声雷达定量测量C_T~2中的几个问题

本文讨论了逾量衰减和风对声线的弯曲作用对声雷达接收功率的影响.分析和计算表明在很多情况下分子吸收、逾量衰减和风的影响对测量C_T~2是不能忽略的,不考虑这些因素有时可使C_T~2低估500％以上.引进有效湍流外尺度后使声散射截面表达式在实际应用中变得合理而简单.由简化的几何模型导出了风衰减因子,经计算和以往的实验事实相符.因此在原声雷达方程中应加入风衰减因子.声雷达方程加上逾量衰减算式、风衰减算式和分子衰减算式,组成了适合定量测量C_T~2的声雷达方程组.利用温度脉动仪测定低高度上的C_T~2值可使估算工作简化,此方法在多普勒声雷达上应用效果会较好,本文并给出了初步实验结果.     

声波逾量衰减的估算

本文计算表明速度场湍流对逾量衰减的作用比温度场湍流的大5倍.在有限宽度声束条件下,引入有效湍流外尺度Le后,计算表明在大多数情况下逾量衰减系数难以忽略.当波束有张角时,同样的大气湍流状态下逾量衰减系数值随离声源距离增大而增大.过量衰减系数与声波波长关系近似为Ke∝λ~(-2).提供了考虑通量衰减作用的计算C_T~2的简单方法并给了几个计算实例.     

声波在湍流大气中的散射衰减

文章推导出了改进的大气声散射截面σ(θ)表达式，确定了大气声散射系数αs的解析表达式，并得出改进的大气声折射指数脉动结构系数c2n表达式。实测结果表明，在潮湿大气中，湿度脉动和温湿相关脉动与温度脉动一样是影响声散射衰减的重要因子，而在干燥大气中，湿度脉动的影响可以忽略。     

山回波的PIA因子在3 cm雷达波衰减订正中的应用

通过比较同一山体在晴雨两种天气条件下的回波反射率,确定在雷达与山体之间的降水赞成山体回波的路径积分衰减因子（Ｐａｔｈ－ｉｎｔｅｇｒａｔｅｄ ａｔｔｅｎｒａｔｉｏｎ ｆａｃｔｏｒ）,以此计算雷达与山体之间降水的真实反射率,从而把降水对Ｘ（３ｃｍ波长）面雷达波的衰减订正掉。     

贵州威宁X波段双偏振雷达反射率因子的衰减订正分析

针对贵州威宁X波段双偏振雷达的衰减订正问题,在利用综合小波去噪方法对差分传播相移ФDP进行质量控制基础上,基于自适应衰减订正算法对雷达反射率因子ZH进行衰减订正分析,订正结果与昭通C波段新一代天气雷达进行对比,结果表明：（1）综合小波去噪能够有效去除ФDP存在的脉动和毛刺,保证ФDP的连续性和平滑度;（2）订正前后雷达反射率因子在距离雷达较近处（20-35 km）强度廓线基本重合,订正前后差别不大,随着对流区（雨区）距离的增加,电磁信号出现衰减,订正后反射率值加强,总体上订正后的反射率值比订正前高1-12 dB,其中在35～75 km距离范围内,订正后的反射率值达到50 dBz以上,雷达反射率因子更加接近真实情况;（3）使用综合小波去噪配合自适应衰减订正算法可以提高雷达反射率因子衰减订正的准确率,方法具有普适性,在今后科研业务中可以采用该订正方法以进一步提高对粒子相态识别、降水估测精度。     

