Four methods to perturb model terrain are demonstrated in a convection-allowing scale (3 km) ensemble prediction system (EPS) for heavy rain forecasts. The impact of methods was examined in terms of ensemble mean, spread-skill relationship and probabilistic forecast. The study was carried out for four major precipitation events. It is found that: (a) using different combinations of topography smoothing and interpolation is a promising way to perturb model topography. Even perturbing the topography alone can greatly improve heavy rain forecasts. The benefit of ensemble forecasts increases with the increase in rainfall amount. (b) Adding random perturbations to model topography is not ideal and could hurt ensemble performance due to a too noisy structure although it increases ensemble spread. Perturbation structure plays a more important role than perturbation size. (c) Using the terrain difference between a higher and lower resolution model to estimate possible errors in model terrain is explored. By adding such terrain differences to the terrain-generating-scheme based perturbation (the control experiment) can not only enlarge perturbation size but also keep a good spatial structure. Such a modified ensemble performed similarly in ensemble mean forecasts but improved the ensemble spread and probabilistic forecast. It is recommended that perturbed model terrain should be included in operational EPSs to improve heavy precipitation forecasts. The simple terrain-generating-scheme based perturbation method, as described in this study, is effective and can be implemented in an existing operational EPS without extra computing cost.

中文翻译：

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用于大雨预报的允许对流集合预报系统中的扰动地形

在用于大雨预报的允许对流尺度（3 公里）集合预测系统 (EPS) 中展示了四种扰动模型地形的方法。从整体均值、传播-技能关系和概率预测方面检查了方法的影响。该研究针对四个主要降水事件进行。发现： (a) 使用地形平滑和插值的不同组合是扰乱模型地形的有前途的方法。即使仅扰动地形也可以大大改善大雨预报。集合预报的效益随着降雨量的增加而增加。(b) 向模型地形添加随机扰动并不理想，并且可能会由于过于嘈杂的结构而损害集成性能，尽管它会增加集成扩展。扰动结构比扰动大小起着更重要的作用。(c) 探索使用高分辨率模型和低分辨率模型之间的地形差异来估计模型地形中可能的误差。通过在基于地形生成方案的扰动（控制实验）中加入这种地形差异，不仅可以扩大扰动的大小，而且可以保持良好的空间结构。这种修改后的集合在集合平均预测中的表现类似，但改进了集合传播和概率预测。建议将扰动模型地形纳入业务 EPS 以改进强降水预报。本研究中描述的简单的基于地形生成方案的扰动方法是有效的，可以在现有的操作 EPS 中实施，而无需额外的计算成本。