光流法及其在临近预报中的应用

The optical flow method and its application to nowcasting

介绍了一种全新的对流临近预报外推算法——光流法。主要是通过计算雷达回波的光流场得到回波的运动矢量场,并基于运动矢量场对雷达回波进行外推从而达到预报的目的。光流法立足于变化,在计算光流场时既考虑连续时次回波的变化,又考虑相邻位置回波的变化。因此,基于该算法的外推临近预报具有一定的物理意义。利用中值滤波等方法对雷达资料进行质量控制,有效地抑制了噪声等的影响。个例对比分析表明:经过中值滤波等质量控制后回波更平滑,边沿更清晰,回波质量明显改善,能够获得比较真实的雷达回波,也得到了方向更加一致、更加平滑的回波运动矢量场。光流法给出的30、60 min内雷达回波的位置、形状的外推预报和对应时次实况较接近,预报结果具有较好的业务指示意义。对光流法和交叉相关法预报结果进行的定量对比评价表明,对移动型局地生成及强度和形状随时间变化很快的回波,光流法预报效果优于交叉相关法。光流法可以弥补传统的交叉相关法的缺陷,能提升对流临近预报系统的性能。

This paper introduces a novel extrapolation algorithm,the optical flow method,for the convection nowcasting.Basi-cally,the forecasting is achieved through the following two steps.The motion vector field of the echo is firstly obtained through the calculation of the optical flow field for the radar echo.Based on this motion vector field,the radar echo can then be extrapolated.The optical flow method focuses on the change of echo.It will consider both the change of the continuous echo when calculating the optical flow field,and the change of the echo for the adjacent position.Physically,this forecast method based on the algorithm is meaningful.The quality of the radar data can be controlled by applying the median filtering method, with suppressing effectively the influence of noises.Case study shows that the quality of the echoes has been significantly im-proved,after the quality control by the median filtering method.The echo becomes smoother with the echo edge clearer.In ad-dition,the echo motion vector field becomes smoother and more consistent.The extrapolating forecast of the location and shape of the radar echo within 30 and 60 min by the optical flow method is close to the real observation,which is valuable for the ref-erence for operational forecast.By quantitatively comparing their forecasts,the optical flow method performs better than the cross-correlation method when forecasting the echo which is generated locally,with quick changes in the intensity and shape. The optical flow method can remedy the defect of the traditional cross-correlation method,and can improve the performance of the convection nowcasting system.Therefore,this new method has great application promotion value in the near future.