对流尺度集合预报中模式地形扰动对其预报技巧的影响研究

Study on the influence of model topography perturbation on prediction skill in a convection-allowing scale ensemble prediction system

基于在重庆市气象局业务运行的对流可分辨尺度(3 km)集合预报系统,在已有初值扰动、模式物理过程扰动和侧边界扰动的基础上,对不同集合成员采用不同地形插值方案和地形平滑方案实现对模式静态地形高度的扰动,体现数值模式中地形转换过程的不确定性,开展集合预报批量平行试验。结果表明:(1)实现对模式静态地形高度的扰动后,各集合成员地形高度的离散度与实际地形的起伏程度对应关系较好,两者空间分布特征非常相似,地形较平坦的平原地区离散度较小,而地形较复杂的高原地区或山区离散度较大;(2)加入模式地形扰动方案后,集合扰动能量总体上有所增大,且低层比中、高层更明显,能量增幅在较短预报时效(12 h)内最显著,随着预报时效延长呈逐渐减小趋势,且能量增幅大值中心主要出现在地形较复杂、集合成员地形高度离散度较大地区;(3)模式地形扰动方案一定程度上能提高降水概率预报技巧及改进集合平均降水预报,在对高空要素和2 m温度、10 m风场等近地面要素的集合平均均方根误差和集合离散度无负面影响的前提下,能一定程度上优化集合分布。

Based on the convection-allowing scale high resolution(3 km) ensemble prediction system which has been operationally implemented in meteorological services of Chongqing and includes initial perturbation, model physical process perturbation and lateral boundary perturbation in it, the perturbation to model static topographic height field is realized by adopting different topographic interpolation schemes and smoothing schemes to generate static topographic height field of each ensemble members to express the uncertainty of the topographic conversions in the numerical model to some extent. Using these perturbations, we have performed a month consecutive ensemble prediction tests and the comparative analysis with respect to the ensemble forecast results without model topography perturbation. The results show that(1) after adding the model topography perturbation scheme into the ensemble prediction system, the topographic height spread among ensemble members has a good correspondence with the real topographic height gradient, and both spatial distribution characteristics are very similar. The topographic height spread in the plain area is small, while that in the plateau area or mountainous area with complicated terrain is large.(2) The addition of model topography perturbation scheme increases the ensemble different total energy(DTE) whose increase ratio in the low-level is larger than that in the mid-and upper-level. The enhancement of DTE in the short forecast lead time(12 h) is the most significant. It gradually decreased with the forecast lead time becoming longer. Large enhancement centers of DTE mainly appear in the areas with complicated topography and the great topographic height spread among the ensemble members.(3) In general, the addition of model topography perturbation scheme can improve the probability forecast skill and ensemble mean forecast skill of 24 h accumulated rain to some extent. It can optimize the ensemble distribution of all the elements in upper air, and 2 m air temperature, 10 m zonal wind, 10 m meridional wind and other elements near the ground level to a certain extent without the negative effect in the ensemble mean forecast error and ensemble spread.