Comparison of Beijing MST Radar and Radiosonde Horizontal Wind Measurements

To determine the performance and data accuracy of the 50 MHz Beijing Mesosphere–Stratosphere–Troposphere(MST)radar,comparisons of radar measured horizontal winds in the height range 3–25 km with radiosonde observations were made during 2012.A total of 427 profiles and 15 210 data pairs were compared.There was very good agreement between the two types of measurement.Standard deviations of difference(mean difference)for wind direction,wind speed,zonal wind and meridional wind were 24.86?(0.77?),3.37(-0.44),3.33(-0.32)and 3.58(-0.25)m s~(~(-1)),respectively.The annual standard deviations of differences for wind speed were within 2.5–3 m s~(-1)at all heights apart from 10–15 km,the area of strong winds,where the values were 3–4 m s~(-1).The relatively larger differences were mainly due to wind field variations in height regions with larger wind speeds,stronger wind shear and the quasi-zero wind layer.A lower MST radar SNR and a lower percentage of data pairs compared will also result in larger inconsistencies.Importantly,this study found that differences between the MST radar and radiosonde observations did not simply increase when balloon drift resulted in an increase in the real-time distance between the two instruments,but also depended on spatiotemporal structures and their respective positions in the contemporary synoptic systems.In this sense,the MST radar was shown to be a unique observation facility for atmospheric dynamics studies,as well as an operational meteorological observation system with a high temporal and vertical resolution.     

Laboratory Studies Of Wind Stress Over Surface Waves

Simultaneous laboratory observations of wind speed, wind stress, and surfacewind-wave spectra are made under a variety of wind forcing patterns using cleanwater as well as water containing an artificial surfactant. Under typical experimentalconditions, more than half of the total stress is supported by the wave-induced stressrather than by the surface viscous stress. When the surfactant reduces the shortwind-wave spectra, the wind stress also decreases by as much as 20–30% at agiven wind speed. When the wind forcing is modulated in time, the wind stresstends to be higher under decreasing wind than under increasing wind at a givenwind speed, mainly because the response of short wind-wave spectra to varyingwind forcing is delayed in time. These examples clearly demonstrate that therelationship between the wind speed and the wind stress can be significantlymodified if the surface wave field is not in equilibrium with the wind forcing.Next, we examine whether the wind stress is estimated accurately if the wave-inducedstress by all surface wave components is explicitly evaluated by linear superpositionand is added to the surface viscous stress. It is assumed that the surface viscous stressis uniquely related to the wind speed, and that the wind input rate is determined by thelocal, reduced turbulent stress rather than the total stress. Our wind stress estimatesincluding the wave contributions agree well with observed wind stress values, evenif the surface wave field is away from its equilibrium with the wind in the presenceof surface films and/or under time-transient wind forcing. These observations stronglysuggest that the wind stress is accurately evaluated as a sum of the wave-induced stressand the surface viscous stress. At very high winds, our stress estimates tend to be lowerthan the observations. We suspect that this is because of the enhancement of wind stressover very steep (or breaking) short wind-waves.     

2012年春季宜春地区降水期间高空风的频谱分析

利用地面和梯度塔的风连续观测数据进行的频谱分析已经开展很多,而受资料限制,高空风的频谱分析仍较欠缺。本文使用风廓线雷达获取的长时间序列连续测风数据,运用傅里叶变换的方法,计算了风的脉动谱密度。脉动谱能够反映不同频率的风速涨落对风动能的贡献。使用2012年4月江西宜春前汛期期间的高空风连续数据,结合地面降水资料进行了1 000～3 000 m高度区间的频谱分析,发现地处前汛期雨带上的宜春地区降水存在着两种不同时间周期的天气系统影响,脉动谱的分布表现出时间周期为5～7 d和2～3 d的峰值区。分别对两种不同时间周期的天气系统频谱进行了分析,并与平稳天气时的频谱进行比较。5～7 d周期峰区的脉动谱密度数值为2～3 d的4～5倍,脉动谱峰区在2 000～3 000 m高度上较强,峰值强度向下迅速降低;2～3 d周期的脉动谱峰区在低层比较明显,峰值强度较弱。风的脉动谱分布与地面降水的时间周期较为吻合。     

