The western U.S. precipitation climatology, simulated by the NA-CORDEX regional climate model ensembles, with spatial resolutions of 0.44° (14 model members) and 0.22° (15 model members), is examined to evaluate the capability of these ensembles to reproduce (1) the annual and semi-annual precipitation cycle of several hydrologically-important western U.S. regions and (2) localized seasonality in the amount and timing of precipitation. Similar to previous organized downscaling projects, NA-CORDEX ensembles continue to over-simulate wintertime precipitation over mountainous regions. However, both NA-CORDEX ensembles showed improvements to the Desert Southwest's summertime precipitation associated with the North American Monsoon. Furthermore, the predominant dry bias found throughout the 0.44° ensemble was improved in many of the higher resolution 0.22° ensemble members; associated with enhanced mesoscale processes and improved thermodynamics driving the model's convective parameterizations. When compared against observation-based gridded precipitation, the ability to capture the annual and semi-annual modes of variability was starkly improved in the higher resolution 0.22° ensemble. The higher resolution members reproduced more consistent spatial patterns of variance featuring lower errors in magnitude—especially with respect to the winter-summer and spring-fall seasonality. A great deal of spread in model performance was found for the semi-annual cycles, although the higher-resolution ensemble exhibited better clustering of performance metrics. Ultimately, NA-CORDEX model performance is still tied to the fidelity of the large-scale parent model although we found performance is also closely related to which regional climate model was used while the future trend was more related to the parent GCM. Additionally, the value-added via finer domain grid spacing is clear. Lastly, future projections of precipitation patterns were analysed to examine trends in the seasonality of precipitation intensity and modes of variability.

中文翻译：

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评估美国西部降水模式和变率模式的 NA-CORDEX 历史表现和未来变化

美国西部降水气候学由 NA-CORDEX 区域气候模型集合模拟，空间分辨率为 0.44°（14 个模型成员）和 0.22°（15 个模型成员），用于评估这些集合的再生能力（1 ) 美国西部几个重要水文地区的年度和半年度降水循环，以及 (2) 降水量和时间的局部季节性。与之前有组织的降尺度项目类似，NA-CORDEX 集合继续过度模拟山区的冬季降水。然而，两个 NA-CORDEX 集合都显示出与北美季风相关的西南沙漠夏季降水的改善。此外，在整个 0. 在许多更高分辨率的 0.22° 合奏成员中改进了 44° 合奏；与增强的中尺度过程和改进的热力学相关，驱动模型的对流参数化。与基于观测的网格降水相比，捕捉年和半年变化模式的能力在更高分辨率的 0.22° 集合中得到了显着提高。分辨率更高的成员再现了更一致的空间方差模式，其幅度误差更低——尤其是在冬夏和春秋季节性方面。尽管更高分辨率的集成表现出更好的性能指标聚类，但在半年周期中发现模型性能有很大差异。最终，NA-CORDEX 模型的性能仍然与大尺度母模型的保真度相关，尽管我们发现性能也与使用哪个区域气候模型密切相关，而未来趋势更多地与母 GCM 相关。此外，通过更精细的域网格间距增加的价值是显而易见的。最后，分析了降水模式的未来预测，以检查降水强度和变异模式的季节性趋势。