Statistical Characteristics of Cloud Precipitation in the Motuo Area Observed by X-band Dual-Polarization Phased Array Radar
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摘要: 第二次青藏高原综合科学考察研究项目在墨脱布设了一部X波段相控阵偏振雷达（X-PAR），实现了首次对河谷地区云降水的雷达连续观测。为了揭示高原东南河谷地区云降水的宏观特征，本文利用墨脱X-PAR 2019年11月至2020年10月的观测数据定量分析了墨脱地区云降水回波强度、回波顶高等参数的月变化、日变化和高度变化，并与那曲地区夏季季风时期多普勒雷达观测数据进行了比较。研究发现：（1）墨脱地区回波顶高、面积、强回波所占比例以及回波分布范围在4～10月大于11～3月，4～10月降水频次高、对流性降水多，其中以6月最为显著。而进入4月后弱回波数量的大幅度增加导致了4～10月回波强度小于11～3月。降水回波月变化特征结合高原季风指数，将一年分为旱季（11～3月）与雨季（4～10月）。（2）雨季降水回波频次、顶高、面积均大于旱季，说明雨季降水频次更高、对流性活动更旺盛。降水回波频次、顶高、面积的日变化表明，旱季日降水主要发生在下午与上半夜，雨季主要发生在下半夜。（3）墨脱降水回波强度大部分小于30 dBZ，旱季在海拔高度3 km以上回波发生频次高，雨季在3 km以下高。（4）夏季季风期间墨脱回波顶高低于那曲，其顶高、面积日变化趋势与那曲不同。夏季季风期间那曲日降水主要集中在下午与上半夜，而墨脱则集中在下半夜。墨脱旱季云降水特征与那曲夏季季风时期特征较为相似。Abstract: During the second comprehensive scientific expedition to the Qinghai Tibet Plateau, an X-band phased array polarimetric radar (X-PAR) was installed in Motuo. For the first time, the most advanced dual-polarization phased array radar is used to continuously observe the precipitation in the valley area. The monthly, diurnal, and altitude variations of echo intensity and echo top height of precipitation in Motuo were quantitatively analyzed using the observation data of Motuo X-PAR from November 2019 to October 2020 to reveal the characteristics of precipitation in the southeast valley of the plateau. The results are then compared to those obtained using Doppler radar during the summer monsoon in Naqu. The results show that: (1) The echo peak height, echo area, proportion of strong echo, and echo distribution range from April to October are greater than those from November to March in Motuo, indicating that the precipitation frequency is high and convective precipitation is more from April to October, particularly in June. However, the increase in the number of weak echoes in April shows that the echo intensity from April to October is less than from November to March. According to the monthly variation characteristics of cloud precipitation in Motuo and the plateau monsoon index, the year is divided into the dry season (November to March) and the rainy season (April to October). (2) The echo frequency, top height, and area of precipitation in the rainy season are higher than those in the dry season. The diurnal variations of echo frequency, top height, and area show that the strongest convection occurs in the afternoon in both seasons. Precipitation occurs primarily in the afternoon and first half of the night during the dry season and in the second half of the night during the rainy season. (3) In Motuo, the echo intensity of precipitation is mostly less than 30 dBZ. The echo frequency is higher both in dry and rainy seasons for altitude >3 km and <3 km, respectively. (4) During the summer monsoon, the echo peak height of Motuo is lower than that of Naqu, and the diurnal variation trend of its peak height and area differs from that of Naqu. Besides, daily precipitation in Naqu is primarily concentrated in the afternoon and the first half of the night. In contrast, precipitation in Motuo is focused mainly in the second half of the night. The characteristics of cloud precipitation in the dry season of Motuo are similar to those in the summer monsoon period of Naqu.
