Macro–Micro Physical Characteristics of Rainfall Clouds in the West Tianshan Mountains Based on Ka-Band Cloud Radar
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摘要: 利用Ka毫米波云雷达与自动气象站降雨资料,研究了西天山地区2019年和2020年5~8月的降雨云宏微观特性。结果表明:(1)降雨主要发生在夜间,累积降雨量集中在21:00(北京时间,下同)至次日07:00,降雨频次和累积降雨量相关系数为0.71。大雨强频次虽最少,但对总累积降雨量贡献较显著。(2)小雨强、中雨强、大雨强平均反射率因子最大值分别为30 dBZ、35.8 dBZ和39.5 dBZ,最大平均液态水含量分别为1.5 g m−3、4.2 g m−3和7.3 g m−3。(3)不同降雨强度对应的反射率因子都有两个集中区域,2.0~4.4 km反射率因子集中在15~26 dBZ,地面附近的小雨强、中雨强、大雨强对应的反射率因子分别集中在24~32 dBZ、29~38 dBZ和31~42 dBZ。1.75 km以下中雨强和大雨强液态含水量小于1 g m−3的频率明显少于小雨强,降雨强度的越大降雨粒子径向速度越集中。Abstract: This study analyzed the physical characteristics of rainfall clouds in the West Tianshan Mountains, from May 2019 to August 2020, based on the Ka-band millimeter-wave cloud radar and rainfall data from automatic weather stations. The findings demonstrate that: (1) Rainfall occurs primarily at night. The cumulative rainfall was concentrated from 2100 BJT to 0700 BJT the next day. There was a significant beneficial correlation between rainfall frequency and accumulated precipitation. The frequency of heavy rainfall was the lowest, but its contribution to total accumulated rainfall was significant. (2) The maximum average reflectivity of light, moderate, and heavy rainfall intensities were 30, 35.8, and 39.5 dBZ, respectively, and the maximum average liquid water content was 1.5, 4.2, and 7.3 g m−3, respectively. (3) There are two concentrated areas for the reflectivity of various rainfall intensities. The reflectivity of 2.0–4.4 km was concentrated in 15–26 dBZ, and the reflectivity of light, moderate, and heavy rainfall intensities near the surface was respectively concentrated in 24–32 dBZ, 29–38, and 31–42 dBZ. The frequency of moderate and heavy rain intensity below 1.75 km, where the liquid water content is less than 1 g m−3, is significantly lower than light rain intensity. The greater the intensity of rainfall, the more concentrated the radial velocity of rainfall particles.
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图 6 新源气象站反射率因子在降雨期间的归一化等频率高度图(2019年5~8月、2020年5~8月数据):(a)00:00~03:00;(b)03:00~06:00;(c)06:00~09:00;(d)09:00~12:00;(e)12:00~15:00;(f)15:00~18:00;(g)18:00~21:00;(h)21:00~24:00
Figure 6. Normalized contoured frequency by the altitude diagram of reflectivity factor during the rainfall period at XY (Data for May–August 2019 and May–August 2020): (a) 0000–0300 BJT; (b) 0300–0600 BJT; (c) 0600–0900 BJT; (d) 0900–1200 BJT; (e) 1200–1500 BJT; (f) 1500–1800 BJT; (g) 1800–2100 BJT; (h) 2100–2400 BJT
图 9 新源气象站降雨云液态含水量归一化等频率高度图(2019年5~8月、2020年5~8月数据):(a)小雨强;(b)中雨强;(c)大雨强
Figure 9. Normalized contoured frequency by the altitude diagram of rainfall cloud liquid water content at XY (Data for May–August 2019 and May–August 2020): (a) Light rain intensity; (b) moderate rain intensity; (c) heavy rain intensity
图 8 新源气象站降雨云反射率因子归一化等频率高度图(2019年5~8月、2020年5~8月数据):(a)小雨强;(b)中雨强;(c)大雨强
Figure 8. Normalized contoured frequency by the altitude diagram of rainfall cloud reflectivity factor at XY (Data for May–August 2019 and May–August 2020): (a) Light rain intensity; (b) moderate rain intensity; (c) heavy rain intensity
图 10 新源气象站降雨云径向速度归一化等频率高度图(2019年5~8月、2020年5~8月数据):(a)小雨强;(b)中雨强;(c)大雨强
Figure 10. Normalized contoured frequency by the altitude diagram of rainfall cloud radial velocity at XY (Data for May–August 2019 and May–August 2020): (a) Light rain intensity; (b) moderate rain intensity; (c) heavy rain intensity
表 1 毫米波云雷达主要参数
Table 1. Main performance indexes of the Ka-band millimeter-wave cloud radar system
参数名称 参数值 工作频率 35 GHz±500 MHz 波束宽度 ≤0.4° 发射功率 ≤500 W 天线增益 ≥52 dB 天线直径 1.8 m 探测范围 0.21~15 km 时间分辨率 1 min 空间分辨率 30 m 发射波长 8.6 mm 表 2 毫米波云雷达三种探测模式(边界层模式、中云模式、卷云模式)对应的工作参数
Table 2. Detailed parameters of the boundary layer, middle cloud, and cirrus observation modes of Ka-band millimeter-wave cloud radar
项目 边界层模式 中云模式 卷云模式 脉冲宽度 0.2 µs 8 µs 24 µs 脉压比 1 40 120 脉冲重复
周期60 µs 120 µs 167 µs 相干积
累数2 1 1 非相干积
累数32 32 32 FTT点数 256 256 256 探测盲区 210 m 1.2 km 3.8 km 探测能力 −18.23 dBZ@5 km −32.54 dBZ@5 km −30.78 dBZ@10 km 表 3 回波分类法经验公式
Table 3. Empirical formula of echo classification
回波强度Z分类 反演方法 经验关系 Z<−17 dBZ LWC=4.564Ze0.5 Atlas −17 dBZ≤Z<5 dBZ LWC=0.457Ze0.19 Baedi Z≥5 dBZ LWC=0.02584Ze0.633 Krasnov -
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