新一代天气雷达网资料的三维格点化及拼图方法研究

STUDY OF METHODS FOR INTERPOLATING DATA FROM WEATHER RADAR NETWORK TO 3-D GRID AND MOSAICS

文中研究了几种把球坐标系下的雷达网反射率体扫资料插值到统一的笛卡尔坐标系下的经纬度网格上以及用多个雷达的反射率网格资料进行三维拼图的方法,并对多个雷达同步观测的反射率因子的空间一致性、系统误差以及雷达等距离线上回波的水平和垂直结构进行了分析。结果发现:在雷达资料格点化过程中,径向和方位上的最近邻居法和垂直方向的线性内插法的结合(NVI方法)是一种有效的雷达资料分析方法,它既能得到空间比较连续的反射率分析场,同时也较好地保留了体扫资料中原有的反射率结构特征;广州雷达和梅州雷达同步观测的空间一致性比较好;在多个雷达资料合成拼图的过程中,距离指数权重平均法能提供空间连续的三维反射率拼图数据,拼图也减轻了由雷达波束几何学引起的各种问题。

The advent of Internet-2 and effective data compression techniques facilitates the economic transmission of base-level radar data from the Doppler radar network to users in real time. The native radar spherical coordinate system and large volume of data make the radar data processing a nontrivial task, especially when data from several radars are required to produce composite radar products. This paper investigates several approaches to remapping and combining multiple-radar reflectivity fields onto a unified 3D Cartesian grid with high spatial and temporal resolutions. The purpose of the study is to find an analysis approach that retains physical characteristics of the raw reflectivity data with minimum smoothing or introduction of analysis artifacts. Moreover, the approach needs to be highly efficient computationally for potential operational applications. Four interpolation schemes were tested for remapping radar data from polar coordinates onto 3 dimension Cartesian coordinates here. The first scheme, nearest neighbor mapping （NN）, is where each grid cell gets a value from the nearest data bin （based on the distance between center of the grid cell to the center of the radar data bin）. The second scheme is a nearest neighbor scheme on range-azimuth planes combined with a linear interpolation in vertical direction （NVI）. The third scheme uses a linear interpolation in vertical direction plus a horizontal interpolation （VHI）. The fourth scheme is dual linear interpolation using data from eight points around grid cell （EPI）. Through comparison of vertical and horizontal reflectivity cross section and estimated one-hour precipitation obtained by use of the four interpolation schemes, it was found that the vertical interpolation scheme with nearest neighbor on the range-azimuth plane is the most reasonable analysis scheme that provides consecutive reflectivity fields and retains high-resolution structure comparable to the raw data. In order to check spatial coherence and calibration differences of two radars when they detect synchronously atmosphere, horizontal and vertical structure of reflectivity fields on equidistant line are analyzed. Result shows the space consistency of synchronous observation of Guangzhou and Meizhou radar is better. After reflectivity fields from individual radars are remapped onto the Cartesian grid, they are combined to produce a unified 3D reflectivity grid. There are many regions, especially at upper levels of the atmosphere, are covered by multiple radar umbrellas. For grid cells with multiple radar coverage, the following three mosaic schemes are tested. The first scheme is the nearest neighbor scheme where the analysis value from a radar that is closest to the grid cell is assigned. The second scheme is to take the maximum among the analysis values from multiple radars. The third scheme is a weighted mean scheme. Two weighting functions, both based on distances between a given grid cell and the multiple radars covering the grid cell, are tested. The first is a Cressman scheme with 300 km radius of influence and the second is an exponential function with 100 km distance scale. Result shows the distance-exponential-weighted mean scheme provides a spatially consistent reflectivity mosaics while retaining the magnitude of the observations from the close radar. Mosaics can mitigate various problems that caused by the geometry of radar beam such as data voids with the cone of silence above the radar and in regions below the lowest beam.