QX气象Meteorological Monthly1000-0526气象编辑部中国北京qx-41-5-57710.7519/j.issn.1000-0526.2015.05.006P412, P458暖季试验研究论文Articles两次降水过程的微降雨雷达探测精度分析Precision Evaluation of Micro Rain Radar Observation in Two Precipitation Events温龙WENLong
MRR is a vertically pointing Micro Rain Radar which can measure vertical profiles of radar reflectivity and drop size distribution (DSD). It plays an important role in understanding precipitation microphysical structure and improving quantitative precipitation estimation. In this study, the observations from an S-band Doppler radar, a 2D-Video-Disdrometer and a rain gauge during two summer precipitation events in Nanjing are used to assess the performance of MRR. The intercomparison analyses of these four instruments in stratiform and convective precipitation are also performed. The results show that, the vertical profile of reflectivity measured by MRR and S-band radar has a good agreement below 4 km with the mean difference less than 1 dB. Above 4 km, however, MRR tends to underestimate the reflectivity due to the precipitation attenuation. The difference of reflectivity between MRR and S-band radar increases with the height. Furthermore, MRR can accurately estimate the rain rate with the reflectivity below 35 dBz, but underestimate the rain rate with the reflectivity above 35 dBz. The rainfall estimation for stratiform clouds is more accurate than that for convective clouds. Especially, MRR exhibits good performance for the weak rainfall below 0.1 mm·h-1, which cannot be measured by rain gauge. Due to the inherent limitation of MRR and the precipitation attenuation, MRR shows high consistency with 2DVD for raindrops with size from 1 mm to 5 mm, but underestimates the concentration of drops larger than 5 mm and overestimates the concentration of drops smaller than 1 mm. Overall, MRR is an effective instrument for the precipitation measurements, especially suitable for quantitative estimation of stratiform.
理解降水结构和内部的微物理过程特征变化,对于提高降水的监测和预报有重要的意义。目前常用的降水探测设备包括多普勒天气雷达、地面雨量筒和雨滴谱仪等(刘晓阳等,2010;丛芳等,2011;朱亚乔等,2013;柳臣中等,2015;明虎等,2014)。多普勒雷达能对大范围降水的三维结构进行高时空分辨率的观测,但缺陷是无法获取降水系统内部雨滴谱分布特征,降水估计精度较低(周黎明等,2015)。同时,由于雷达波束随着距离抬高,在远距离处无法测得近地面的降水回波信息(Joss et al, 1990;杨金红等,2013)。地面雨量筒可以准确测量地面降水,而地面雨滴谱仪可进一步探测到近地面的雨滴谱分布,有助于理解地面降水的微物理特征(王可法等,2011;罗俊颉等,2012;刘西川等,2014)。然而,地面雨滴谱仪无法观测降水的垂直结构,因此无法深入研究降水的微物理过程。垂直指向雷达能够测量从近地面至高空的反射率因子和雨滴谱分布特征,对研究和分析降水微物理结构,改进雷达降水估计精度有重要作用(Diederich et al, 2004a;Wagner et al, 2004)。
