A Study to Determine Enhancement Potential for Convective-Stratiform Mixed Precipitation Based on Polarimetric Radar
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摘要: 本文统计分析了北京地区近三年的有效降水，重点研究了积层混合云降水特点并对其分类，发现积层混合云降水出现频次约占总降水次数的61%，其中积层混合云降水以积层连结型和水平混合型为主，二者之和占近80%。重点分析了积层混合云中对流和层云两种不同特点降水类型的宏微观结构，确立了反射率因子Z、温度T、粒子含水量M、催化剂AgⅠ（碘化银）活化率NE和粒子相态HTC（hydrometeor type classification）为人工增雨潜力识别指标及这些识别指标的取值范围，同时也根据研究现状和人工影响天气需求总结制定出人工增雨潜力等级。利用偏振雷达构建模糊逻辑识别算法对积层混合云三种降水类型进行增雨潜力区域识别研究，结果表明：（1）对于播撒碘化银增雨来说，积层混合云的增雨潜力区在垂直方向上可分为上、中、下三层，上层（增雨等级为“不适合”）和下层（零度层及以下）分别受含水量和温度等影响不适合增雨，中间层（增雨等级大于等于“等级一”）是可增雨区域；（2）积层混合云中层云区增雨潜力较小，对流云区可增雨潜力要远大于层云区，开式流场型与积层连结型可增雨潜力要大于水平混合型；（3）当降水云中识别出霰粒子时，其附近的大部分区域会有较好的增雨潜力。通过偏振雷达实例检验和数值模式模拟在积层混合云不同部位播撒碘化银催化试验发现，在增雨潜力较好的区域催化有很明显增雨效果，模拟试验结论与偏振雷达识别增雨潜力区结果也基本一致，说明基于偏振雷达的增雨潜力区识别方法和结果是具有参考意义的。Abstract: Statistical analysis on effective precipitation enhancement in Beijing area over the past three years was conducted. Characteristics and classification of convective-stratiform mixed clouds were also studied in this paper. Results show that the frequency of mixed clouds precipitation accounted for 61% of the total frequency. Convective-stratiform connected type and horizontally mixed type are the main types of mixed clouds precipitation and the two types of precipitation account for 80% of the total. This study mainly analyzed the macro-and micro-physical structures of convective clouds and stratus clouds in convective-stratiform mixed clouds. Identification indexes, including the reflectivity factor Z, temperature T, particle water content M, AgⅠ nucleation efficiency (NE), hydrometeor type classification (HTC), and their ranges were determined. Levels of rain enhancement potential were established based on the research situation and weather modification demands. Fuzzy models and algorithms based on polarimetric radar data were established to identify regions of rain enhancement potential for convective-stratiform precipitation. The results show that three layers in the vertical (upper, middle, and lower) could be found in the rain enhancement potential area. Both the upper and lower layers are not suitable for artificial rain enhancement because of the influence of water content and temperature. Thereby only the middle layer is fit for artificial rain enhancement. It was also found that the stratus clouds in the mixed clouds have a small rain enhancement potential, whereas the convection clouds have a large potential. Of different types, the open airflow type and convective-stratiform connected type have a higher potential for artificial rain enhancement than the horizontally mixed type. Furthermore, when graupels are detected by radar, the nearby areas will have a large potential for artificial rain enhancement. Cloud seeding with AgⅠ in different layers was simulated using a numerical model. The result of the numerical model is basically consistent with the result of radar detection. This indicates that the identification method based on polarimetric radar has great implication for assessing artificial rain enhancement potential.