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图 1 青藏高原地形（阴影，单位：m）与墨脱站地理位置（29°18′46″N，95°19′03″E，海拔高度为1305 m）以及2019年11月至2020年10月平均整层水汽通量（箭头，单位：kg m−1 s−1）
Figure 1. Topography of Qinghai–Tibet Plateau (shaded, units: m), geographical location of Motuo (29°18′46″N，95°19′03″E, 1305 m ASL), and average whole layer water vapor flux from November 2019 to October 2020 (arrow, units: kg m−1 s−1)
图 3 2020年10月4日04:53:46 X-PAR观测第7层仰角（10.7°）的（a）ZH，（b）ZDR，（c）ρhv，（d）KDP，除掉地物杂点回波的（e）ZH，（f）ZDR，（g）ρhv，（h）KDP。红圈位置为地物和杂点
Figure 3. Plan position indicator (PPI) of (a) ZH, (b) ZDR, (c) ρhv, and (d) KDP at the elevation of the 7th layer (10.7°) observed by X-PAR on October 4, 2020, at 0453:46 BJT; (e) ZH, (f) ZDR, (g) ρhv, and (h) KDP are those whose ground clutter and spurious echo have been removed. The position of the red circle is the echo of ground clutter and spurious
图 4 部分高度层X-PAR有效探测回波范围（黑色为可探测到的区域）：（a）2 km；（b）3 km；（c）4 km；（d）5 km；（e）9 km；（f）10 km；（g）11 km；（h）12 km
Figure 4. Effective detection echo range of X-PAR is in some altitude layers (black area represents the detectable area): (a) 2 km; (b) 3 km; (c) 4 km; (d) 5 km; (e) 9 km; (f) 10 km; (g) 11 km; (h) 12 km
图 5 墨脱X-PAR海拔高度可探测点数廓线图（高度分辨率：100 m）
Figure 5. Profile of detectable points of Motuo X-PAR with altitude (Height resolution: 100 m)
图 6 不同月份（a）回波强度与（b）回波顶高发生频率的分布
Figure 6. Frequency distribution of (a) echo intensity and (b) echo top height in different months
图 7 墨脱站月降水量随时间变化
Figure 7. Time variation diagram of monthly precipitation at Motuo station
图 8 观测数据箱型图的逐月变化，箱型图中最高最低两点分别为最大值和最小值，盒子上下横线分别为上四分位数点（75%）与下四分位数点（25%），盒子中间横线为中位数点。（a）回波强度，图中折线为月平均回波强度的连线；（b）回波顶高，折线为月平均回波顶高的连线；（c）回波面积，折线为月平均回波面积值连线；（d）强回波发生频率，折线为每月强回波发生频率平均值的连线
Figure 8. Monthly variation of box chart of observational data; the highest and lowest points in the box diagram indicate the maximum and minimum values, upper and lower horizontal lines of the box represent the upper quartile points (75%) and lower quartile points (25%), respectively, and the middle line of the box represents the median point. (a) echo intensity, the line is the average of echo intensity per month in the figure; (b) echo top height, the line is the average of echo top height per month; (c) echo area, the line is the average of echo area per month; and (d) frequency of strong echo, the line is the average of the frequency of strong echo per month
图 9 不同时间回波强度、顶高发生频率分布：（a）旱季回波强度；（b）雨季回波强度；（c）旱季回波顶高；（d）雨季回波顶高
Figure 9. Frequency distribution of echo intensity and echo top height at different time. Echo intensity in (a) dry season and (b) rainy season; echo top height in (c) the dry season and (d) the rainy season
图 10 观测数据平均值的日变化：（a）回波强度；（b）回波顶高；（c）回波面积；（d）强回波发生频率
Figure 10. Daily variation of the average of the observed data: (a) Echo intensity; (b) echo top height; (c) echo area; (d) frequency of strong echo
图 11 观测时间段内回波强度的垂直分布：（a）旱季；（b）雨季
Figure 11. Vertical distribution of echo intensity in observation period during (a) the dry season and (b) the rainy season
图 12 旱季回波强度垂直分布的逐时变化
Figure 12. Hourly variation of the vertical distribution of echo intensity in the dry season
图 13 雨季回波强度垂直分布的逐时变化
Figure 13. Hourly variation of the vertical distribution of echo intensity in the rainy season
图 14 夏季季风期间（6~8月）(a)平均回波顶高、(b)平均回波面积日变化曲线与不同时间(c)回波强度与(d)回波顶高的分布
Figure 14. Diurnal variation of the average of (a) echo intensity and (b) echo area, and the distribution of (c) echo intensity and (d) echo top height at different time during the summer monsoon (June–August)
表 1 X-PAR雷达参数
Table 1. The parameters of X-PAR
主要参数 参数设置 工作频率 9.3～9.5 GHz 峰值功率 256 W 观测用时 92 s 最大探测距离 42 km 最小距离分辨率 30 m 仰角扫描范围 0.9°～20.7°，以1.8°的步进角扫描 波束宽度 3° 阵面法向角 15° 观测模式 VRHI 偏振体制 双发双收 
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