THE CISK MECHANISM AND THE INERTIAL GRAVITATIONAL WAVES IN SHEAR FLOW

By using the symmetric equations of atmospheric dynamics in y-z plane with vertical and horizontal shear of wind, the nonlinear ordinary differential equation is derived with the method of travelling wave.Its stability is discussed by using the theory of nonlinear stability and the KDV equation is solved. The effects of linear CISK, nonlinear CISK, inertial stability and vertical shear of wind on the amplitude and the width of isolated inertial gravitational wave are discussed. In order to understand deeply the formation and development of meso-scale synoptic systems, such as the squall line, MCC, the cold surge of Asia high and typhoon, the factors of development of the isolated inertial gravity wave are analysed.     

Wind variability in the northwestern part of the Black Sea from the offshore fixed platform observation data

The statistically ensured estimates of characteristics of temporal variability of wind speed and wind direction are obtained on the basis of observations carried out in 1996–2001 at the offshore fixed platform in the northwestern part of the Black Sea. The maximum values of monthly mean wind speed (more than 8 m/s) are registered in the cold half-year and the minimum ones (～4 m/s), in summer. The moderate winds of northern and southern directions dominate during the whole year. Using the Pearson’s chi-squared test, it is demonstrated that the wind speed follows the normal distribution law during the most part of observation period. The largest deviations from the normal law are timed to the periods of existence of intensive large-scale anomalies in the ocean-atmosphere system. A significant alternation of both synoptic and lower-frequency intramonthly wind speed fluctuations with typical periods of ～10–15 days is revealed. Their peak amplitude was registered in the fall and winter season of 1997/98, i.e., it was observed during the mature phase of one of the most intensive El Niños during the whole period of instrumental observations. At that time, the energy of intramonthly low-frequency wind speed fluctuations (～4 m²/s²) was equal to the energy of fluctuations within the synoptic range of the spectrum.     

Evaluation of wind resource using numerically optimized data in the southwestern Korean Peninsula

The wind distribution over the Korean Peninsula was analyzed using numerically optimized wind data to reduce the uncertainties in estimating the wind resources. The simulated data were validated by a comparison with surface wind observations and three statistical indexes. According to the simulated surface winds, mesoscale circulation, such as land-sea breeze and mountain-valley winds affect the wind characteristics of the hub height at coastal and inland regions. However, the prevailing winds are strongly associated with the synoptic forcing at the island and mountainous regions, not the regional circulation. On the other hand, the atmospheric stability definitely affects the strength of the daytime and nocturnal wind speed at a hub height. Overall, there was a significant difference between the numerical and logarithmic method to estimate the wind energy at hub height. Moreover, the discrepancy in the wind density estimated using the two methods becomes clear over inland and mountainous areas.     

On the conversion of Pasquill categories for use over sea

Pasquill stability categories or similar classifications can be used to estimate dispersion of airborne material. Such categories are defined in terms of synoptic surface-layer variables. In order to transpose experience gained over land to sea, the different boundary conditions over land and sea must be considered. Using the assumption that the turbulence intensity and hence diffusion will be the same over land and sea if the boundary conditions expressed by sensible heat flux and shear stress are equal, a conversion is derived. With a reasonable degree of accuracy, synoptic parameters, mean wind speed and air sea temperature difference can be used at sea to specify stability categories.During the course of this research, it was found that Golder's relation between stability categories and Monin-Obukhov length can not be used at sea, since this relationship must depend on the Bowen ratio and albedo.     

