气象   2020, Vol. 46 Issue (2): 269-277.  DOI: 10.7519/j.issn.1000-0526.2020.02.012

### 引用本文 [复制中英文]

[复制中文]
XIE Qiyu, WEI Guocai, ZHANG Lingzhen, et al, 2020. Characteristics of Doppler Radar Radial Velocity Images for Heavy Rainfall Events in Xining[J]. Meteorological Monthly, 46(2): 269-277. DOI: 10.7519/j.issn.1000-0526.2020.02.012.
[复制英文]

### 文章历史

2018年10月24日收稿
2019年12月27日收修定稿

1. 青海省西宁市气象台，西宁 810016
2. 青海省气象台，西宁 810016

Characteristics of Doppler Radar Radial Velocity Images for Heavy Rainfall Events in Xining
XIE Qiyu1, WEI Guocai2, ZHANG Lingzhen1, ZHAO Juan1
1. Xining Meteorological Observatory of Qinghai Province, Xining 810016;
2. Qinghai Meteorological Observatory, Xining 810016
Abstract: The characteristics of heavy rainfall events on the Doppler weather radar velocity images and their early warning indicators in Xining from 2010 to 2016 are analyzed. The results show that the adverse wind region (AWR) on radial velocity images often occurs in the case of heavy rainfall events in Xining. Type-Ⅰ-AWR is slightly more for the mixed heavy rainfall and type-Ⅱ-AWR is mostly for the convective heavy rainfall during the directly affected heavy rainfall events. The AWR moves mostly northeastward in general. The occurring times of the AWR have certain lead times before the beginning of heavy rainfall events,and heavy rainfall falls near the AWR and its moving path,that is,the AWR is not only a time-based criterion for issuing early warnings of heavy precipitation but also a useful criterion for identifying falling areas of heavy rainfall. When the shear flow field in the AWR is convergent (divergent),it will enhance (weaken) the development of heavy rainfall,and the time of the convergence (divergence) is ahead of schedule the beginning (end) of heavy rainfall. At the time 1 h before the beginning of heavy rainfall,the vertical wind shear is the largest in low level and the smallest in middle level of the mixed heavy rainfall. However,it is obviously large in all the levels during convective heavy rainfall. The changes in wind direction mainly show deep and warm advection for the mixed heavy rainfall,while they are not obvious or there is warm advection in the low and middle levels for the convective heavy rainfall. The maximum echo heights and zero speed layers are higher for the convective heavy rainfall than for the mixed heavy rainfall,but there are smaller differences on the whole for the convective heavy rainfall. The indicators for early warning of heavy rainfall by comprehensive application of the radial velocity are obtained,and the accuracy rate reaches 78.6%.
Key words: Doppler radar radial velocity    vertical wind field    heavy rainfall    early warning

1 资料选取

2 西宁强降水的多普勒雷达径向速度图特征 2.1 强降水的径向速度特征

2.1.1 逆风区与强降水出现的时间和距离

 \begin{aligned} L=&\left\{\left[\left(\alpha_{1}-\alpha_{1}\right) \times 111 \times \cos \left(\frac{\beta_{1}+\beta_{2}}{2}\right)\right]^{2}+\right.\\ &\left.\left[\left(\beta_{1}-\beta_{2}\right) \times 111\right]^{2}\right\}^{\frac{1}{2}} \end{aligned} (1)

2.1.2 逆风区强度与强降水

 $f_{r}=\frac{V_{r_{2}}-V_{r_{1}}}{r_{2}-r_{1}}$ (2)

 $f_{\theta}=\frac{V_{\theta_{2}}-V_{\theta_{1}}}{r \theta_{2}-r \theta_{1}}$ (3)

2.1.3 逆风区与强回波中心

 图 1 2013年5月7日18:09(a, c)和2012年7月29日21:17(b, d)的逆风区与强回波中心位置 (a, b)径向速度，(c, d)回波强度 Fig. 1 Position of AWR and centers of strong echoes during heavy rainfall events at 18:09 BT 5 July 2013 (a, c), 21:17 BT 29 July 2012 (b, d) (a, b) radial velocity, (c, d) radar echo intensity
2.1.4 逆风区对强降水的预警能力检验

 图 2 2013年5月7日强水过程逆风区(▲)与强降水中心(○)的时间和位置变化 Fig. 2 Time and position change of AWR (▲) and centers (○) during heavy rainfall event on 7 May 2013

2013年5月7日过程共有西宁、湟源、湟中三个国家级测站(多个区域站)达到了强降水标准。18:09湟源站附近首先出现了较强的逆风区，属于负速度包围正速度的Ⅰ类逆风区，逆风区与其后部的环境风速度构成辐合，此时湟源尚无降水出现；18:16湟源附近的逆风区明显扩大并略东移；19:10降水开始，逆风区的出现较强降水开始时间提早了61 min，逆风区与强降水中心的距离只有1.7 km。19:20湟中站西北部出现了负速度包围正速度的Ⅰ类逆风区；20:20湟中降水开始，逆风区的出现较强降水开始时间提早了60 min，逆风区与强降水中心的距离为13.4 km；之后逆风区沿东北方向移动，可推断强降水将继续东北上进入西宁，实况是20:03逆风区进入西宁，21:00西宁降水开始，逆风区到达时间较强降水开始时间提早了57 min，逆风区与强降水中心的距离为12.1 km(表 2)。可见，逆风区对强降水的监测和预警具有很好的指示作用，强降水过程中逆风区存在发生、发展过程，即有一个从小变大并移动进而造成强降水的逆风区，不仅逆风区出现或到达的时间均较强降水时间有较明显的提前量，而且根据逆风区可以判断强降水的位置和移动路径，即推断下一时刻强降水的移动方向和出现地区，从而通过逆风区的移动速度和方向外推做出强降水时间及落区预警。

2.2 强降水前的垂直风场特征 2.2.1 垂直风切变及冷暖平流

 $|\Delta V|=\sqrt{V_{1}^{2}+V_{2}^{2}-2 V_{1} V_{2} \cos D}$ (4)

2.2.2 最大回波高度及零速度层

3 西宁强降水的预警指标

4 结论与讨论

(1) 西宁强降水的径向速度往往会出现逆风区，占比达到92.9%，且直接受影响的混合性强降水Ⅰ类逆风区略多，对流性强降水多为Ⅱ类，总体上以东北向移动为最多，占比为55.6%，其次为东向。

(2) 逆风区较强降水有20~152 min的提前量，逆风区出现或经过到达的地方与强降水中心的距离为1.7~25.8 km，逆风区既是强降水预警的时效性判据，也是识别强降水落区及路径的有用判据。若逆风区的切变流场为辐合和气旋型，将促进强降水的发展；若逆风区的切变流场转为辐散时，降水将减小；且辐合、辐散的出现提前于强降水的开始、结束。

(3) 西宁强降水前1 h垂直风切变混合性强降水低层最大、中层最小，对流性强降水各层均明显大；低—中—高层冷、暖平流的配置混合性强降水主要表现为深厚暖平流，对流性强降水各层均为NW或N，即平流不明显或低、中层有暖平流。强降水前1 h的最大回波高度以10.7~13.7 km为最多，且对流性强降水略高；零速度层以4.9~5.5 km为最多，也是对流性强降水略高，但其总体的差别较小。

(4) 综合应用雷达径向速度产品确定强降水临近预警指标，西宁14例强降水中准确预报11例，漏报3例，无一空报，准确为78.6%。