallow inversion in the first 2–3 m above the ground surface.The morning dilution of the katabatic flow is apparently different from common experience over simple slopes and may be attributed to the steep upper portion of the slope in association with its easterly orientation, which results in strong non-uniformity of the solar heating along the slope.     

Evidence for Nearly Complete Decoupling of Very Stable Nocturnal Boundary Layer Overland

Concentrations of ²²²Rn at 0.1 m and 6.5 m height above ground level and ²²²Rn flux density were measured during nights characterized by strong cooling, light winds and clear sky conditions in the Carpathian Basin in Hungary. A very stable boundary layer (vSBL) formed on 14 nights between 15 August and 3 September 2009. On 12 nights, an estimated 72% (s.d. 20%) of ²²²Rn emitted from the surface since sunset was retained within the lowest 6.5 m above the ground until sunrise the following morning. On two nights an intermittent increase in wind speed at 9.4 m height was followed by a rise in temperature at 2.0 m height, indicating a larger atmospheric motion that resulted in ²²²Rn at 0.1 m around sunrise being the same as around the preceding sunset. It does not seem to be rare in a large continental basin for a vSBL to be nearly completely decoupled from the atmosphere above for the entire period from sunset to sunrise.     

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     

北极低空急流和低层逆温特征观测分析

利用北冰洋冰表面热量平衡计划1997年10月中旬至1998年10月上旬的探空气球探测结果,分析了北极地区近地层逆温和低空急流特征.结果表明,96％的观测时次(11:15和23:15,协调世界时)出现近地层逆温,其中22％的逆温为贴地逆温,70％的逆温厚度在250～850 m之间,冬半年贴地逆温发生频率、逆温层厚度和逆温层内的温度变化都明显要大于夏半年.全年间低空急流出现频率为41％,平均高度为520 m,最大频率出现在150 m附近,70％的急流出现在600m高度以下.急流平均风速为10.6m·s-1,风速在4～13 m·s-1范围内的急流约占总数的75％,东和东南方向为全年急流的主导风向.根据对急流核和地面风速之间转换角分布的分析,惯性震荡可能是北极低空急流的主要成因.     

A Two-Day Case Study: Comparison of Turbulence Data from an Unmanned Aircraft System with a Model Chain for Complex Terrain

The airborne measurement platform MASC-3 (Multi-Purpose Airborne Sensor Carrier) is used for measurements over a forested escarpment in the Swabian Alps to evaluate the wind field. Data from flight legs between 20 and 200 m above the ground on two consecutive days with uphill (westerly) flow in September 2018 are analyzed. In the lowest 140 m above the ground a speed-up is found with increased turbulence and changes in wind direction directly over the escarpment, whereas in the lowest 20 to 50 m above the ground a deceleration of the flow is measured. Additionally, simulation results from a numerical model chain based on the Weather Research and Forecasting (WRF) model and an OpenFOAM (Open Source Field Operation and Manipulation) model, developed for complex terrain, are compared to the data captured by MASC-3. The models and measurements compare well for the mean wind speed and inclination angle.

     

河西走廊风能变化及储量

利用河西走廊地区1970~2004年风速气候资料和2004年9月至2005年8月风塔精细资料,研究了河西走廊风能资源特征。结果表明:河西走廊地区环境风速比较稳定,在气候上没有明显的减小趋势;绿洲内风速下降明显。风能分布存在区域差别,存在2个风能大值区域。全区风能普遍较高,10 m层内风能密度都在100 W/m2以上,大部分地方在150 W/m2以上,大值地带在200 W/m2以上;在10~70 m高度层风能随高度按线性关系增长,平均每升高10 m风能增加28 W/m2;70 m层普遍在300 W/m2以上。河西走廊风能存在日和年变化,2~6月是风能密集阶段;距地面10 m高度处风能在07:00前后处于低值,从08:00以后不断增大,到18:00前后达到日最大,然后逐渐减小。河西走廊年有效风速时数普遍在6000 h以上,大值区接近7500 h。     

塔中一次强沙尘暴过程边界层风场变化特征

利用风廓线雷达探测资料对2010年4月19日塔中一次强沙尘暴过程中的边界层三维风场进行分析研究。沙尘暴爆发前,塔中1 000 m高度内空中风主导风向由偏东风转为偏西风;沙尘暴爆发时,地面至1 500 m高度内为偏东风。近地面风速在沙尘暴爆发初期迅速增大至18.3 m/s,中后期逐渐变小,但依然保持10 m/s左右的较大风速;300～1 000 m高度,沙尘暴爆发时段的风速小于过程前后;1 000～2 000 m高度内,沙尘暴爆发前风速达到最大,然后随时间变化呈递减趋势;3 000 m以上高空风在沙尘暴爆发期间风速可达20 m/s。沙尘暴过程中塔中上空存在明显的沙尘颗粒沉降运动,平均下沉速度为1.2 m/s。     

