机载W波段测云雷达回波强度衰减订正仿真研究

考虑到大气和云对W波段雷达波的衰减,数值仿真4种非降水云的机载W波段雷达(94GHz,3.2 mm)回波强度;基于雷达回波强度Z与云衰减系数k之间的经验关系,进行雷达回波衰减订正.结果表明,在云厚、云滴粒子浓度和含水量参数的一般取值情况下,可以获得比较好(相对误差＜15%)的订正效果.随着云体加厚、云中大滴增多和含水量...     

结合毫米波雷达提取降水条件下风廓线雷达大气垂直速度的研究

风廓线雷达主要是利用大气湍流对电磁波的散射作用,在晴空条件下对大气风场等进行探测。在降水天气下,风廓线雷达能同时接收到大气湍流回波和雨滴的散射回波信号,其探测到的回波功率谱中降水信号谱和大气湍流信号谱叠加在一起,使得大气的运动被雨滴的运动信息所掩盖,给后续的大气风场反演带来误差。而毫米波云雷达在降水天气下仅能探测到云雨粒子的回波而无法探测到大气湍流回波,基于这一差异结合毫米波云雷达资料对风廓线雷达功率谱数据进行订正,剔除其中的降水回波信息,进而获取正确的大气运动垂直速度。通过一次典型弱降水天气过程的雷达资料对该方法进行了可行性验证,并将计算得出的大气垂直速度与传统双峰法提取的大气运动垂直速度及原始风廓线雷达垂直速度进行了对比分析,显示在弱降水天气下该方法能有效消除降水对风廓线雷达垂直速度测量的影响,提高弱降水天气下测速准确率,并且在湍流谱极其微弱的情况下该方法也能准确地获取到大气运动垂直速度信息。但是云雷达回波在降水时会有衰减,虽然是弱降水也会导致在高层距离库上的订正效果变差,故目前只适用于弱降水时低距库处的降水订正。      

联合遥感技术对X波段雷达反射率的衰减订正

由于在使用X波段雷达时,必须要对雷达反射率进行衰减订正,本文发展了X波段雷达和微波辐射计联合遥感技术对雷达反射率进行衰减订正的方法。并选用两个不同降水系统个例(积层混合云降水和层状云降水),分析了联合遥感方法的订正效果,结果表明,联合遥感方法可对雷达反射率进行有效的衰减订正,订正后雷达回波的空间结构、回波强度和路径分布较为合理,订正后雷达反射率因子平均值在两个个例中分别增加1.71 dBZ和0.84 dBZ,略小于双偏振方法订正后的结果。     

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。     

星载雷达DPR与地基雷达CR的匹配对比及系统偏差初探

将GPM(Global Precipitation Measurement)卫星搭载的双频降水雷达DPR(Dual-frequency Precipitation Radar)和广东龙门观测站的地基Ka/Ku双波段雷达CR(Cloud Rader)在2019年4—9月的观测数据进行时空匹配,结合微雨雷达MRR (Micro Rain Rader)的雨滴谱数据对CR衰减订正处理,比较DPR与CR探测回波强度差异,最后对两者的系统偏差进行分析。结果表明:(1) DPR和CR探测到的回波强度随高度变化的趋势大体一致,零度层亮带高度基本一致,同样高度下DPR探测到的回波强度明显大于CR。(2)统计弱的非降水云算得CR双频系统偏差约为3.1 dBz。(3)雨强越强,CR天线罩水膜厚度越大,回波衰减越大。(4)逐库订正过程中利用雨滴谱数据计算衰减系数,CR在Ka与Ku波段的反射率因子订正量不超过4 dBz。(5)综合所有适配数据,CR与DPR的反射率因子在Ka波段内订正后平均系统偏差为15.7 dBz,在Ku波段内订正后平均系统偏差为14.4 dBz。虽然经订正处理后两部雷达间的系统偏差偏大,但对改善地基Ka/Ku双波段雷达的数据质量有明显的改善。     

网络化多普勒天气雷达反射率因子的衰减订正

利用国内第一个网络化雷达试验平台的观测资料,基于网络化雷达反射率因子衰减订正算法,对观测数据进行衰减订正。并与单部雷达自适应约束算法和常州S波段雷达观测资料进行比较分析,结果显示订正方案合理。个例分析表明,经过雨区后单部雷达探测的回波强度较弱甚至探测不到,而网络化雷达可以利用多部雷达的共同观测,弥补单部雷达探测上的不足,体现出网络化雷达衰减订正的优势。     

