Development of a hurricane boundary-layer wind model

Summary Hurricanes cause a variety of damage due to high winds, heavy rains, and storm surges. This study focuses on hurricanes’ high winds. The most devastating effects of sustained high winds occur in the first few hours of landfall. During the short period, hurricanes’ rainfall often increases, while the low-level pressure gradients continue to weaken. Latent heating does not appear to strengthen the surface winds. The indicator is that dry mechanisms such as the boundary layer processes and terrain are responsible for the damaging winds in the coastal areas. In this study, the design of a dry hurricane boundary layer wind model is described. The goal is to develop a forecast tool with near-real time applications in expeditious wind damage assessment and disaster mitigation during a hurricane landfall event. Different surface roughness lengths and topographic features ranging from flat land to the mountainous terrain of Taiwan were used in the model simulation experiments to reveal how the coastal environment affected the hurricane surface winds. The model performed quite well in all cases. The experiments suggested that the downward transfer of high momentum aloft played a significant role in the maintenance of high wind speeds at the surface. The surface wind maximums were observed on the lee sides of high terrain. The surface streamline analyses showed that the high mountains tended to block the relatively weak flow and caused small eddies, while they forced the stronger flow to turn around the mountains. Due to great difficulty in data collection, the hurricane boundary layer over land remains one of the least understood parts of the system. The dry model proves to be an effective way to study many aspects of hurricane boundary layer winds over a wide range of terrain features and landfall sites. The model runs efficiently and can be run on a medium-size personal computer. Received March 16, 2001 Revised September 10, 2001     

Statistical characteristics of instantaneous dense gas clouds released in an atmospheric boundary-layer wind tunnel

Wind tunnel experiments were performed to examine the behavior of suddenly released volumes of dense gas in a turbulent shear layer. Instantaneous concentrations were measured with hot-wire katherometers. Multiple replications of each cloud volume, density, and velocity combination produced statistics for plume arrival time, arrival of maximum concentration time, plume departure time, and maximum concentrations. Probability distributions and standard deviations of each plume property permit prediction of hazard risks.     

Indications of topographically-induced eddies in stratified flow during a severe air pollution event

During the last week of June 1978, Melbourne experienced a prolonged period of stagnation over the city, resulting in high levels of air pollutants. Internal Froude numbers through the period ranged from approximately 0.1 to 0.3, indicating that flow in the area should be strongly stratified. Wind patterns determined using data from thirteen anemographs within and immediately adjacent to the city revealed a pattern of eddies across the city each afternoon in a cavity in the lee of upstream topography. That pattern evolved with time, and changed between days as the controlling meteorological conditions altered. The available mean sea level pressure data from the region for the same times indicated perturbations in that field which were consistent with the observed wind patterns.The eddy patterns were replaced at the surface overnight by katabatic winds as cooling took place and a ground-based inversion became reestablished. However, mean sea level pressure data and the anemograph records suggest that the eddy pattern may have continued overnight, aloft.Considerable recycling of air occurred, both within individual circulations and by wind reversal between daytime and nocturnal regimes. This aided the persistence of high pollutant concentrations.     

A boundary-layer model for the determination of hourly surface wind characteristics in a representative tropical African region

A quantative transposition model is introduced which determines hourly wind speeds in a representative tropical region (Central Sudan). The model consists of two parts. Firstly, a local boundary-layer model, based on the energy balance equation and the Businger-Dyer equations, is used to compute the average diurnal cycle of various characteristic boundary-layer parameters. Secondly, a horizontal transposition method is introduced to calculate wind speed behaviour at an arbitrary station from that at a reference station. This method is based on assumed spatial constancy of the turbulence parameter u _* in the period November–April in a region of about (700 × 800) km² in Central Sudan. The constancy of u _* is concluded from the very stationary character of the climate. Model-computed hourly wind speeds are consistent with the potential wind speeds (at 10 m over open country) calculated from the measured data, and provide better local wind estimates than the conventional procedure which assumes constant regional hourly wind speeds.     

Some medium-path infrared observations of atmospheric boundary-layer structure during light wind

Variations in water-vapour absorption were measured using a recently constructed field spectrophotometer, operated at a wavelength of 1.38 gm and with path lengths of 15 and 20 m. Observations were made 1 m above the ground near sunset, dawn, and during the morning destruction of nocturnal atmospheric stability. A diurnally consistent evolution of water-vapour turbulence regimes was observed. The presence of ordered events was revealed. These fluctuations indicate changes of vapour pressure, spatially averaged over the base-line, as large as about +7 mb in 4 s. Classification of light-wind observing conditions by using six classes of distinct non-random events appears possible. Such a classification system may reduce some of the characteristic scatter of micrometeorological results which arises during stable observing conditions or during comparison of real-world observing sites.On sabbatical leave (1975) at the Royal Military College of Science, Shrivenham, Wilts., England.     