德国METEK公司生产的微降水雷达(Micro Rain Radar,简称MRR)是常用的一种垂直指向雷达,用于观测降水垂直结构。其采用连续调频技术(FM-CW),工作频率为24 GHz, 波长为12.5 mm(K波段)。MRR利用多普勒效应,通过雨滴大小和散射截面、下落速度之间的关系,测量不同高度(垂直30层)的雨滴大小分布,并导出降水率、液态水含量、粒子下落速度和雷达反射率因子等数据。国外学者利用地面雨量筒、雨滴谱仪和多普勒天气雷达对MRR的性能进行了系统评估,发现由于MRR使用了Gunn等(1949)提出的粒子直径与下落速度关系算法,只能测量0.246~5.03 mm间的粒子,因此对于中等强度降水有较高的测量精度,但对强降水则存在一定的测量误差(Löffler-Mang et al, 1999;Peters et al, 2002)。Peters等(2002)还分析了环境风对MRR测得粒子降落速度的影响。由于MRR算法中忽略了环境风的影响,在下沉气流区会高估粒子下落速度,导致高估粒子直径和低估粒子浓度,以及低估降水率。近年来,随着MRR硬件和算法的不断改进,其逐渐被用于定量降水估计和降水微物理结构特征研究。Wagner等(2004)和Peters等(2005)分析了波罗地海沿岸MRR测得雨滴谱的垂直分布特征,提出了两种常规天气雷达定量降水估计的改进方法。Bendix等(2006)则利用MRR对南美热带山谷地区降水日变化的动力学机制进行研究。Cha等(2009)利用MRR的长期观测资料对韩国山区和海岸附近层状云降水的0℃层亮带特征进行对比研究。García-Vila等(2009)利用MRR雷达、雨量筒和雨滴谱仪分析了马德里地区对流性降水和层状云降水的传播特性。Diederich等(2004b)和Harikumar等(2012)分别利用MRR对荷兰Cabauw地区和印度南部西南季风系统控制下的热带地区雨滴谱随高度-时间演变特征进行了分析,发现降水后期大粒子增多是粒子在降落过程中与小粒子合并,导致小粒子降低、大粒子增加所致。
同国外相比,我国在MRR的应用和相关研究相对较少。2007—2010的中德合作项目“不同放牧强度对内蒙古草原生态系统物质流的影响”首次引进5部MRR雷达,结合地面雨量筒研究锡林河流域小尺度区域内降水的分布特征(刘辉志等,2009;Chen et al, 2014)。陈勇等(2011)利用这5部雷达的观测资料,评估了环境垂直气流和雷达垂直倾角对MRR降水强度探测的影响,并提出了一种雷达间相互校准的方法。蔡嘉伦等(2012)利用台湾地区四次降水过程,结合业务雷达和一维撞击式雨滴谱仪,定量评估MRR对副热带地区层云和台风降水的探测精度。结果表明,层状云降水环境中,MRR与其他仪器一致性很好,资料可信度很高。2014年6—7月,科技部国家重点基础研究发展计划(973计划)项目“突发性强对流天气演变机理和监测预报技术研究(2013CB430101)”在南京地区开展了强对流野外观测试验。试验在南京江宁区的国家级地面观测站架设了一台第二代MRR雷达和二维视频雨滴谱仪(2DVD),用于观测降水系统的微物理结构特征。相比于一维雨滴谱仪器,2DVD利用高速摄像机直接测量通过采样空间内的每个降水粒子直径和下落速度,能更准确观测粒子的尺寸和垂直速度,因此具有更高的精度(Schönhuber et al, 2007)。本文选择试验期间两次降水过程,利用我国业务S波段天气雷达、2DVD雨滴谱仪和地面雨量筒(RG),对MRR的观测结果进行对比分析,定量评估MRR对我国东部夏季降水系统的探测性能。本文也利用地面雨量观测,将降水分为对流性降水和层状云降水,定量分析MRR对不同类型降水的探测精度。本文也为应用MRR研究东亚夏季季风系统控制下的我国东部地区不同类型降水的云微物理特征提供依据。
The time-height profiles of radar reflectivity factor from MRR (top), NJRD (middle) and CZRD (bottom) for (a) the 31 May to 1 June and (b) the 16 June 2014 rain events
Hourly accumulated precipitation measured by MRR radar at 200 m (dashed line) and 400 m (dot-dash line), 2DVD (gray line) and RG (black solid line) for (a) the 31 May to 1 June and (b) the 16 June 2014 rain events
2014年两次降水个例中MRR和2DVD雨滴谱的降水率变化
Rain rate comparison between measurements from MRR (black dashed line) at 200 m (a, b) and 400 m (c, d) and 2DVD (gray line) for (a, c) the 31 May to 1 June and (b, d) the 16 June 2014 rain events
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