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图 1 AgⅠ活化率随温度T变化的曲线
Figure 1. Variation of AgⅠ nucleation efficiency with temperature (T)
图 2 2013年6月28日21:09的RHI (垂直高度扫描) 扫描图：(a) 反射率因子；(b) 粒子相态识别结果；(c) 改进后的含水量 (单位：g m-3) 估测。图中两个红色矩形框分别表示距离雷达站18~28 km (层云区)、42~52 km (对流云区) 的区域
Figure 2. Radar scan images of RHI (range height indicator) at 2109 BJT (Beijing time) 28 June 2013: (a) Reflectivity factor; (b) particles phase identification; (c) improved particle water content. The red rectangle areas represent 18-28 km (stratiform clouds) and 42-52 km (convective clouds) from radar, respectively
图 5 模糊逻辑增雨潜力等级识别系统框图
Figure 5. Flow diagram of rain enhancement potential levels based on the fuzzy model and algorithm. Z, M, T, NE, and HTC represent reflectivity factor, particle water content, temperature, nucleation efficiency, and hydrometeor type classification. Inputs can belong to different fuzzy sets with different degrees of membership defined by a membership function (MBFi_j). Rain enhancement potential levels (RS) is constructed by the product of the individual propositions (PSi_j)
图 7 2013年6月28日21:09时 (a) 反射率Z、(b) 含水量M、(c) 粒子相态HTC和 (d) 增雨潜力识别结果的垂直分布。方位角：91°，俯仰角：1°~50°
Figure 7. Vertical distributions of (a) reflectivity factor Z, (b) particle water content M, (c) hydrometeor type classification (HTC), and (d) identification results of rain enhancement potential levels at 2109 BJT 28 June 2013. Azimuth: 91°; elevation angle: 1°-50°
图 10 2011年9月7日15:22(左)、15:25(中)、15:32(右) 增雨效果分析 (红框表示催化作业区域)。方位角：77°，俯仰角：1°~50°
Figure 10. Effects of rain enhancement at 1522 BJT (left column), 1525 BJT (middle column), and 1532 BJT (right column) on 7 September 2011. Red box: cloud seeding area. Azimuth: 77°; elevation angle: 1°-50°
图 11 模拟的2012年5月29日12:00叠加在温度与流场上的雷达回波垂直剖面图。实 (虚) 线代表温度的正 (负) 值，黑框代表对流云区，蓝框代表对流入口区，红框代表层云区
Figure 11. Simulated vertical cross section of radar echo superposed on the temperature and airflow field at 1200BT on 29 May 2012. Solid (dashed) lines: positive (negative) temperature; black box: convective area; blue box: convective entrance; red box: stratiform cloud area
图 12 2012年5月29日12:00催化作业影响区内催化作业与未催化作业地面累积雨量差值变化
Figure 12. Differences of accumulated surface rainfall between seeded and unseeded by AgⅠ at 1200BJT on 29 May 2012
表 1 X波段偏振雷达主要性能指标
Table 1. Main specifications of X band polarimetric radar
序号 指标项 详细说明 1 雷达体制 双线偏振、全相参、单发双收/双发双收/脉间变极化 2 工作频率 9370±30 MHz (X波段) 3 探测范围 水平探测距离：75 km、100 km、150 km、300 km，方位扫描范围：0°~360°，俯仰扫描范围：-2°~+90° 4 探测要素 水平反射率、径向速度、速度谱宽、差分反射率、差分传播相移、零相关系数 5 探测精度 强度：≤1 dBZ，速度：≤1 m s-1，谱宽：≤1 m s-1，差分反射率：≤0.2 dB，线性退偏振比：≤0.2 dB，单位差分传播相移：≤1°/km 6 探测模式 VOL (立体扫描)、PPI (平面位置扫描)、RHI (垂直高度扫描) 及sPPI (扇形扫描)，天线转速：0.5~4 rpm 
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表 2 2012~2014年3~10月北京地区降水类型统计
Table 2. Statistics of precipitation types in Beijing area from March to October in 2012-2014
年份 积层混合云
类型出现
频次占积层混合云
降水比例占3~10月总
降水比例2012 开式流场型 4 17% 59% 积层连结型 11 48% 水平混合型 8 35% 2013 开式流场型 2 7% 71% 积层连结型 16 53% 水平混合型 12 40% 2014 开式流场型 9 21% 57% 积层连结型 20 48% 水平混合型 13 31%
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表 3 2009~2015年偏振雷达冰雹识别与地面降雹对比统计结果
Table 3. Statistics of comparison results of polarimetric radar observations and surface hail records
年份 地面降雹/次 偏振雷达冰
雹识别/次未识别原因 2009 7 4 两次不在观测范围内，一次断电 2010 7 5 一次波束阻挡，一次断电 2011 12 11 波束阻挡 2012 8 7 断电 2013 15 13 两次波束阻挡 2014 16 15 雷达停机维修 2015 18 14 平谷区波束阻挡四次
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表 4 增雨潜力等级对应参数的范围
Table 4. Parameter ranges corresponding to rain enhancement potential (REP) levels
增雨
潜力参数范围 Z/dBZ M/g m-3 T/℃ NE/m-3 HTC 不适合 -5~50 0.01~3 -3~40 0~3 雨滴 一级 20~40 0.05~0.2 -15~-2 1.6~8 霰、融化的雪、雪 二级 30~45 0.1~0.4 -20~-5 4~12 霰、冰晶 三级 30~55 0.2~1 -40~-8 8~16 霰、冰晶、过冷水、冰雹 注：Z表示反射率，M表示粒子含水量，T表示温度，NE表示AgⅠ活化率，HTC表示粒子相态识别。
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