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交叉相关外推算法的改进及其在对流临近预报中的应用
Improvement and Application Test of TREC Algorithm for Convective Storm Nowcast
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摘要: 交叉相关外推算法是目前对流临近预报的主要算法之一。它主要是通过计算雷达回波等资料在连续时次的空间最优相关, 得到对流系统不同位置的移动矢量特征, 并基于这些获得的移动矢量对雷达回波等进行外推, 从而达到预报的目的。因为算法既考虑了回波移动矢量大小和方向的变化, 也考虑了整个回波在移动过程中的形变, 所以, 基于该算法的外推临近预报具有一定的物理意义。该文介绍了交叉相关外推预报算法的基本原理, 对算法进行了一系列的质量控制, 包括抑制地物杂波和噪声的影响、对“丢失”计算的点进行矢量补充以及对追踪的矢量进行平滑处理等多个步骤。个例对比分析表明:算法在通过质量控制和优化处理后, 追踪的回波移动矢量质量明显改善。利用优化后的算法, 对2004年夏季和2005年夏季发生在京津冀地区的4个强对流个例, 包括2次飑线过程、1次雹暴过程和1次强雷暴过程, 进行了外推临近预报试验, 并且对算法在对流临近预报中的效果做了初步的检验评估。结果表明:该算法对强对流天气过程的临近预报具有一定效果, 算法可以给出未来30 min或者60 min内雷达回波和雷暴位置以及形状的外推预报, 预报结果与实况比较接近。Abstract: At present, cross-correlation extrapolation is one of the main algorithms for convective storm now cast. Motion vectors of convective storm for every divided equal-sized two-dimensional arrays of radar echo or other data measured at two times several minutes apart by calculating optimal spatial cross-correlation are obtained in the algorithm. The obtained motion vectors are customarily called TREC (tracking radar echoes by correlation) vectors or TREC winds. And then, storm now cast can be achieved by extrapolating radar echoes or other data based on the obtained TREC vectors. The algorithm results involve not only the changing characteristics of magnitude and direction of the motion vectors, but also shape varieties of the whole echoes in the course of their movement. So the result of storm now cast based on the algorithm is assuredly reasonable and significant in meteorology. The basic principle of TREC is introduced firstly. And a number of methods to improve the algorithm result are presented, including noisy vector restriction and clutter contamination removal, discarded or missing motion vector supplement, vector smoothing, and so on. Analysis results of two cases indicate that the tracked motion vectors can be markedly improved after quality control and optimization processes to TREC algorithm. Finally, based on the optimized TREC algorithm and Tianjin radar data, storm nowcast tests and verifications of four intense convective storms that occur in Beijing-Tianjin-Hebei areas during 2004 and 2005 summertime, including two squall line cases, a hailstorm case and a strong thunderstorm case, are described in detail. The results indicate that the improved algorithm is available for convective storm nowcast.The algorithm can automatically produce 30-minute or 60-minute forecast of location and shape of radar echoes or storm characteristics based on the extrapolated vectors. The forecast results are close to what is actually happening. Because the algorithm can automatically produce forecast results in real time mode, it is helpful for convective storm now cast and warning.The forecast results are also clear at a glance, so abilities of forecasters for strong convective storm identification and forecast can be enhanced by the algorithm. An expectation is that the improved algorithm can be used for operational storm now cast in the near future.
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Key words:
- cross-correlation;
- TREC;
- now cast;
- radar echo
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图 1 2005年8月8日19:44天津雷达PPI强度回波与由交叉相关算法追踪得到回波移动矢量 (仰角:0.4°)(a) 质量控制前, (b) 质量控制后
Fig. 1 Coloured plan position indicator (PPI) of Tianjin radar reflectivity with elevation angle of 0.4° at 19:44 on August 8, 2005 and overlaid TREC vectors without quality control (a) and with quality control (b)
图 2 交叉相关外推预报算法对2004年7月29日飑线过程大于等于35dBz的反射率因子09:38(a) 和09:50(c) 的30min外推预报，以及相应的30min后10:08(b) 和10:20(d) 的强雷达回波 (仰角:0.4°)
Fig. 2 30-min forecast of reflectivity echo (≥35 dBz) at 09:38(a) and 09:50(c) on July 29, 2004 based on TREC vectors, and reflectivity observation (≥35 dBz)30-min later from Tianjin radar with elevation angle of 0.4 at 10:08(b) and 10:20(d)
图 3 交叉相关外推预报算法对2004年7月29日飑线过程的35 dBz以上反射率因子峰值可能出现范围在09:19(a) 和09:44 (b) 的30 min外推预报 (黑色线范围), 以及30 min后的实况分析 (彩色阴影区)
Fig. 3 30-min forecast of extension of reflectivity maximum (≥35 dBz) at 09:19(a) and 09:44(b) on July 29, 2004(black contour) based on TREC vectors, and actual extension of reflectivity maximum (≥35 dBz)30-min later (shaded color)
图 4 交叉相关外推预报算法对2005年5月31日雹暴过程35 dBz以上反射率因子在11:18的30 min外推预报 (a) 以及相应的30 min后强雷达回波的实况 (b)(仰角:0.4°)
Fig. 4 30-min forecast of reflectivity echo (≥35 dBz) at 11:18(a) on May 31, 2005 based on TREC vectors, and reflectivity observation (≥35 dBz)30-min later from Tianjin radar with elevation angle of 0.4° at 11:18(b)
图 5 交叉相关外推预报算法对2005年6月13日雹暴过程35 dBz以上反射率因子在08:47的30 min外推预报 (a) 以及相应的30 min后强雷达回波的实况 (b)(仰角:0.5°)
Fig. 5 30-min forecast of reflectivity echo (≥35 dBz) at 08:47(a) on June 13, 2005 based on TREC vectors, and reflectivity observation (≥35 dBz)30-min later from Tianjin radar with elevation angle of 0.5° at 09:18(b)
图 6 交叉相关外推预报算法对2005年6月13日飑线过程的35 dBz以上反射率因子峰值可能出现范围在08:47的30 min外推预报 (黑色线范围), 以及30 min后的实况分析 (彩色阴影区)
Fig. 6 30-min forecast of extension of reflectivity maximum (≥35 dBz) at 08:47on June 13, 2005(black contour) based on TREC vectors, and actual extension of reflectivity maximum (≥35 dBz)30-min later (shaded color)
图 7 交叉相关外推预报算法对2005年6月13日飑线过程的反射率因子在08:41的60 min外推预报 (a) 以及60 min后的实况 (b)
Fig. 7 60-min forecast of reflectivity echo at 08:41(a) on June 13, 2005 based on TREC vectors, and reflectivity observation with elevation angle of 0.5° at 09:36(b)
图 8 交叉相关外推预报算法对2005年7月14日强对流过程35 dBz以上反射率因子在20:36(a) 和20:47(c) 的30 min外推预报以及相应的21:03(b) 和21:15(d) 强雷达回波实况 (仰角:0.5°)
Fig. 8 30-min forecast of reflectivity echo (≥35 dBz) at 20:36(a) and 20:47(c) on July 14, 2005 based on TREC vectors, and reflectivity observation (≥35 dBz) 30-min later from Tianjin radar with elevation angle of 0.5° at 21:03(b) and 21:15(d)
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