用改进的光流法和卫星TBB资料进行对流临近预报

CONVECTIVE NOWCASTING USING IMPROVED OPTICAL FLOW METHOD AND SATELLITE TBB DATA

关键词:  光流法  半拉格朗日外推  COTREC  FY-4A  TBB

Key words:  optical flow method  semi-Lagrangian extrapolation  COTREC  FY-4A  TBB

摘要:

为了在缺少雷达观测的地区开展对流临近预报，利用光流法和半拉格朗日外推法进行了卫星云图外推实验，同时利用无辐散约束改进光流矢量来避免云图辐散失真。(1) 光流法反演的风场能够准确反映出台风涡旋环流结构，采用半拉格朗日方案进行外推，可以保持云系的旋转特性，具有良好的稳定性和精度，但随着外推时间的增加光流矢量中的噪声会导致云图辐散失真。(2) 无辐散约束减少了风矢量杂乱无序现象，弥补了缺失的光流，还能对风速进行平滑，使风速空间梯度更平滑。(3) 用改进后的风场进行外推，避免了云系辐散失真，在保持其形态不被破坏的同时，还能减少云团TBB虚假增长。(4) 检验表明改进后的外推预报方案，具有更小的平均绝对误差，MAE提高了4%，临界成功指数提升了9%。

Abstract:

To carry out convective nowcasting in areas lacking radar observations, the present study uses the methods of optical flow and semi-Lagrangian extrapolation to conduct satellite cloud image extrapolation experiments. Moreover, the optical flow vector is improved by using non-divergence constraint to avoid divergence and distortion of the cloud image. Case studies show that: (1) The wind field retrieved by using the optical flow method can accurately reflect the typhoon vortex circulation structure. The semi-Lagrangian scheme is employed for extrapolation as it can maintain the rotation characteristics of the cloud system, and has good stability and accuracy; however, as the extrapolation time increases, the noise in the optical flow vector will lead to divergence and distortion of the cloud image. (2) The non-divergence constraint reduces the disorder of the wind vector, makes up for the missing optical flow, and can also smooth the wind speed to make the wind speed spatial gradient smoother. (3) The improved wind field is used for extrapolation to avoid cloud divergence and distortion. The cloud shape will not be damaged, and the false growth of cloud cluster TBB will be reduced. (4) The test shows that the improved extrapolation forecasting scheme has a smaller average absolute error, with MAE increased by 4%, and critical success index increased by 9%.