W波段和Ka波段云雷达探测回波对比分析

通过分析W波段和Ka波段云雷达同时探测回波的差异,验证了W波段云雷达初样机的探测性能。结果表明：1）W波段云雷达初样机工作稳定,两个波段雷达都可以探测云层、云的边界、云厚等宏观参数,也可以反映出云的精细结构及云内微物理参数的变化,回波强、速度小、谱宽大的冰晶云含有上升气流及较多过冷水。2）增强模式的W波段云雷达在近地面探测雾、霾的能力比Ka波段云雷达强;两部云雷达对云层较薄的云探测能力基本相当,对多层云、云层较厚、含水量较多的云及降水的探测,由于强衰减的作用,W波段雷达所测云厚度小、云顶低、回波强度小,并且非瑞利散射也会造成W波段雷达的回波强度降低。     

基于风廓线雷达资料的亮带识别订正算法研究

在降水过程中,固态降水粒子下落穿越0℃等温线的融化效应会引起雷达反射率因子增大,产生亮带。文章基于北京房山地区的边界层风廓线雷达的探测资料,对2014年8—10月四次典型的层状云降水和以层状云为主的混合性降水过程进行特征统计,提出了适用于该季节、该地区的0℃层亮带自动识别订正算法,并使用这种算法对一次层状云为主的混合性降水天气过程进行了识别研究,通过与探空资料、多普勒天气雷达资料的对比检验,结果表明该算法可以有效识别该季节、该地区的0℃层亮带,通过亮带订正,融化区的回波强度高值区得到了有效抑制,原亮带高度附近回波强度廓线的显著弯曲消失,融化区之外的回波强度基本没有变化。     

机载W波段雷达衰减订正方法的不同云型模拟对比研究

为了探讨适用于机载W波段(94 GHz,3.2 mm)测云雷达的衰减订正算法,根据大气和云的毫米波衰减理论,模拟在5种典型参数情况下的非降水云雷达回波强度,基于回波强度Z与云衰减系数k_c之间的关系,使用解析法等5种衰减订正算法进行回波订正试验,寻求最优k_c-Z关系系数,同时研究改变云液态水含量和云滴数密度(使典型云变成为非典型云)对订正效果的影响。结果表明,W波段k_c-Z关系系数a=5.096 5和b=0.491 9适用的云型接近于典型层积云,而逐库类法订正效果优于解析法和迭代法。在使用最佳k_c-Z关系系数情况下,5种订正算法对典型云的订正效果都较好,即使云厚增加到典型云厚度的2倍,订正误差仍小于5%。对于改变云液态水含量、云滴数密度、云厚度而表示的非典型云,虽然逐库法订正结果略优于逐库近似2法和逐库近似1法,但效果相近,在云不太厚时订正误差均小于10%。如果用“φ≤10%或20%”来评价其适用范围更广些,优于解析法和迭代法。但逐库类法对云液态水含量较敏感,导致“可订正厚度”减小、误差增大。因此,要尽量做好云型和云液态水含量的估算。      

94/220 GHz星载雷达双波长比对非球形冰晶云参数敏感性分析

利用不同形状冰晶的散射特性,获得了非球形冰晶云的94/220 GHz测云雷达双波长比,探讨了非球形冰晶云的双波长比与云内微物理参数的关系,分析了衰减前后的星载雷达反射率因子及双波长比的垂直廓线。结果表明:（1）双波长比可以反映小到0.1 mm中值尺度的冰粒子,对粒子总数、谱的形状参数不敏感,对粒子大小、形状、云衰减较敏感。（2）雷达灵敏度一定时,星载雷达可测云厚与雷达波长、冰含水量（IWC）的垂直分布、云厚及衰减有关;没有进行衰减订正时,双波长比和衰减有关,冰含水量越大,波长越短,衰减越大,双波长比最大值与可探测云厚有关。两部雷达可探测冰含水量为0.001—0.1 g/m3、厚2 km的冰云;当云厚5 km、冰含水量垂直分布在0.001—0.2 g/m3时,云厚的94%基本可以被220 GHz云雷达探测到。（3）如果两部雷达气象方程中用水的介电因子,测量回波强度应进行介电因子的订正后再计算双波长比。      

