CINRAD/CC雷达发射功率在线测量误差探讨

雷达发射功率是雷达一个非常重要的指标,雷达发射功率Pt的测量误差将直接影响反射率因子Z的测量精度.CC雷达使用微波脉冲信号峰值检测仪对雷达发射功率进行实时在线测量,在实际运用中,发现该仪器测量值与实测值有一定误差,本文首先对降水计算原理和自动标校原理进行阐述,对微波脉冲信号峰值检测仪的结构特点进行了介绍,对波导定向耦合...     

自适应高斯频域滤波器在天气雷达中的应用

多普勒天气雷达的基数据(反射率因子、速度和速度谱宽)经常会被地物杂波所污染,这对基数据估计的准确性影响很大,进而又会影响导出产品。因此,抑制地物杂波是所有多普勒天气雷达获取高质量数据产品的必要步骤。作者首先介绍了自适应高斯频域滤波器的研究背景,其次分析了多普勒天气雷达地物杂波及天气回波信号的特征,再次给出了IIR椭圆地物杂波滤波器的原理以及自适应高斯频域滤波器的滤波算法,最后根据实际的雷达回波信号,分析了IIR椭圆地物杂波滤波器和自适应高斯频域滤波器的杂波抑制性能,并对结果进行了分析和比较。结果表明:不使用杂波图的自适应高斯频域滤波器的杂波抑制性能优于使用杂波图的IIR椭圆滤波器,且满足实时处理的要求。     

相关法从常规天气雷达信号中获取多普勒诺宽的模拟试验

理论上,对常规天气雷达信号进行相关分析可得多普勒谱宽,但此法是否有实用价值即实测精度如何是一个需要探讨的问题。用模拟方法对此进行研究是比较理想的既经济又灵活。本文模拟了一个可以改变谱形参量的接近实际的随机多普勒复频谱,对复频谱进行傅里叶反变换得到天气雷达回波的时域信号。然后对时域信号进行相关分析以求得自相关曲线和多普勒标准差。通过不同条件下多次试验结果的分析,得出不同的信噪比及多普勒谱形对相关法测量多普勒谱宽精度的影响,这对于用相关法从实际的常规天气雷达信号中获取多普勒谱宽有一定的指导意义。     

广义余弦窗对全固态天气雷达弱目标探测的影响

气象雷达脉冲压缩技术的应用关键是要尽可能地抑制脉冲压缩后的距离旁瓣。具有钟形特性的广义余弦窗加权函数能有效的平滑频谱边缘,提升主旁瓣比,因而这类窗被广泛地应用于线性调频信号的脉冲压缩旁瓣抑制技术中。文章采用广义余弦窗作为加权网络对线性调频脉压信号进行抑制旁瓣的处理,并对其探测弱天气目标的能力做了对比分析。文章首先介绍了脉冲压缩技术以及评判旁瓣抑制效果的主要性能指标,其次对广义余弦窗的频谱特性进行了仿真和分析,最后将广义余弦窗引入全固态天气雷达真实回波数据的信号处理中,得到处理后的回波图并结合现有的多普勒天气雷达反射率因子回波图给出了对比分析结果。     

用频谱仪检修CINRAD/CC雷达接收系统故障

针对云南省普洱雷达站新一代天气雷达接收系统出现的一次特殊故障,通过对主要性能指标进行标定和分析,判断雷达接收系统出现故障,并根据接收系统的工作原理和信号流程,运用频谱分析仪对接收系统的各信号频谱进行测试分析,完成故障定位和排除。此次故障排除过程充分体现了频谱分析仪在接收系统故障处理中的重要性。     

FM-CW多普勒雷达

FM-CW(调频连续波)雷达自1969年问世以来,一直用于晴空探测。最初是对雷达反射率的结构进行定性显示。晴空雷达回波往往出现波动,信号频率是取决于目标距离和目标速度,因此一般认为FM-CW雷达不能得到离散目标的多普勒速度谱。但是重复扫描发射频率信号的相仕只取决于目标速度,所以用分析功率频谱的方法可以得到多普勒频谱。1975年进行的FM-CW雷达多普勒处理试验表明:EM-CW雷达除了可以探测晴空雷达散射,还可以探测水汽凝结体的多普勒频谱。     