盆地环流型及冷湖的形成和消散的数值研究

本文建立了一个三维原始方程数值模式，模拟三维盆地中的环流，重点讨论在热力强迫环流型的转变过程中能量场的动量场的配置情况及环流演变过程。模拟结果表明，夜间地面强烈的辐射冷却过程引起盆地各坡面冷迳流的发展，盆地中成为冷空气湖，逆温层结建立。日出后对流边界层从大气低层向上发展，逆温从底部随之消蚀，仅在上层存在了夜间逆温残余，即所谓稳定核，上坡风在对流边界层发展，而稳定核内仍维持夜间下坡风环流。     

Observations of complex terrain flows using acoustic sounders: Echo interpretation

We examine methods for the interpretation of sodar facsimile records obtained in the study of complex terrain flows. Acoustic scattering theory is presented first and then interpreted using a simpolified second-order turbulence closure scheme. The use of this theory suggests the strong sensitivity of acoustic scatter to changes in the wind shear. With this introduction, detailed sodar facsimile records, temperature and wind profiles, and model calculations follow. Characteristic scattering patterns are described for simple drainage jets, complex basin flows, convection with a capping inversion, stratus, and dynamical instabilities. Examples are also shown of bistatic facsimile records detailing the strong temporal and spatial variability in small-scale turbulence.     

淮河洪涝年7月上旬环流特征与近10年平均环流特征差异分析

利用NCEP 1998-2007年分析资料,分析了2007年、2005年和2003年3个淮河大洪水年7月上旬逐日各层的位势高度、温度、水汽和风场等资料.分析结果表明:这3个洪水年和近10年平均场的大尺度环流背景、高低空急流、水汽输送特征等存在显著差异.东亚夏季风异常是这3年淮河流域洪涝出现的主要原因.同时,这3年西北太平洋副热带高压均较历史同期偏强,西伸脊点偏西;华北南部到黄淮距平风场上有较常年明显的偏北气流,副热带高压西北侧850hPa低空急流较常年平均偏强6m·s-1以上,南北两支气流交汇区正好位于淮河流域;在200hPa上淮河流域处于异常的高空急流入口南侧或受较常年偏强的反气旋环流控制之下;从整层水汽通量距平分析,南海是这3个淮河洪涝年的主要水汽源地.     

Observations of the Relation Between Upslope Flows and the Convective Boundary Layer in Steep Terrain

Slope flow mechanisms are crucial for the transport of air pollutants in complex terrain. Previous observations in sloping terrain showed upslope flows filling the entire convective boundary layer (CBL) and reducing air pollution concentrations by venting air pollutants out of the CBL into the free atmosphere. During the Pacific 2001 Air Quality Field Study in the Lower Fraser Valley, British Columbia, Canada, we observed slope flows during weak synoptic winds, clear skies, and strong daytime solar heating. With a Doppler sodar we measured the three wind components at the foot of a slope having an average angle of 19° and a ridge height of 780 m. We operated a scanning lidar system and a tethersonde at a nearby site on the adjacent plain to measure backscatter of particulate matter, temperature, wind speed, wind direction, and specific humidity. Strong daytime upslope flows of up to 6 m s⁻¹ through a depth of up to 500 m occurred in the lower CBL, but with often equally strong and deep return flows in the upper part of the CBL. The mass transport of upslope flow and return flow approximately balanced over a 4-h morning period, suggesting a closed slope-flow circulation within the CBL. These observations showed that air pollutants can remain trapped within a CBL rather than being vented from the CBL into the free atmosphere.     

On the Turbulence in the Upper Part of the Low-Level Jet: An Experimental and Numerical Study

The characteristics of low-level jets (LLJ) observed at the “Centro de Investigacion de la Baja Atmósfera” (CIBA) site in Spain are analysed, focussing on the turbulence generated in the upper part of the jet, a feature that is still to be thoroughly understood. During the Stable Boundary Layer Experiment in Spain (SABLES) 1998, captive balloon soundings were taken intensively, and their analyses have highlighted the main characteristics of the jet’s wind and temperature structure, leading to a composite profile. There are indications that the turbulence has a minimum at the level of the wind maximum, with elevated turbulence in a layer at a height between two and three times that of the LLJ maximum, but no direct measurements of turbulence were available at these heights. In September 2001, a 100-m tower at the same site was re-instrumented to give turbulence measurements up to 96.6 m above ground level. All occurrences of LLJ below this height between September 2002 and June 2003 have been selected and significant turbulence above the LLJ has been found. Simulations with a single-column turbulence kinetic energy model have been made in order to further investigate the generation of elevated turbulence. The results correlate well with the measurements, showing that in the layer above the LLJ, where there is significant shear and weakly stable stratification, conditions are conducive to the development of turbulence.     

