CMIP6模式水平分辨率对模拟我国西南地区夏季极端降水的影响评估

An Evaluation for Impacts of the Horizontal Resolution of CMIP6 Models on Simulating Extreme Summer Rainfall over Southwest China

我国西南地区的地形地貌非常复杂,当前的气候模式对该地区降水状况特别是极端降水的模拟技巧是比较低的。本文基于台站和卫星观测的逐日降水资料以及欧洲中心第五代再分析(ERA5)降水资料,通过与CMIP6高分辨率模式比较计划(HighResMIP)中的12个模式高、低分辨率模拟结果的对比分析,评估了当前气候模式对西南地区夏季降水的模拟性能特别是模式水平分辨率对极端降水模拟的影响。结果表明:(1)在夏季降水气候态方面,各HighResMIP模式模拟与台站观测之间的空间相关系数均超过0.75,总体性能较CMIP5有明显提升,但仍有超过半数模式明显低估了四川盆地降水。模式分辨率提高使横断山脉地形陡峭区的降水空间分布和强度更接近观测和ERA5资料,但对四川盆地降水的改进效果不佳。(2)在夏季极端降水方面,HighResMIP模式对极端降水频率和强度模拟差异较大。CNRM-CM6、FGOALS-f3、GFDL和HadGEM-GC31等4个模式对极端降水的各项指标模拟总体较好,但受气候态模拟偏差影响,前三者模拟的极端降水在四川盆地偏弱,而HadGEM-GC31在广西明显偏强。ECMWF-IFS、EC-Earth3P、IPSL-CM6A、MPI-ESM1-2和MRI-AGCM3-2等5个模式中极端降水发生频率明显偏低。提高分辨率可以一定程度改进降水强度的模拟,主要体现在提高地形陡峭区的降水强度,但对地形平坦区如四川盆地降水强度改进不大。

Due to complex topography in Southwest China(SWC),state-of-the-art climate models cannot capture sufficiently the distribution and intensity of summer precipitation,especially extreme rainfall(ER)over SWC. Thus,this study is to evaluate how well the current climate models could reproduce the climate mean summer precipitation and to what extend the horizontal resolutions might impact the ER simulations over SWC,based on daily rain-gauge station-observed,satellite-observed and ERA5-reanalysed rainfall datasets and 12 models available from CMIP6 High-Resolution Model Inter-comparison Project(HighResMIP). Each HighResMIP model contains one high-resolution and one low-resolution simulation with the same suite of physical processes and external forcing. Results show that almost all models can reproduce the climate-mean state of summer rainfall over SWC,with an area correlation coefficient(ACC)greater than 0. 75 between the rain-gauge observed rainfall and simulated rainfall by each model. Over,the performance of CMIP6 HighResMIP models is better than that of CMIP5 models,but over half of the CMIP6 HighResMIP models still underestimate the summer rain amount over the Sichuan Basin. As the model resolution increased,the intensity and spatial pattern of the simulated summer rainfall over the Hengduan mountains are much closer to the observational dataset,especially to the ERA5 reanalysis. However,the underestimating biases over Sichuan Basin are not improved obviously with a higher horizontal resolution. In terms of ER,large spreads exist in the ER intensity and occurrence frequency over SWC among CMIP6 HighResMIP models. The four models,including CNRM-CM6、FGOALS-f3、GFDL-CM4 and HadGEM-GC31,exhibit better performances in capturing ER days and percentage. Even so,the first three of the above four models underestimate the ER days over SWC,but HadGEM-GC31 overestimates ER intensity over Guangxi Province. In contrast,the ER frequency is much lower than that of observation in the models ECMWF-IFS,EC-Earth3 P,IPSL-CM6 A,MPI-ESM1-2 and MRI-AGCM3-2. The higher resolution simulations can improve the simulation in the rainfall intensity to a certain degree,manifesting mainly in enhancing the rainfall intensity over the mountainous region rather than the flat-terrain area such as the Sichuan Basin.