不同数字滤波方法在风暴轴研究中的性能比较

比较了Butterworth带通滤波器和不同截断方式下的对称滤波器在风暴轴研究中对天气尺度瞬变波动的滤波性能。理论分析表明：当n≥9时,在天气尺度为2．5～6．0d的定义下,对称滤波器对天气尺度瞬变波动的滤波性能优于Butterworth带通滤波器。针对逐日观测资料,对称滤波器在研究天气尺度瞬变波动问题时取截断参数n=15相对最优。在此截断参数条件下,正弦截断对称滤波器的滤波性能优于梯形截断对称滤波器,且两者均优于Butterworth带通滤波器,这种优势在风暴轴偏强的厄尔尼诺年体现得更为明显。在样本序列较短的情况下,为了得到天气尺度瞬变波动,利用Butterworth带通滤波器并选取带通滤波参数为2．5～8．0d的做法是可取的。     

CINRAD/SA雷达标定技术研究

从雷达数据质量需求出发,在4个方面对CINRAD/SA标定技术进行了研究。1结合雷达回波强度定标原理,分析了目前回波强度定标过程中存在的问题,在此基础上详细制定了回波强度定标测试方法、操作步骤和技术要求,实现了SA雷达回波强度定标方法和操作流程的规范化、标准化,从而消除将人为因素或测试原因引入雷达系统造成定标误差。2对全省雷达波导长度进行实际测量,并修正了各雷达发射支路馈线损耗出厂测试数据;研制了能注入微波信号的专用测试波导,实现台站准确测量出SA雷达收、发支路馈线的实际损耗数值,应用于雷达回波强度定标中。3将太阳作为微波信号源,根据发布的太阳能流密度等数据,结合实际测量接收到的太阳射电功率,来检验全链路雷达接收系统回波强度。4取相邻雷达等距离线(中点)的低仰角同步观测反射率因子数据,开展两部雷达、多部雷达对比观测检验等。从而提高了回波强度定标的客观性和一致性,更好地为组网雷达提供一致性的雷达数据。     

CINRAD/CC型新一代天气雷达激励源故障分析

激励源是全相参多普勒天气雷达发射分系统射频激励调制脉冲信号和标定分系统射频测试调制脉冲信号的提供者,如果激励源输出的RF调制脉冲信号的功率下降比较多,或者时序控制脉冲幅度和脉宽发生变化,或者进入激励源的本振1、本振2的频率和功率发生变化,都将引起发射机发射功率的大幅度下降,从而造成回波信号异常等故障现象.通过2010年10月27日新疆阿克苏天气雷达激励源故障现象,并结合激励源的基本原理及雷达正常运行和非正常情况下关键点波形的对比分析揭示了CINAD/CC天气雷达激励源故障的成因,给出了故障诊断依据和测试排除的具体办法,为保证天气雷达业务实时运转提供了保障.     

新一代天气雷达发射机寄生频谱分析与处理方法

针对新一代天气雷达(CINRAD)发射机频谱中存在寄生谱线的现象,建立了雷达发射信号寄生调制的数学模型,根据CINRAD雷达发射机组成原理框图,分析了寄生谱线产生的原因,并提出了解决雷达发射机寄生谱线问题的方法,利用该方法在某CINRAD雷达系统联调和测试中基本消除了发射机输出频谱的寄生谱线,提高了发射机的频谱纯度.     