基于微波链路的雷达回波衰减订正个例分析

降雨对微波的衰减作用在雷达探测和信号通信中有着不可忽略的影响。本文利用近年来出现的微波链路测量新方法,研究了利用微波链路进行天气雷达回波衰减订正的方法。微波链路可以直接获取传播路径上的总衰减,基于此建立衰减订正的约束条件,并利用前向迭代法对天气雷达回波进行逐库订正。首先从现有衰减订正的理论出发,通过确定微波链路衰减参考值、雷达衰减订正算法和衰减系数,建立基于微波链路的天气雷达衰减订正模型;搭建9.47GHz微波链路测量试验系统,对X波段雷达进行衰减订正试验。针对一次个例进行分析,与自动雨量站同步对比观测试验表明:订正后的雷达反射率因子明显高于雷达实测反射率因子;订正后雷达估测的降雨量与自动雨量站的观测差值减小;累积降雨量绝对偏差由-6.11mm减小到1.04mm,相对偏差由0.99%减小到0.17%;订正后估测的降雨强度与雨量计的相关性得以提高。该订正算法主要通过对参数的调整,使得k-Z关系更符合实际,从而提高了探测精度,验证了该方法的可行性与准确性。     

不同波段垂直指向雷达功率谱密度对比

衰减对回波的影响是雷达探测中的一项重要课题,常用订正方法还有很大提升空间。由于垂直指向雷达观测的功率谱数据包含了非常丰富的微物理及动力信息,该文尝试在功率谱层面对衰减影响机理进行深入讨论,以期改进衰减订正方法。对2014—2016年广东阳江和广东龙门获取的Ka波段毫米波雷达、C波段调频连续波雷达、Ku波段微雨雷达功率谱数据进行对比表明:3部雷达功率谱谱型基本一致,具有一定可靠性。米散射效应反映在功率谱上即经过某一速度后回波强度谱密度的陡降,当降水下落末速度达临界速度时,会对衰减量的计算造成影响。雷达波长越短,受衰减影响越大,衰减对功率谱的影响表现为谱的整体下移,最终导致回波强度小于真值。对于对流云降水,单纯使用经验订正公式获得的结果可能与真实值相差很大,此时需要考虑降水粒子谱分布,在功率谱层面上对订正进一步改善。     

结合风廓线雷达的毫米波衰减特性初步研究

使用中国气象局大气探测综合试验基地35 GHz毫米波云雷达和L波段风廓线雷达2016年5月1日-7月31日在降水条件下的观测数据,根据不同观测模式下两部雷达得到的数据,计算在一定高度区间内不同下落速度的降水粒子反射率因子变化量,初步分析不同下落速度的降水粒子对毫米波衰减的影响。结果表明:在持续时间较长的层状云降水且降水粒子在雷达观测范围内均匀分布条件下,毫米波衰减与降水粒子下落速度呈近似线性关系,且毫米波经过的路径长度越长,衰减越大;毫米波在经过1110~2430 m,1110~3510 m的高度区间时,下落速度处于3.5~7.5 m·s-1之间的降水粒子对毫米波的衰减作用导致毫米波云雷达所测的等效反射率因子分别减小约1~7 dB和2~11 dB。     

网络化衰减订正方法在北京X波段网络化雷达中的应用

衰减是影响X波段雷达数据质量的主要因素之一,直接影响到X波段雷达在强对流监测预警中的应用效果。文中在分析中、外雷达衰减订正方法的基础上,选择并改进了一种利用雷达网中不同雷达相互订正的方法,该方法通过不断迭代运算,使订正误差达到最小,形成了适合于网络化X波段雷达的反射率因子衰减订正方法。订正结果与原始雷达数据以及差分相位（${\phi _{{}_{{\rm{DP}}}}}$）订正法结果进行对比,并且还同时与北京S波段雷达观测数据进行了对比。结果表明：衰减订正对X波段雷达穿透强降水后的回波以及探测距离较远的回波效果比较明显,订正后的回波与S波段雷达观测结果比较吻合。     

雨区衰减及谱变化影响雷达测雨精度的数值模拟

用随机函数的方法，并假定雨滴谱为Ｇａｍｍａ分布，模拟并讨论了雨滴谱变化及雨区衰减对雷达和双线偏振雷达测雨误差的影响。结果表明：双线偏振雷达测雨精度受雨区衰减及滴谱变化的影响均小于普通雷达。雨区衰减使双线偏振雷达测雨产生小雨高估，大雨影响不大的现象，其主要原因可能是差反射率因子ＺＤＲ的测量误差。     