Wind forcing due to synoptic storm activity over the North Pacific Ocean

Abstract

Six‐hourly surface wind analyses over the North Pacific Ocean covering the 10‐year period 1969–78 are used to describe synoptic storm activity in terms of parameters that are directly related to the atmospheric forcing of the ocean. The cube of the atmospheric friction velocity, u³ _* and the curl of the surface wind stress, curl τ, are used because of their relationship to turbulent vertical mixing and Ekman pumping in the ocean, respectively. In an attempt to isolate synoptic disturbances from mean fields, the time series of surface wind components at each individual grid point are partitioned into “high‐pass” (periods shorter than 10 days) and “low‐pass” (periods longer than 10 days) components by means of conventional filtering procedures. The two quantities u³ _* and curl τ are then calculated from (a) the high‐pass filtered wind components only, (b) a combination of the filtered wind components that include the interaction between the high‐ and low‐pass fields, and (c) the unfiltered wind components. These quantities describe the atmospheric forcing of the ocean that is attributable to (a) synoptic storm activity by itself, (b) synoptic storm activity in the presence of the low‐pass (mean) flow, and (c) the total spectrum of wind forcing, respectively.

Maps of the long‐term (10‐year) monthly mean u³ _* calculated from (a) and (b) are coherent across the mid‐latitude North Pacific and appear to coincide with the normal seasonal evolution of synoptic storm activity in that region. In mid‐latitudes, the values of u³ _* calculated from (a) and (b) are 27 and 83%, respectively, of the value of u³ _* itself. Thus, a major fraction of the production of turbulent energy available for mixing in the upper layers of the ocean comes from synoptic disturbances with a period shorter than 10 days. Maps of the long‐term monthly mean wind stress curl are quite different in that the mean wind stress curl calculated from (a) is essentially negligible. However, the mean curl calculated from (b) closely resembles the pattern of total curl (c), but with a magnitude of only 41% of (c). Thus, synoptic disturbances with a period shorter than 10 days are also responsible for a significant fraction of the Ekman pumping of the ocean.

Future studies with these data will attempt to determine whether a relationship exists between synoptic storm activity, as measured by the parameters developed in this study, and large‐scale sea‐surface temperature anomalies.     

Regional shear stress of broken forest from radiosonde wind profiles in the unstable surface layer

Mean wind speed profiles were measured by tracking radiosondes in the unstable atmospheric boundary layer (ABL) over the forested Landes region in southwestern France. New Monin-Obukhov stability correction functions, recently proposed following an, analysis by Kader and Yaglom, as well as the Businger-Dyer stability formulation were tested, with wind speeds in the surface sublayer to calculate the regional shear stress. These profile-derived shear stresses were compared with eddy correlation measurements gathered above a mature forest stand, at a location roughly, 4.5 km from the radiosonde launch site. The shear stress values obtained by means of the newly proposed stability function were in slightly better agreement with the eddy correlation values than those obtained by means of a Businger-Dyer type stability function. The general robustness of the profile method can be attributed in part to prior knowledge of the regional surface roughness (z ₀=1.2 m) and the momentum displacement height (d ₀=6.0 m), which were determined from neutral wind profile analysis. The 100 m drag coefficient for the unstable conditions above this broken forest surface was found to beu _* ² /V ₁₀₀ ² =0.0173.     

中国东部7类暴雨异常环流型

近年来的研究发现,瞬变扰动天气图上的扰动场天气系统对区域暴雨的落区指示能力强于传统天气图上的总场天气系统。为供预报员在业务预报中参考,本文划分1998年发生在中国东部地区的41日次区域暴雨为7类扰动场天气系统。与区域暴雨相联系的7类异常环流型分别是:华南切变线、华南涡旋、华南倒槽、长江切变线与槽、沿江涡旋、华北涡旋和东北涡旋。无论是在对流层的垂直剖面上,还是在850 hPa水平分布上,扰动天气图上位势高度低值和风扰动辐合处并配合大的水汽扰动对应有区域暴雨,而传统天气图上的低值系统和高水汽区与暴雨之间存在位置上的偏移。由此建议,用实况大气变量和中期数值模式产品绘制扰动天气图有助于预报员确定区域暴雨落区。     