高分辨率合成孔径雷达卫星反演风场资料在中国近海风能资源评估中的应用研究

由于海上测风非常昂贵,实地测风资料严重不足,而卫星反演海面风场资料可以有效地弥补这一缺陷,对近海风能资源评估具有至关重要的意义。近年来,欧洲太空局2002年3月发射的Envisat卫星搭载合成孔径雷达（SAR）图像产品在欧洲近海风能资源评估中得到了广泛应用。文中探讨了空间分辨率约1 km×1 km的SAR卫星反演海面风场资料应用于杭州湾近海风能资源评估的技术方法,通过杭州湾海域的实测风速与SAR卫星反演海面风速数据的对比分析发现：（1）14个实测站点中13个站的相对误差小于20%,其中7个站的相对误差小于10%,平均标准差为2.29 m/s;（2）以SAR卫星反演风速数据为基础计算的风能参数（形状参数和尺度参数）与实测数据计算结果的一致性较高;（3）将该数据同化进入WRF数值模式中,与控制试验相比,大部分检验站点的风速相关系数明显提高,标准差和相对误差也得到改善。SAR卫星反演风场资料可用于中国的近海风能资源评估。     

在空气污染研究中应用风廓线参数之探讨

本文使用对数律和幂指数律公式,计算了年平均和不同稳定度、不同风速等级下的丘陵山地的风指数(α)和综合粗糙度(Z_0)值,对公式及计算结果的适用性进行了探讨。结果表明,对于地面边界层(SBL),此两公式在丘陵山区条件下均有较好的适用性。可利用某地年平均地面风速(V_(10))和数次实测烟囱口高度的风速(V_烟),应用对数律或幂指数律推算该地各高度上的风速。对于丘陵山区,对数律可适用于地面以上300m层次内。     

A wind tunnel boundary-layer simulation of wind flow over complex terrain: Effect of terrain and model construction

The report presents the results of a wind-tunnel study of the flow of the natural wind over complex terrain. A 1:4000 undistorted scale model of Gebbies Pass in the South Island of New Zealand was prepared and tested in the boundary-layer wind tunnel in the Department of Mechanical Engineering at the University of Canterbury.Three forms of construction, viz., terraced, contoured and roughness-added, were compared. Velocity and turbulence profiles, Reynolds stresses and spectra were measured, and correlation of results between different types of construction was calculated. The terraced form was much simpler to construct but was found to be unsatisfactory. The correlation between the contoured and roughness-added models was as high as 0.94, although the roughness-added model made a significant difference to the results in the lower 20%; of the boundary layer. The results of these tests will be compared with results from the field in a future report.     

山东沿海风能资源分布特征及评估

利用山东省沿海测风塔70 m高度完整1 a的观测资料计算分析风能资源参数特征.结果表明：山东沿海地区平均风速与有效风功率密度分布特征相似,烟台沿海区域平均风速及有效风功率密度最大分别达到6.7 m/s、463.5 W/m²,沿海北部地区风能资源最为丰富,日照地区最少;受海陆风作用,春季风能资源最好,其次是冬季,夏季最差,风速最大值基本出现在14-16时;年有效风能时数及百分率分别为7 440 h、85%;风能密度分布基本以偏北或偏南方位较大.沿海区域风能资源分布特征与长年代评估结果及数值模拟结果基本一致.     

An assessment of radiosonde launch conditions affected by the surface wind

Carried out is an analysis of radiosonde launch conditions affected by the horizontal surface wind. Mathematically stated is a condition of successful launch agreed qualitatively with the known practical methods and upper-air sounding techniques. This condition is defined by several main factors taking account of the clamping force impact on the balloon, adaptation of the balloon speed to the wind speed, and approaching of the instrumental box of radiosonde to the surface immediately after the launch due to the deviation of the harness string from the vertical. The results obtained provide a theoretical basis for analyzing the launch parameters in order to increase the reliability of upper-air sounding and to develop the ways of its successful use under conditions of strong winds.     

Synoptic and local scale atmospheric circulation associated with air pollution episodes in an urban Mediterranean area

Air pollution episodes in urban coastal areas follow certain pre-determined patterns, being associated with certain local meteorological conditions and emission of primary pollutants. In this study, the synoptic and local scale atmospheric circulation that prevails during air pollution episodes in a coastal major city in Greece, Thessaloniki, is examined for a period of 15 years (1989–2004). The study signifies the importance of studying air pollution meteorological patterns between coastal areas with different terrain characteristics. For Thessaloniki, it was found that the episodes occur mainly during the cold period of the year, while four types of synoptic scale circulation were recognized (I, II, III, IV) and five patterns of the local scale circulation (A1, A2, B1, B2 and B3). The highest percentage of episodes is associated with the presence of an anticyclone over the northern Greece (types I and IV), being characterized by weak or very weak surface pressure gradient intensity, according to the position and extension of the anticyclone. Moreover, a temperature increase of at least 1°C during the previous 3 days is required in the lower troposphere. Consistent with the synoptic conditions, the development of the sea breeze plays a crucial role in the occurrence of the episodes, even in the cold period of the year, when the sea breeze can still develop with smaller frequency and intensity. Finally, it was found that a small number of episodes is related with the advection of polluted air masses from the industrial area in the northwest of the city and from the Eordaia area in the west, which is the largest lignite producing area of Balkans.     