Radar and sodar probing of waves and turbulence in statically stable clear-air layers

This paper reviews the remote sensing of waves and turbulence in statically stable atmospheric layers, utilizing sodar and microwave radar echoes from the small-scale inhomogeneities in gaseous refractive index caused by localized fluctuations in temperature, humidity, and velocity. Scattering theory and sounding methodology are reviewed briefly, and the relative performance of typical radar and sodar systems compared. The main section of the paper takes the form of a summary and discussion of experimental progress since 1969, showing how the echo patterns obtained may be applied to the interpretation of multiple layering, gravity waves, internal fronts and the details of dynamic instability and the genesis of turbulence in stably stratified shear layers. In addition, methods for the measurement of the intensity of the small-scale ( /2) variability of wind, temperature and water vapor from the observed radar or sodar echo intensities, and the use of Doppler techniques for the measurement of mean velocity and turbulence are discussed.SODAR from SOund Detection And Ranging in analogy to RAdio Detection And Ranging.     

Use of a High-Resolution Sodar to Study Surface-layer Turbulence at Night

Measurements in the atmospheric surface layer are generally made with point sensors located in the first few tens of metres. In most cases, however, these measurements are not representative of the whole surface layer. Standard Doppler sodars allow a continuous display of the turbulent thermal structure and wind profiles in the boundary layer up to 1000 m, with a few points, if any, in the surface layer. To overcome these limitations a new sodar configuration is proposed that allows for a higher resolution in the surface layer. Because of its capabilities (echo recording starting at 2 m, echo intensity vertical resolution of approximately 2 m, temporal resolution of 1 s) this sodar is called the surface-layer mini-sodar (SLM-sodar). Features and capabilities of the SLM-sodar are described and compared with the sodar. The comparison of the thermal vertical structure given by the SLM-sodar and the sodar provides evidence that, in most cases, the surface layer presents a level of complexity comparable to that of the entire boundary layer. Considering its high vertical resolution, the SLM-sodar is a promising system for the study of the nocturnal surface layer. The nocturnal SLM-sodar measurements have shown that, depending on wind speed, the structure of the surface layer may change substantially within a short time period. At night, when the wind speed is greater than 3 m s⁻¹, mechanical mixing destroys the wavy structure present in the nocturnal layer. Sonic anemometer measurements have shown that, in such cases, also the sensible heat flux varies with height, reaching a peak in correspondence with the wind speed peak. Under these conditions the assumption of horizontal homogeneity of the surface layer and the choice of the averaging time need to be carefully treated.     

Observations of Atmospheric Solitary Waves in the Urban Boundary Layer

The simultaneous operation of a three-axis Doppler sodar system in the centralurban area of Rome and two similar systems in the suburban area, forming atriangle about 20 km on each side, provided evidence of solitary-type wavesin the urban boundary layer. Three events, each lasting from a few minutes toabout 30 min, and ranging in depth from the minimum range of the sodar (39 m) to over 500 m, are reported here. Two events were recognizable onall three sodar records while the third event could be observed at the urbanlocation only. Time-height acoustic echo intensity records showed no-echoregions within the wave indicating transport of trapped recirculating air.This is typical of large amplitude solitary waves. The time series plots ofsodar-derived vertical wind velocity revealed a maximum peak-to-peakvariation of about 5 m s^-1 during periods of wave-associated disturbance.The vertical velocity is found to increase with height up to the top of the closedcirculation within the wave and decreases further above. The normalisedamplitude-wavelength relationship for the two events indicates that theobserved waves are close to a strongly nonlinear regime.     