雨滴谱仪与风廓线雷达反射率对比试验

针对风廓线雷达估算的反射率数据需要进行验证。开展雨滴谱仪与风廓线雷达反射率对比试验,通过两种不同探测设备观测数据的对比,以验证风廓线反射率数据的可靠性和可用性。结果表明:确定了以风廓线低模360~1440 m采样体积内的反射率与3 min雨滴谱反射率数据对比方法能最大程度的减少时空差异;在雨滴谱仪反射率小于40 dBz时,对应的风廓线雷达反射率数据是可靠和可用的;同时由于风廓线雷达有限的动态范围造成反射率低估的现象,使得风廓线雷达反射率在大气垂直结构以及微物理特性等方面应用受到一定的局限性。     

基于滴谱分布的机载雷达波束匹配情况研究

基于机载双频测雨雷达外场校飞试验中获取的有效降水测量数据,利用Ku波段的雨衰系数和等效雷达反射率因子的比值估算了雨滴谱分布参数.在此基础上,通过米散射模拟计算了Ka波段在该滴谱分布下的等效雷达反射率因子,并和机载Ka波段衰减订正后的等效雷达反射率因子进行了比对.结果表明:两者的廓线基本重合,相关系数超过0.95,两部雷达探测的目标物一致,测量的雷达反射率因子的差异主要是由雷达频率的不同所引起的后向散射以及衰减的不同所造成的,波束匹配精度较高.此外,单频滴谱参数的估计依赖于单频衰减订正的精度.机载雷达外场试验中探测的都是弱降水,Ku波段衰减订正不会存在太大的误差,估计的滴谱较为可靠.必须指出的是,精确的参数估计仍然要借助双频雷达的反演方法.     

星载双频测雨雷达航空校飞试验降水反演分析

2010年9月6日到10月20日,在江苏东台黄海附近进行了中国首次星载双频(Ku和Ka波段)测雨雷达的外场校飞试验,获得了宝贵的机载雷达数据。利用所探测的有效降水资料,对机载雷达的降水探测能力进行了性能分析。对比机载测雨雷达和同步星载测雨雷达(Precipation Radar,PR)探测的反射率因子廓线可以发现,机载雷达反射率因子廓线在1.55 km高度的探测结果和PR较为一致,尤其是融化层一致性更好,表明机载雷达有探测降水垂直结构的能力。为了进行Ka波段机载雷达的降水反演,利用卫星数据模拟器(Satellite Data Simulator Unit,SDSU)计算了k-Z_e和R-Z_e关系的系数,并在此基础上进行了衰减订正以及雨强的反演。结果表明,Ku和Ka波段反演的雨强廓线基本一致,证实了反演系数的合理性。     

ANALYSIS AND CASE STUDY ON THE GENERATION OF RADAR ECHO BRIGHT BAND

In this study, the vertical profiles of radar refractive factor (Z) observed with an X-band Doppler radar in Jurong on July 13, 2012 in different periods of a stratiform cloud precipitation process were simulated using the SimRAD software, and the contributions of each impact resulting in the bright band were analyzed quantitatively. In the simulation, the parameters inputted into SimRAD were updated until the output Z profile was nearly consistent with the observation. The input parameters were then deemed to reflect real conditions of the cloud and precipitation. The results showed that a wider (narrower) and brighter (darker) bright band corresponded to a larger (smaller) amount, wider (narrower) vertical distribution, and larger (smaller) mean diameter of melting particles in the melting layer. Besides this, radar reflectivity factors under the wider (narrower) melting layer were lager (smaller). This may be contributed to the adequate growth of larger rain drops in the upper melting layer. Sensitivity experiments of the generation of the radar bright band showed that a drastic increasing of the complex refractive index due to melting led to the largest impact, making the radar reflectivity factor increase by about 15 dBZ. Fragmentation of large particles was the second most important influence, making the value decrease by 10 dBZ. The collision–coalescence between melting particles, volumetric shrinking due to melting, and the falling speed of raindrops made the radar reflectivity factor change by about 3–7 dBZ. Shape transformation from spheres to oblate ellipsoids resulted in only a slight increase in the radar reflectivity factors (about 0.2 dBZ), which might be due to the fact that there are few large particles in stratiform cloud.     

