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The Evaluation of Wind Measurement Accuracy by Wind Profile Radar
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摘要: 采用风廓线雷达5波束探测模式的数据对测风精度进行评估分析，用垂直波束和其中两个相邻倾斜波束的探测数据构成一对计算因子，通过对同一距离高度上的4对计算因子进行误差分析，评估风廓线雷达的测风精度，得到水平风在垂直指向连续高度上的精度。对北京延庆CFL-08风廓线雷达2010年3，6，9，12月4个典型代表月份逐日连续探测资料进行了处理分析，结果表明：该雷达满足风速误差不大于1.5 m·s-1、风向误差不大于10°探测精度要求的最大探测高度6月、9月为8 km，3月、12月为6 km，基本符合该雷达探测高度的设计要求。信噪比、大气风场的不均匀性是影响雷达测风精度的主要因素：信噪比影响了高空的测风精度，-15 dB可以作为判断雷达测风可信数据最大探测高度的阈值；晴空大气出现的风场不均匀性对风廓线雷达的测风精度影响不大，降水出现时环境风场不均匀性造成水平风向、风速的测量误差较大，不能满足测风精度要求，特别是对流性降水发生前的1~2 h，水平风向、风速的方差增长迅速，可以作为强降水出现的预警指标。Abstract: As a new type of detection instrument, wind profile radar (WPR) can detect meteorological factors such as wind profiles. The special detecting ability of WPR decides its broad application in atmospheric science research, climate research, meteorological operation application, aviation security and many other areas. Data quality control and accuracy of WPR for application of comprehensive developing meteorological operation has the vital significance.Based on basic data of WPR and meteorological background information, a new method of evaluating wind measurement precision of WPR is proposed. Combined with PB-type Ⅱ troposphere WPR detection data from Yanqing Meteorological Observatory of Beijing, the feature of WPR data is analyzed, processing all continuous observation data of four months (March, June, September and December) of 2010, except for some missing measurements. According to rain gauge data hour by hour, all data are divided into two categories: Clear sky and rainfall, rainfall data are extended to five hours before and after the precipitation. Yanqing is located in the north mountain areas of Beijing, few days of precipitation are observed by WPR site, only 15 times in all. A total of 34380 individual observation cycle data of clear sky and 2580 individual observation cycle data of rainfall are statistically analyzed, respectively.Results show that the maximum height is 8 km in June and September, 6 km in March and December when the velocity error is less than 1.5 m·s-1, and the wind direction error is less than 10°, which basically meets the design requirements of the radar detection height. Quality of radar return signal and inhomogeneity of atmosphere are the two influencing issues for wind measurement accuracy of radar.Signal quality directly affects the detection height, and the SNR (signal to noise ratio) of radar return signal has influences on wind measurement precision on the upper air, where -15.0 dB of SNR can be judged as the threshold of maximum detection height of radar wind measurement reliability. Inhomogeneity of atmosphere underclear air conditions affects the precision of wind measurement slightly.Larger errors of horizontal wind direction and speed appear when it rains, making the wind accuracy unacceptable especially. 1 or 2 hours before the convective precipitation, horizontal wind direction and speed of the variance increase rapidly, which could be a sign of strong precipitation.This new method and the evaluating approaches have the following advantages: Four groups of three-beam detected pattern are used to evaluate effective detected height and accuracy of wind measurement under clear sky conditions.The wind levels are determined through the threshold value combined horizontal speed and direction.The thresholds are based on different months and different heights under clear air conditions.The influences of wind measurement precision are given by analyzing two different types of precipitation.Inhomogeneity of atmosphere is found obviously before rainfall, which can be seen as a warning indicator before strong precipitation.
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图 2 水平风速、风向标准差分布频数图
(a) 高模式风速标准差, (b) 高模式风向标准差, (c) 低模式风速标准差, (d) 低模式风向标准差
Fig. 2 The standard deviation of horizontal wind speed and direction for four-month data in high and low modes
(a) standard deviation of wind velocity in high mode, (b) standard deviation of wind direction in high mode, (c) standard deviation of wind velocity in low mode, (d) standard deviation of wind direction in low mode
图 3 晴空信噪比与水平风速标准差及高度的变化曲线
Fig. 3 Two curves of clear air SNR with horizontal wind speed standard deviation, and clear air SNR with height in high mode
图 4 晴空水平风速标准差的日变化
Fig. 4 The diurnal variation of horizontal wind speed standard deviation in clear sky for four months
图 5 晴空水平风向标准差日变化
Fig. 5 The diurnal variation of horizontal wind direction standard deviation in clear sky for four months
图 6 晴空测风质量评估等级
Fig. 6 Distribution of wind measurement precision in clear sky for four months
图 7 典型的层状云降水个例 (阴影区表示水平风速标准差；黑色折线表示降水量)
Fig. 7 The examples of typical uniform precipitation
(the shaded denotes the standard deviation of horizontal wind speed; black polyline denotes precipitation)
图 8 典型的对流性降水个例 (阴影区表示水平风速标准差；黑色折线表示降水量)
Fig. 8 The examples of typical convective precipitation
(the shaded denotes the standard deviation of horizontal wind speed; black polyline denotes precipitation)
表 1 风廓线雷达参数
Table 1 Parameters of wind profile radar
雷达参数 探测模式 高 低 脉冲重复周期/μs 100 20 脉冲宽度/μs 4.0 0.8 发射功率/kW 7.7 7.7 天线增益/dB 29 29 相干积分次数 50 200 谱平均数 12 6 谱变换数 512 216 距离库长/m 240 120 噪声系数/dB 2 2 
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表 2 测风质量等级
Table 2 Rank of wind measurement precision
序号 测风质量等级 水平风标准差约束条件 风速/(m·s-1) 风向/(°) 1 优秀 σv≤0.5 σφ≤5 2 良好 0.5＜σv≤1.0 5＜σφ≤10 3 达标 1.0＜σv≤1.5 10＜σφ≤15 4 超标 1.5＜σv≤2.0 15＜σφ≤20 5 误差大 σv＞2.0 σφ＞20
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