若干声雷达回波图象的分析

本文分析了近些年由声雷达观测到的海（陆）风、大风、下坡风和雾的回波图象和相应气象参数变化的特征。     

Foehn signals detected by sodar wind and turbulence measurements in the Rhine Valley, Austria, during the MAP field phase

Summary Within the Mesoscale Alpine Programme MAP conducted in autumn 1999, the vertical structure and the temporal evolution of the planetary boundary layer (PBL) in the Rhine Valley 2km south of Lake Constance were observed with a Remtech PA2 sodar (sound-detection-and-ranging instrument) rendering half-hour averages of the three-dimensional wind profile within the lowest kilometre above ground. During Foehn events, tethered balloon soundings and wind profiler measurements were conducted in addition to the rawinsonde network which was built up for the MAP field campaign.The remote sensing instrument renders a surprisingly high number of valid data during south Foehn. Due to the frequent formation of a cold air pool with stable conditions below the Foehn flow with near-neutral static stability, even more sodar data is valid during Foehn periods than during no Foehn periods. A significant reduction of the sodar data quality is only observed during Foehn events with grounding of the Foehn at the sodar site due to high background noise. At higher levels, a Foehn signal can be detected from the sodar wind and turbulence intensitiy information. With Foehn, higher wind speeds and larger turbulence intensities occur than without Foehn. Comparisons to rawinsonde and tethersonde soundings and wind profiler measurements at sites nearby reveal the spatial inhomogeneity of the Foehn flow within this part of the valley as well as instrumental short-comings. Different methods to determine the mixing height using the vertical sounding devices lead to some uncertainty of mixing height estimates which however can reasonably be explained.     

Detection of Inversions and Mixing Height by REMTECH PA2 Sodar in Comparison with Collocated Radiosonde Measurements

Summary A REMTECH PA2 Doppler Sodar is operated regularly at the Czech Hydrometeorological Institute (CHMI) observatory in Prague, collocated with a routine rawinsonde sounding system. The Air Pollution Control Division of CHMI utilises the sodar data in air pollution studies and as an information support for the smog warning system operated in Prague. Besides of the basic software for echo strength and wind profile evaluation, optional routines for deriving parameters such as inversion and mixing height, stability class etc. were delivered by the sodar manufacturer. Based on a sufficiently large data set (more than one year) of synchronous sodar and rawinsonde measurements, an analysis and comparison of inversion and mixing heights provided by both sounding systems have been accomplished in order to evaluate the correctness and accuracy of sodar estimates of these parameters. In contrast to the wind speed and wind direction data, for which a satisfactory agreement with other kind of measurements has been reported by many studies, the results for inversion and mixing height detection were totally disappointing. A direct applicability of inversion height and mixing height data provided by the REMTECH’s automatic routines in air pollution studies or smog warning systems is quite problematical with the present “state of the art”. Received November 3, 1998 Revised April 20, 1999     

近山地区大气边界层声雷达回波的季节特征

本文根据北京附近某一近山地区冬季和夏季的声雷达探测结果,对比了大气边界层声雷达回波图象在季节上的相似性和差异,分析了冬夏逆温层和对流混合层的生消演变特征,并对声雷达回波图象的几种类型与边界层风廓线的关系作了初步探讨。     

Wavy Vertical Motions in the ABL Observed by Sodar

Some characteristics of wavelike motions in the atmospheric boundary layer observed by sodar are considered. In an experiment carried out in February 1993 in Milan, Italy, Doppler sodar measurements were accompanied by in situ measurements of temperature and wind velocity vertical profiles using a tethered balloon up to 600 m. The oscillations of elevated wavy layers containing intense thermal turbulence, usually associated with temperature-inversion zones, were studied by using correlation and spectral analysis methods. The statistics of the occurrence of wavelike and temperature-inversion events are presented. The height distributions of Brunt–Vaisala frequency and wind shear and their correlation within elevated inversion layers were determined, with a strong correlation observed between the drift rate of the wavy layers and the vertical velocity measured by Doppler sodar inside these layers. Spectral analysis showed similarities regarding their frequency characteristics. The phase speed and propagation direction of waves were estimated from the time delay of the signals at three antennae to provide estimates of wavelength. Moreover, wavelengths were estimated from the intrinsic frequency obtained from sodar measurements of the Doppler vertical velocity and oscillations of wavy turbulent layers. The two wavelength estimates are in good agreement.