Strong Updraft at a Sea-Breeze Front and Associated Vertical Transport of Near-Surface Dense Aerosol Observed by Doppler Lidar and Ceilometer

To study the wind field within the atmospheric boundary layer over the Tokyo metropolitan area, Doppler lidar observations were made 45 km north of Sagami Bay and 30 km west of Tokyo Bay, from 14 May to 15 June 2008. Doppler lidar on 27 May 2008 observed the vertical and horizontal wind structure of a well-developed sea-breeze front (SBF) penetrating from Sagami Bay. At the SBF, a strong updraft (maximum w approximately equal to 5 m s⁻¹) was formed with a horizontal scale of about 500 m and vertical scale of 2 km. The spatial relationship between the strong updraft over the nose of the SBF and prefrontal thermal suggests that the strong updraft was triggered by interaction between the SBF and the thermal. After the updraft commenced, a collocated ceilometer observed an intense aerosol backscatter up to 2 km above ground level. The observational results suggest that the near-surface denser aerosols trapped in the head region of the SBF escaped from the nose of the SBF and were then vertically transported up to the mixing height by the strong updraft at the SBF. This implies that these phenomena occurred not continuously but intermittently. The interaction situations between the SBF and prefrontal thermal can affect the wind structure at the SBF and the regional air quality.     

Modelling Near-Surface Low Winds over Land under Stable Conditions: Sensitivity Tests,Flux-Gradient Relationships,and Stability Parameters

Low or weak wind-speed conditions, roughly defined as the periods when the mean wind speed at 10 m above the ground is 2 ms⁻¹ or less, are of considerable practical interest. However, they are not readily amenable to treatment within prognostic meteorological models and, consequently, difficult to predict, especially when the ambient stability is strong. In this paper, we apply an E − ε prognostic meteorological model to simulate near-surface meteorology and, focusing on low wind speeds, compare the predictions with measurements from two independent datasets. A sensitivity analysis is performed to investigate the possible reasons for the relatively inferior model performance for low winds when the atmosphere is stably stratified. A comprehensive data analysis is carried out to study low wind stable conditions, concentrating on the validity of various forms of flux–gradient relationships for momentum and heat within the framework of the Monin-Obukhov similarity theory, which models employ for calculating surface fluxes. The observed behaviour of various stability parameters, such as the Richardson number, is investigated. The results point to inadequacies of the current flux–gradient relationships, especially regarding momentum, under strongly stable conditions as being a dominant reason for the poor low wind predictions. The modelling issues identified are not just restricted to the present model, but are general in nature. The use of an alternative stability function for momentum under strongly stable conditions is explored. It results in improved model performance for low winds; however, further research is needed to better understand strongly stable flows in the lower atmosphere and to develop methods that can translate that understanding to operational meteorological modelling.     

Flow dynamics in a trough blowout

The dynamics and geomorphological development of a trough blowout located at Fiona Beach in the Myall Lakes National Park in NSW, Australia are examined. Wind velocities and flow structure were measured utilising an array of miniature Rimco cup anemometers, Gill bi-vane and UVW instruments, and wind vanes. Flow measurements indicate that when the wind approaches the trough blowout parallel to the throat orientation, jets occur both in the deflation basin and along the erosional walls, relative flow deceleration and expansion occur up the depositional lobe, jets are formed over the depositional lobe crest accompanied by downwind flow separation on the leeward side of the lobe, and flow separation and the formation of corkscrew vortices occur over the crests of the erosional walls. Maximum erosion and transport occur up the deflation basin and onto the depositional lobe. Trough blowout morphologies are explained as a function of these flow patterns.When the wind approaches the blowout obliquely, the flow is steered considerably within the blowout. The degree and complexity of topographic steering is dependent on the blowout topography. The flow is usually extremely turbulent and large corkscrew vortices are common. The local topography of a blowout can be very important in determining flow patterns, overall sand transport and blowout evolutionary conditions and paths.Estimates of potential sand transport within the blowout may be up to two orders of magnitude lower than actual rates if remote wind data are used.     

基于不同时间尺度风切变指数推算风资源的对比分析

利用2014—2018年辽宁省探空资料分析了水平风速的垂直风廓线分布特征。用2座代表性测风塔逐时梯度风观测分析了采用不同高度组合方案计算出风切变指数的月、日变化特征, 分别用月、小时、年风切变指数推算高层风速和风功率密度, 并与实测对比。结果表明: 沈阳相较于大连地区风速随高度增加较快, 180 m高度以上风速基本保持不变, 而大连因其纬度低且靠近海洋, 300 m以下风速均匀上升。在非复杂地形情况下, 距地面10 m高度以上间隔一定高度设立4层风观测, 基本可以满足近地层风资源评估需求。受太阳辐射、下垫面、海陆热力性质差异等影响, 辽宁省风切变指数日变化特征比月变化更显著。利用小时风切变指数推算高层风速和风功率密度的方案优于采用月、年风切变指数方案。风切变指数日变化越显著, 采用逐时风切变指数推算方案越优于其他计算方案。