Observation of Precipitating Systems over Complex Orography with Meteorological Doppler Radars: A Feasibility Study

Summary  This paper concerns the use of airborne or ground-based Doppler radars to observe precipitating systems over complex orography. As nearly all of the previous experiments involving Doppler radars were conducted over flat surfaces over the continents or the oceans, new techniques are needed firstly to separate ground clutter from meteorological signal and, in the case of airborne Doppler observations, to deduce navigational errors. Secondly, it is necessary to take the atmospheric circulation induced by orography into account in the three-dimensional wind field analysis. Variational techniques are presented to solve these problems. The proposed methods are tested with simulated ground-based and airborne Doppler radar observations for analytic flows over analytic terrains and for numerically simulated wind and reflectivity fields for the Brig event (22 September 1993) of heavy precipitation over the southern flank of the Alps (Cosma and Richard, 1998), and with actual airborne Doppler data relative to weak snow showers over the Rocky Mountains on 12 March 1995. Received March 22, 1999/Revised June 1, 1999     

Application of a Three-dimensional Variational Method for Radar Reflectivity Data Correction in a Mudslide-inducing Rainstorm Simulation

Various types of radars with different horizontal and vertical detection ranges are deployed in China, particularly over complex terrain where radar blind zones are common. In this study, a new variational method is developed to correct threedimensional radar reflectivity data based on hourly ground precipitation observations. The aim of this method is to improve the quality of observations of various types of radar and effectively assimilate operational Doppler radar observations. A mudslide-inducing local rainstorm is simulated by the WRF model with assimilation of radar reflectivity and radial velocity data using LAPS(Local Analysis and Prediction System). Experiments with different radar data assimilated by LAPS are performed. It is found that when radar reflectivity data are corrected using this variational method and assimilated by LAPS,the atmospheric conditions and cloud physics processes are reasonably described. The temporal evolution of radar reflectivity corrected by the variational method corresponds well to observed rainfall. It can better describe the cloud water distribution over the rainfall area and improve the cloud water analysis results over the central rainfall region. The LAPS cloud analysis system can update cloud microphysical variables and represent the hydrometeors associated with strong convective activities over the rainfall area well. Model performance is improved and the simulation of the dynamical processes and moisture transport is more consistent with observation.     

用机载多普勒雷达资料进行三维风场解析反演 I. 多重解析多普勒方法在机载雷达情况下的推广及模拟验证

多重解析多普勒（MANDOP）方法是用解析表达式进行三维风场反演。它成功地应用于处理地基多普勒雷达资料，获取对流系统层状降水区域的风场结构。本文将MANDOP方法在机载雷达情况下的进行推广并通过模拟研究径向速度测量中信号噪声、雨滴降落速度估计误差、平流速度估计误差、尤其是由于机载特性的引入产生的雷达束定位误差等对方法的影响。模拟结果显示，在机载雷达的技术参数下，MANDOP方法能够提供误差可以接受的三维风场反演。这为利用MANDOP方法处理在CaPE实验中获取的历史上第一次机载双束多普勒雷达资料建立了可靠的基础。     

SIMULATION AND ANALYSIS ON OBSERVATIONAL ERRORS OF CLOUD INTENSITY AND STRUCTURE WITH TRMM PR AND GROUND-BASED RADAR*

In order to explain theoretically the observational biases of reflectivity and structure of precipitation systems by TRMM Precipitation Radar (TRMM PR) and ground-based radar,the effects of wavelengths,incident direction of radar waves and radar beam width on the reflectivity observation are simulated.The results show that the error due to the different wavelength and incident direction of radar wave is within 2.0 dB,TRMM PR can observe a larger reflectivity than ground-based radar in echo center.TRMM PR smoothes the cloud structure,overestimates and underestimates reflectivity by 3-5 dB in strong and week echo areas,respectively.Beam width and long distance from TRMM PR to target cause it to overestimate the large echo area and area integrated rainfall amount,and to underestimate the averaged refleetivity.The theoretical results above can only explain part of observational facts,meaning that the comparison of observation results between TRMM PR and ground-based radar is complicated,the attenuation of radar wave within precipitation area is the main factor to affect the observed result.     

RTTOV散射模块在FY-3 MWHS资料同化中的应用

一直以来,受同化系统中观测算子的限制,云区的卫星微波观测资料在使用时都常会做晴空假设,只考虑大气的吸收作用而忽略散射作用,从而会造成对大气中水汽含量的高估。为了克服这一缺点,文中设计了RTTOV微波散射模块在WRF 3DVAR系统中的应用方案,使得计算辐射能衰减的同时考虑大气的散射作用,并在此基础上通过4组试验进行了风云3 A星微波湿度计亮温以及多普勒雷达反射率因子的同化研究对比。结果表明：RTTOV微波散射模块的引入,能有效抑制晴空辐射模块对大气水汽含量的高估。若先同化雷达反射率因子后再继续同化卫星资料,会发现同化雷达后,同化区域内大气水凝物含量明显增加,受此影响同化卫星资料时大气的散射作用也会随之增大。并且雷达同化区域覆盖率越大,对降水预报的改善效果也愈明显。