低纬大气滤波模式研究

本文从描写低纬大气运动的方程组出发,采用半地转模式和准地转模式对低纬快速波动进行了滤除,这些模式仅含低频的Rossby波。分析指出:低纬半地转模式可以滤去高频波动,但得到的是波数k→0的Rossby波;而低纬准地转模式不但可以滤去高频波动,而且得到的是不受歪曲的Rossby波。 在滤波的基础上,本文分析了由Rossby波引起的低频振荡周期。      

数字滤波初始化方案在高分辨率GRAPES区域模式中的应用研究

高分辨率同化系统中引入的高频率高密度观测信息会造成分析变量之间的不平衡,因而在模式积分中产生的虚假重力波会严重影响模式预报质量和运行稳定。为了抑制虚假重力波在模式中快速增长,文中通过在高分辨率GRAPES区域模式中应用非绝热数字滤波和增量数字滤波初始化方案,研究不同滤波截断周期的初始化方案对分析预报的影响。试验结果表明,不同初始化方案的分析预报效果依赖于截断周期,截断周期越长滤除的高频信息越多,对于3 km分辨率模式,30 min与15 min截断周期的初始化方案既滤除了高频噪音,也基本保留了有意义的分析信息。不同截断周期的数字滤波方案在GRAPES-MESO系统中对气象要素预报场的影响基本可忽略;而在GRAPES-RAFS系统中,初始化方案对分析和预报的影响会逐渐累积,15 min截断周期的初始化方案比2 h截断周期的初始化方案的风场预报和降水预报质量更优。      

GRAPES-3DVar高阶递归滤波方案及其初步试验

背景误差协方差矩阵B及其逆的求解是三维变分同化研究的核心问题之一.在GRAPES区域三维变分同化系统(GRAPES-3Dvar)中背景误差协方差矩阵的水平变换部分,假定各向同性并进行递归滤波运算.原有方案中采用一阶递归滤波器,但收敛不够迅速,每次循环同化时需滤波10次才能使目标函数收敛.根据Purser等2003年的研...     

NUMERICAL EXPERIMENTS AND ANALYSIS OF DIGITAL FILTER INITIALIZATION FOR WRF MODEL

Initialization and initial imbalance problem were discussed in the context of a three-dimensional variational data assimilation system of the new generation “Weather Research and Forecasting Model”. Several options of digital filter initialization have been tested with a rain storm case. It is shown that digital filter initialization, especially diabatic digital filter initialization and twice digital filter initialization, have effectively removed spurious high frequency noise from initial data for numerical weather prediction and produced balanced initial conditions. For six consecutive intermittent data assimilation cycles covering a 3-day period, mean initialization increments and impact on forecast variables are studied. DFI has been demonstrated to provide better adjustment of the hydrometeors and vertical velocity, reduced spin-up time, and improved forecast variables quantity.     

Atmospheric data assimilation on the equatorial beta plane

Abstract

Kalman filter theory shows great promise when applied to the assimilation of atmospheric observations. Previous work has concentrated on extratropical dynamics, and tropical aspects have not yet been seriously tackled. In this article, a Kalman filter is applied to the linearized shallow water equations on an equatorial beta plane. The system or model error is constructed from the slow eigenmodes of the model and is based on an expansion in parabolic cylinder functions. The resulting second‐moment statistics are discussed in some detail. The Kalman filter is applied to a special observation network that allows the diagonalization of the system. Following Daley and Ménard (1993), it is then possible to obtain the complete space and time solution for the second‐moment forecast and analysis error statistics. The slow (low‐frequency) and fast (high‐frequency) error statistics are examined separately for both the optimal and suboptimal cases.     

A Variable Mesh Spacing for Large-Eddy Simulation Models in the Convective Boundary Layer

A variable vertical mesh spacing for large-eddy simulation (LES) models in a convective boundary layer (CBL) is proposed. The argument is based on the fact that in the vertical direction the turbulence near the surface in a CBL is inhomogeneous and therefore the subfilter-scale effects depend on the relative location between the spectral peak of the vertical velocity and the filter cut-off wavelength. From the physical point of view, this lack of homogeneity makes the vertical mesh spacing the principal length scale and, as a consequence, the LES filter cut-off wavenumber is expressed in terms of this characteristic length scale. Assuming that the inertial subrange initial frequency is equal to the LES filter cut-off frequency and employing fitting expressions that describe the observed convective turbulent energy one-dimensional spectra, it is feasible to derive a relation to calculate the variable vertical mesh spacing. The incorporation of this variable vertical grid within a LES model shows that both the mean quantities (and their gradients) and the turbulent statistics quantities are well described near to the ground level, where the LES predictions are known to be a challenging task.     

