【作者】 宗蓉；

【导师】 银燕； 刘黎平；

【作者基本信息】 南京信息工程大学 ， 大气物理学与大气环境， 2013， 博士

【摘要】 和传统天气雷达相比,毫米波雷达的波长短,对小云滴更敏感,同时空间分辨率小,能获取更为精细的云结构,因此是探测非降水云和弱降水云的有效工具。毫米波不仅能提供直观的云高云厚等宏观信息,更为云微物理参数的反演提供数据。本文利用中国气象科学研究院的一部35GHz双偏振多普勒毫米波雷达(HMBQ),进行云的宏观结构分析和云微物理参数反演方法的研究,具体研究内容如下：(1)对毫米波雷达的发展状况和探测特点做简要概述。在此基础上,介绍HMBQ的系统组成和系统参数,讨论和分析其探测性能,并给出其观测个例,对其探测能力做初步验证；(2)通过HMBQ和人工影响天气飞机对云的联合探测试验,进一步检验HMBQ的探测能力。将HMBQ探测的反射率因子和利用飞机实测滴谱计算得到的反射率因子作比较,发现在暖云中,两个反射率因子值较为接近,而冰云中,有较大偏差。由于HMBQ和飞机的滴谱仪的采样目标不可能完全一致,并且冰云中可能有过冷水存在,雷达探测值和飞机估值存在差异是合理的。计算飞机在云层中部采样期间两个反射率因子的均方根误差,暖云和冰云分别为5.5dBZ和6.1dBZ,可见,HMBQ具有准确探测云层的能力。利用滴谱计算云滴和降水粒子的反射率因子,通过寻找这两个反射率因子的概率密度函数的交点,得到云滴的反射率因子通常低于-5dBz,而降水粒子高于-20dBZ,暖云的降水粒子反射率阈值在-15～-12dBZ之间；(3)选取两部毫米波雷达和一部微脉冲激光雷达(MPL)联合观测的层积云、高层云、高积云和卷积云个例,对三个仪器探测到的云边界进行对比。两部毫米波雷达探测的结果有很好的一致性。但是MPL和毫米波雷达的探测结果往往存在差异。云粒子对毫米波电磁波和激光光束的散射机制不同以及两个设备判定云底的方法不同是差异产生的主要原因。通常,MPL探测的云底高度高于毫米波雷达。对于云顶的小粒子和云内的小冰晶,MPL的探测能力强于毫米波雷达,但对于发展深厚云层,MPL会因衰减的影响探测不到真实的云顶,而毫米波雷达可以探测到完整的云层。在上述分析的基础上,制定毫米波雷达和激光雷达联合遥感确定云边界的标准,并用联合观测数据对云宏观结构进行统计。(4)介绍多普勒谱的概念和理论模型,同时对HMBQ探测到的多普勒谱进行实例分析,发现绝大部分多普勒谱满足单峰或者双峰高斯分布。其结构与雷达照射体积内包含的粒子有关。双峰的多普勒谱对应雷达照射体积内有两种不同粒径分布的粒子,比如,云滴和降水粒子、液滴和冰晶共存的情况。在对原始多普勒谱进行平均、平滑和去除系统噪声后,通过非线性最小二乘拟合的方法将多普勒谱分解为单峰或者双峰的高斯分布,进而可求得不同类别粒子的反射率因子和速度。对一次观测个例中探测的双峰多普勒谱,用分解多普勒谱的方法,得到云滴和降水粒子的谱参数。通过将云滴看做空气运动的示踪,利用云滴落速对空气速度进行估计。再用利用Frisch模型分别得到这两种粒子的云微物理参数。更多还原

【Abstract】 In comparison with widely used centimeter wavelength radars, millimeter wavelength radars have certain advantages for detecting nonprecipitating and lightly precipitating clouds. The sensitivity to small hydrometeors and excellent spatial resolution make them provide important information about the macro and micro cloud properties. Data with a35GHz dual-polarization Doppler radar (HMBQ) is studied to analyze the qualitative information about the presence of clouds and to develop a method to retrieve cloud microphysical properties. The main contents and conclusions are as follows:(1) The general development and main advantage of millimeter wavelength radars is summarized. Furthermore, the components and parameters of HMBQ are introduced in order to evaluate its detecting capability. In addition, the results from recent cloud experiments are presented to reveal the intricate structure of a wide variety of clouds obtain by HMBQ.(2) The capacity of HMBQ was investigated based on an aircraft and cloud radar co-observation. The reflectivity calculated from aircraft measurements is compared with the simultaneous radar observation in detailed. It shows that the two reflectivities are comparable in warm clouds, but in ice cloud there are more discrepancies which are probably associated with the occurrence of overcooled liquid water. Because of the difficulty of exactly matching up the collocation between the two sensors and their distinct sample volumes, the deviation is inevitable. Based on the dataset collected in warm clouds in this experiment, the threshold of reflectivity to diagnose drizzle and cloud particles is studied by analyses of the probability distribution function of reflectivity due to cloud particles and drizzle drops. The reflectivity of cloud particles is usually below-5dBZ which the value of drizzle is above-20dBZ. A value between-15and-12dBZ can be used as the threshold of drizzle.(3) Determination of cloud base and top heights from millimeter wavelength radar are compared to those from the Micro Pulse Lidar (MPL) as a means to evaluate the accuracy of both cloud radar and MPL retrievals, as well as to ascertain their limitations. Four cases are studied, including a stratocumulus, a altostratus, a altocumulus and a cirrocumulus. The study reveals that the cloud boundaries detected by both cloud radars agree well with each other. However, the MPL usually reports higher cloud base heights than cloud radar. Statistical comparison reveals that the differences between their mean cloud base heights are large for nonflat-based altostratus but small for other cases. The differences are mainly due to the different scattering mechanism for light and microwave and the distinct methods of cloud boundaries identification. The MPL sometimes see higher cloud tops than the cloud radar when the lidar signal can completely penetrate the clouds. However, for deeply developed clouds the cloud radar measure reliable cloud tops while MPL suffers heavy attenuations. Furthermore, the MPL has a superior in some optical thin clouds detection. An integration of cloud boundary is determined based on these analyses. Using the radar/lidar synergetic observations, frequency distributions of cloud macro structure are calculated.(4) A detailed description of the Doppler spectra is given in theory. One size particles in the radar resolution volume produce a uni-mode Gaussian spectrum. Different size particles produce a spectrum that is due to the linear superposition of their Gaussian signals. Several examples of Doppler spectra detected by HMBQ are presented to show that the structures of Doppler spectra bring information on the size range of droplets. Commonly double-model structures are related to cloud droplets of different size, ice crystals with different habit and size, or cloud droplets mixed with ice crystals. In order to evaluate the moments of the Doppler spectra due to different ensembles of particles, a method of decomposing Doppler spectra is applied. Firstly, the measured spectra are averaged over10s and smoothed the turbulence by a three-point window. Then the spectra is decomposed by fitting as Gaussian distribution. Using the decomposed spectra to study a stratocumulus. The air motion is estimated by treating the small cloud particles as the tracer of air motion. The microphysical properties are retrieved by means of the Frisch model.更多还原