云雷达回波强度谱密度定标及云内大气垂直运动速度反演试验

Cloud Radar Doppler Spectra Calibration and Air Vertical Velocity Retrieval Experiment

云内大气垂直运动在云动力和微物理过程中扮演了重要角色,云雷达功率谱数据是其反演的有效数据,但前提必需对谱线的回波强度进行准确定标,其中噪声电平的计算是关键步骤。本文首先提出了一种云雷达功率谱的模拟方法,利用模拟数据定量评估了三种噪声电平计算方法的准确性,对实测谱线回波强度进行了定标。基于以上方法,采用小粒子示踪法反演了2014年6月广东阳江对流云和层状云内的大气垂直运动速度,并提出了假定数浓度和直径下的谱线回波强度临界阈值,利用该阈值对反演结果进行定量检验。结果表明:(1)利用云雷达的高斯白噪声特征和云信号满足高斯分布特征,结合实测数据统计得到的涨落程度,可以模拟出给定谱参数的功率谱,模拟数据与实测十分相近,可作为功率谱定量研究的有效数据源。(2)三种噪声电平计算方法中,分段法的准确性和稳定性最好,最大速度法受噪声起伏的影响,客观法计算结果偏高,当谱宽和多普勒速度较大时客观法和最大速度法误差会达到很大。(3)谱线回波强度临界阈值的检验结果表明,该对流云内所有示踪谱线的回波强度均远小于临界阈值,云内100%距离库为被反演,反演结果可靠;该层状云内,除了少数距离库,大部分示踪谱线的回波强度在临界阈值以下,云内96.23%距离库可被反演,反演结果可靠。

Fine detection of cloud and precipitation is still a great interest topic in atmospheric research. In recent years,millimeter-wavelength cloud radars have become the major instrument for cloud and light-precipitation observation. Because of their short wavelengths,cloud radars have excellent sensitivity to small cloud droplets and ice crystals,can be configured to have high temporal and spatial resolutions,and can operate with antennas that have narrowbeamwidths and limited sidelobes. In a vertically pointing mode,cloud radar can record the Doppler spectral data which can be benefited for the microphysical and dynamic study of cloud and precipitation system. As an important prerequisite,cloud radar Doppler spectra calibration is crucial for accurately calculating radar measurements,in result,this article present a calibration method for a new35 GHz cloud radar,produced by Chinese Academy of Meteorological Sciences and plan to participate the 3td Tibetan Plateau Atmospheric Science Experiment from 2014. The accuracy of three radar noise-level compute method are assessed by using simulated data and spectra processing technologies are researched in this research. Another interesting work in this paper is the air vertical velocity retrieval by cloud radar Doppler spectra. At last,retrieval result and reliability of convective and stratiform cloud collected at Guangdong Yanjiang in June 2014 are verified and analyzed. Mainly conclusions are as follows：（1）base on gaussian characteristic of signal and white noise of cloud radar,combined with statistical fluctuation extents,the method can simulate the cloud spectra closed to real,and they can be used as an effective source data for spectra quantitative study.（2）The segment method is the most accurate and stable method,the max-speed method can be affected by noise fluctuations,the objective method is overrated as Doppler velocity and spectral width become large,as long as can cause an apparent error.（3）Inspection results by using the spectral line intensity threshold showthat,all traced spectral line intensity in convective cloud are less than this threshold,100% gates in cloud can be inverted and results are reliable. Apart from a fewgates,most of traced spectral line intensity in stratiform cloud are less than this threshold too,96% gates in cloud can be inverted reliably.