降雨空间不均匀性与雷达测雨空间平滑误差

Rain Spatial Nonuniformity and Spatial Smoothing Error of Radar Rainfall Measurement

用X波段气象雷达对降雨精细结构观测,获得了125m高距离分辨率的降雨反射率数据。用此数据模拟计算了在与地基、空基的雷达分辨尺寸相当的模拟雷达空间平滑窗口内的不均匀性强度、基于Z-R关系的雷达测雨空间平滑误差等参数,并分析了这些参数间可能的相关关系。结果表明,降雨的小尺度不均匀性非常显著,甚至在1km的雷达分辨尺寸内反射率不均匀度超过10dB以上并不罕见,由此产生的雨强平滑误差不可忽略,甚至对于与地基雷达分辨率相近的0.5km模拟雷达分辨率,其相对相误差可达30%以上。结果比较显示,用通常的1km低分辨率雷达数据模拟计算会低估测雨平滑误差,其中对于4km和3km分辨率约低估25%,对于2km以下分辨率约低估40%以上。

In order to observe fine structures of rainfall, the rainfall radar reflectivity data with a high range resolution of 125 m were obtained by using a ground-based X-band meteorological radar. Using these high resolution radar data, several parameters within simulated radar spatial smoothing windows whose sizes correspond to the resolutions of ground-based and spaceborne radars, such as rain nonuniformity strength, the spatial smoothing error of Z-R relationship based radar rainfall measurement, etc., were computed. The possible correlations between these parameters were also analyzed. The results show that, the small-scale rain nonuniformity is significant, and even in a spatial smoothing window of 1 km size, the reflectivity excursion (defined as the difference between maximum and minimum) above 10 dB is common. The significant rain nonuniformity results in that, the rainrate smoothing error cannot be neglected, and even for a smoothing window of 0.5 km size which is similar to resolutions of ground-based radars, can reach up to above 30%. Comparisons of the smoothing errors computed from radar data with between the 125 m high resolution and the 1 km low resolution shows that the smoothing errors from the 1 km resolution data would be underestimated. The underestimation would be about 25% for smoothing windows of 4 and 3 km, and about 40% for that of 2 km.