Evaluation of Cut-Off Frequency and Correction of Filter-Induced Phase Lag and Attenuation in Eddy Covariance Analysis of Turbulence Data

A recursive filter adopted for online eddy covariance analysis of turbulence data inthe atmospheric surface layer is revisited, and its properties and performanceare evaluated by means of concepts and methods developed in digital signal analysis.A rigorous estimate of effective cut-off frequency is derived along with an estimateof filter induced phase lag. Accordingly, filter design criteria are revised and variouseffects of parameter choice are assessed. Furthermore suitable corrections for compensating filter induced distortion, i.e., phase lag and attenuation, are proposed. The modified filter is tested on: (a) An artificial time series (random noise superimposed to `slow' sinusoidal signal); (b) turbulence data from field measurements. In case (a) the retrieval of input signal parameters is appreciably improved. In case (b) a better agreement with a Gaussian moving average is obtained, at lower computational cost.     

Organized Structure of Active Turbulence Over an Array of Cubes within the Logarithmic Layer of Atmospheric Flow

We investigate the coherent structure of atmosphere turbulence over very large roughness within a fully rough, high Reynolds number turbulent flow. The horizontal distributions of coherent turbulence were determined by multipoint measurements of velocity fluctuations using sonic anemometers in a comprehensive outdoor scale model experiment for urban climate (COSMO). COSMO is made up of 512 cubical obstacles, each 1.5 m on a side, arranged in a rectangular pattern on a flat 50 m × 100 m concrete plate. A total of 15 sets of sonic anemometers were aligned horizontally within the logarithmic layer above this site. The velocity fluctuations observed in COSMO were decomposed into active and inactive contributions by applying a spatial-filtering method, and which used a simple moving average along the spanwise direction of the predominant flow as a filter function. The size of the filter should be between the sizes of the active and inactive fluctuations. This method potentially eliminates the considerable portion of low frequency modes included in the horizontal velocity fluctuation, while preserving well the Reynolds stress. The structural characteristics of the active turbulence were qualitatively similar to those measured over various surface configurations. Overall, the observed structures of the active turbulence are composed of very large streaks of low momentum fluid elongated in the streamwise direction with some sub-structures included in the streaks. The sub-structures were the main cause of the ejections, which accompany horizontal vortices. The active motion, including the streaky structures, did not reproduce the lower frequency peak of the bi-modal distribution of the horizontal velocity spectra, but reproduced the higher frequency mode that robustly follows inner-layer similarity (i.e. Monin–Obukhov similarity).     

一种数字化天气雷达回波原始资料的数据压缩方法

采用方向滤波编码法压缩数字化天气雷达回波的原始资料。首先对回波资料进行二维傅里叶变换，分别出雷达回波的低频分量和高频分量；然后用方向滤波器从回波的高频分量中提取出回波的边缘和纹理，最后对雷达回波的低频分量和边缘，纹理组成的各方向图分别编码，以达到压缩雷达回波原始资料的目的。     

在天气雷达信号处理器中用IQ信号消减地物杂波

地物杂波的检测和处理对雷达数据质量有重要的影响。常规的地物杂波检测依赖于晴空条件下的杂波图,人工判别,后来采用GAMP滤波器进行全程自适应滤波处理,该法在对地物抑制的同时对天气信号的损失较大,现探究比较多的方案是先自动识别地物,后结合滤波器对地物进行处理,在抑制地物的同时减少对天气信号的损耗。该算法基于IQ信号,利用已在C波段天气雷达中进行实验的SCI算法的4个判别因子,引入模糊逻辑理论,重新设计算法识别流程,并应用于我国S波段天气雷达中。通过实例对比杂波消减决策（CMD）算法对同一天气过程的识别与抑制的结果,发现该算法对气象回波的误判率低于CMD算法,对地物的识别率与CMD算法相当。