钦州市一次春季连续性大雾气象特征分析

对2011年2月6～9日钦州市连续性大雾天气过程进行分析,结果表明：有利的大气环流形势为大雾形成提供有利天气背景;持续的东南风和适宜的风速为大雾的形成提供了有利水汽条件;与稳定度有关的物理量场及与水汽条件有关的相对湿度场对预报大雾天气有一定的指导意义;逆温层结对水汽的聚集有重要贡献。     

THE TEMPORAL AND SPATIAL DISTRIBUTION OF SEA SURFACE WIND ALONG THE COAST OF GUANGDONG PROVINCE IN CHINA

Temporal and spatial distribution characteristics of sea surface wind in Guangdong''s coastal areas were analyzed with data from four offshore observational stations between 2012 and 2015. The results are shown as follows: (1) The probability distribution of wind speed was basically consistent with Gaussian distribution characteristics; winds of Beaufort force 6 or higher were observed mainly in far offshore stations from October to March. (2) The probability distribution of wind direction was represented well by Weibull distribution. The deviation of wind direction of far station was relatively small for it was mainly controlled by monsoon over the South China Sea, while the near offshore station had a relatively large diurnal variation because of the influence of local synoptic systems such as sea-land breeze. (3) There were significant seasonal differences in wind speed and direction observed by different offshore observational stations. In strong wind seasons, the deviation of wind direction was relatively small while the deviation of wind speed was relatively large, and vice versa. In contrast with Class I station, the other three stations exhibited approximately normal distribution of wind direction and wind speed deviations. (4) Wind direction diurnal variation was moderate in windy periods, while it was obvious in relatively lower speed conditions. The deviation of wind speed in windy periods was generally greater because it was influenced by mesoscale weather systems for 10-20 h, and the influence was complicated, resulting in greater local differences in wind speed.     

Turbulent fluxes over inhomogeneous landscape

Mesoscale surface turbulent fluxes over a complex terrain surrounded by oceans have been investigated using a 3-D numerical mesoscale model, under conditions with and without synoptic flows. The study indicated that under synoptically calm condition, the allocation and intensity of mesoscale surface turbulent fluxes (MSTFs) were greatly impacted by the thermally forced mesoscale circulation (TFMC) over mesoscale heterogeneous landscape. The max-imum values of sensible (Hs) and latent (LE) heat fluxes were located over the convergent zones and considerably im-pacted by the soil wetness (M), but did not depend strongly on the atmospheric background thermal stability (β0). The simulated results suggested that the sensible heat flux was closely proportional to the square of wind speed in the surface layer. By the action of synoptic flow, the allocation of LE was shifted to downwind, its intensity increased.     

Low-Level Wind Shear Characteristics and Lidar-Based Alerting at Lanzhou Zhongchuan International Airport,China

Lanzhou Zhongchuan International Airport [International Civil Aviation Organization(ICAO) code ZLLL] is located in a wind shear prone area in China, where most low-level wind shear events occur in dry weather conditions.We analyzed temporal distribution and synoptic circulation background for 18 dry wind shear events reported by pilots at ZLLL by using the NCEP final(FNL) operational global analysis data, and then proposed a lidar-based regional divergence algorithm(RDA) to determine wind shear intensity and location. Low-level wind shear at ZLLL usually occurs in the afternoon and evening in dry conditions. Most wind shear events occur in an unstable atmosphere over ZLLL, with changes in wind speed or direction generally found at 700 hPa and 10-m height. Based on synoptic circulations at 700 hPa, wind shear events could be classified as strong northerly, convergence, southerly, and weak wind types. The proposed RDA successfully identified low-level wind shear except one southerly case, achieving94% alerting rate compared with 82% for the operational system at ZLLL and 88% for the ramp detection algorithm(widely used in some operational alert systems) based on the same dataset. The RDA-unidentified southerly case occurred in a near neutral atmosphere, and wind speed change could not be captured by the Doppler lidar.     