Short-term prediction of local wind conditions

Using Numerical Weather Prediction (NWP) models it has been shown that they, combined with models (either physical or statistical) taking local effects into account, can be used to predict the wind locally better than the models commonly used today (as eg persistence). By local is meant at one distinct spot, as eg the location of a meteorological mast. The physical model of local effects takes the following into account: shelter from near-by obstacles, the effect of roughness changes and the effect of the local orography. The large-scale flow is linked to the surface flow by the geostrophic drag law, and the logarithmic wind profile. The predictions are made up to 36 hours ahead. The model is tested on data from 50 meteorological stations scattered all over Europe.     

An objective classification system of air mass types for Szeged, Hungary, with special interest in air pollution levels

Summary This paper determines the characteristic air mass types over the Carpathian Basin for the winter (December, January, and February) and summer (June, July and August) months dependant on levels of the main air pollutants. Based on the ECMWF data set, daily sea-level pressure fields analysed at 00 UTC were prepared for each air mass type (cluster) in order to relate sea-level pressure patterns with the level of air pollutants in Szeged. The data comprise daily values of twelve meteorological and eight pollutant parameters for the period 1997–2001. Objective definition of the characteristic air mass types is achieved by using the methods of Factor Analysis and Cluster Analysis. According to the results, during the winter months five air mass types (clusters) were detected based on higher concentrations of primary pollutants that occur with high irradiance and low wind speed. This is the case when an anticyclone is found over the Carpathian Basin and over the region south of Hungary, influencing the weather of the country. Low levels of pollutants occur when zonal currents exert influence over Hungary. During the summer months anticyclones and anticyclone ridge situations are found over the Carpathian Basin. (During the prevalence of anticyclone ridge situations, the Carpathian Basin is found at the edge of a high pressure centre.) As a result of high irradiance and very low NO levels, secondary pollutants are highly enriched.     

Estimates of wind speed perturbation in boundary-layer flow over isolated low hills — Solutions

A computationally efficient model — the MS3DJH — was used to predict the near-surface wind-speed variations over a given idealised twin hill. Results given compare the model predictions with those of trained meteorologists. Further application of the model to assessment of e.g. power-producing turbine sites is discussed.     

A study of wind speed modification and internal boundary-layer heights in a coastal region

Wind profile data within the first two kilometres of a coast have been used to study the wind field modification downstream of this surface discontinuity. The land area is generally very flat, having an overall roughness length of 0.04 m. A wind model, suitable for practical applications and inexpensive to run, has been tested against the data and was found to give satisfactory results. Knowing the climatological statistics of wind and stratification, e.g., at the coast, the model may thus be used to estimate, on a climatological basis, how the wind field is modified with distance inland, at least in areas with only minor topography. This type of information is of great importance when locating wind turbines. It is in these cases also important to know the statistics of the internal boundary-layer (IBL) height, as the turbulence intensity may be quite different in and above the IBL, which in turn may influence load and fatigue calculations. Using the wind profile data, the IBL height was clearly discernible in the majority of cases. Having very unstable stratification over land, the IBL height could, however, not be determined from the wind profiles, as the wind in these cases did not decrease inland. This result was also obtained using the wind model. A simple model of the type z _IBL = a · x ^b, was instead tested, and was shown to give reasonable results.     

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     

An application of simple models to Sahelian convective boundary-layer growth

Profiles of temperature were measured in the atmospheric boundary layer (ABL) over the West African Sahel on eleven days at three different times of the year. The final depth of the mixed layer was shown to vary substantially during the year, but it was often greater than 2.5 km. Measured values of the sensible heat flux at the surface are used to force several simple slab models of the ABL. It is found that these simple models can describe the increases of height and temperature of the ABL during the day.     

The use of mesoscale numerical models to assess wind distribution and boundary-layer structure in complex terrain

Mesoscale models which can be used to assess wind and turbulent structure in complex terrain are overviewed. The different types of models — diagnostic and prognostic are discussed and the significant physical processes which each can handle realistically are reviewed. Examples of specific applications of these models are presented.     

Automated estimation of local wind enhancement in Novorossisk

A new concept of automated evaluation of local wind maximum speed in mountain regions is considered. The approach is based on taking into account the empirical dependence between the maximum registered wind enhancement in a particular district and prognostic evaluations of background meteorological parameters less detailed in space. Empirical dependencies are established for the Novorossisk region from the data of spring and fall of 2008. It is shown that the new approach complements the technology of maximum wind speed evaluation at the ground currently in use. In most cases, it allows rather accurate evaluation of expected local wind enhancement up to 20–30 m/s in Novorossisk in warm period.