十一运会沿海赛地大风预报技术研究

选取了十一届全运会比赛地烟台、威海、日照3站1971—2008年逐日10min平均最大风速,统计分析了比赛期间(9—11月)大风气候特征。结果表明:秋季大风随着年代的变化,大风日数逐渐减少,共同点是9—11月大风日数逐渐增多;不同区域大风发生几率差异较大,位于北部沿海的威海、烟台出现大风的几率明显大于南部沿海的日照,尤其以威海几率最大。利用2001—2008年的天气图资料,对产生大风的天气系统进行了普查,结果表明,9—11月造成烟台、威海、日照大风的主要有低槽冷锋、气旋、热带气旋等天气系统以及东高西低和南高北低两种形势场。利用NCEP再分析资料,对产生大风的各类天气系统天气形势进行了合成分析,并给出各类天气系统产生大风时平均形势场。     

南海北部风浪特征的计算

探讨南海北部海域风浪成长时有效波高与风速、风时、风区之间的关系,同时分析了5种风浪要素的推算方法,探讨其在南海北部海域的适用性。结果表明：1）在南海北部,风速和风时呈现线性增长的关系,风速越大,风浪从过度状态成长到充分成长状态所需风时就越长;风速大小和风区长度之间满足平方关系,风速越大,风浪充分成长所需风区长度就越长。2）在南海北部,有效波高的大小与风速的大小、风时的长短、风区的长度3者密切相关。3）SMB方法、W ilson IV方法和青岛方法,在计算南海北部的风浪关系中体现出了一定的稳定性和适应性。     

从单Doppler速度场反演风矢量场的VAP方法

从单Doppler速度场反演风矢量场的VAP(Velocity Azimuth Processing)方法是在假定相邻方位角的风矢量相等的前提下,从单部Doppler雷达所测得的各个距离圈上的径向速度随方位角分布的廓线推算风向和风速的方法。这个假定可以保证反演所得的风矢量场中仍保留次天气尺度的风场信息。 对VAP方法反演公式的误差分析表明,反演出的风向误差为20—30度,与常规测风的误差相近。风速误差一般为10~0m/s,但在某些特定的情况下可达到与风速相同的量级(10~1m/s)。 用VAP方法反演风矢量场前必须首先消除原始Doppler速度资料中的脉动。它可以通过简单的一维线性平滑实现。 文中给出了一个反演实例,从中可以分析出次天气尺度的切变线和辐合线。常规天气图和UHF雷达的风廓线观测表明,这些次天气尺度系统是存在的。它们与反射率因子分布的配置还表明这些次天气尺度系统是和降水相联系的。     

On the relationship of radar backscatter to wind speed and fetch

The physics of the interaction of electromagnetic waves with the ocean surface has been an active area of research for a number of years. We present here the results of satellite and aircraft experiments to investigate the ability of active microwave radars to infer surface wind speeds remotely. Data obtained from the recent National Aeronautics and Space Administration (NASA) Skylab experiment are compared with surface wind speeds measured by low-flying aircraft and ships-of-opportunity and found to give useful estimates of the oceanic wind field. We also investigate the influence of varying wave height on radar measurements of wind speed by measuring the backscattering cross-section for constant wind speed but variable wave conditions. We conclude that this effect is of little importance.     

New evidence for a relation between wind stress and wave age from measurements during ASGAMAGE

Data from the 1996 ASGAMAGE experiment, performed in the southern North Sea at research platform Meetpost Noordwijk (MPN), are analysed for the parameters affecting the momentum flux. The stress turns out to be quadratically related to the 10-m wind speed and linearly to the wind speed at a wavelength related level. The Charnock parameter (dimensionless roughness length) shows a pronounced correlation with wave age. This implies, due to a coupling between wave age and the steepness of the waves, a connection between the stress and the steepness. We find that our North Sea results are consistent withopen ocean observations. For a given wind speed the mean stress at MPN turns out to be higher because the wave age there is in general lower. We define and give an expression for a drag coefficient at a wavelength related level that can be calculated straightforwardly from the wave age and then reduced to a standard level.