CAM6 simulation of mean and extreme precipitation over Asia: Sensitivity to upgraded physical parameterizations and higher horizontal resolution
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Abstract. The Community Atmosphere Model version 6 (CAM6) released in 2018, as part of the Community Earth System Model version 2 (CESM2) modeling framework, is a major upgrade over the previous CAM5 that has been used in numerous global and regional climate studies in the past six years. Since CESM2/CAM6 will participate in the upcoming Coupled Model Intercomparison Project phase 6 (CMIP6) and is likely to be adopted in many future studies, its simulation fidelity needs to be thoroughly examined. Here we evaluate the performance of a developmental version of the Community Atmosphere Model with parameterizations that will be used in CMIP6 (CAM6) with the default 1 horizontal resolution (0.91.25, CAM6-1) and a higher resolution simulation (approximately 0.25, CAM6-0.25), against various precipitation observational datasets over Asia. The CAM6 performance is also compared with CAM5 with the default 1 horizontal resolution (CAM5-1). With the prognostic treatment of precipitation processes (which is missing in CAM5) and the new microphysics module, CAM6 is able to better simulate climatological mean and extreme precipitation over Asia, to better capture the heaviest precipitation events, to reproduce the diurnal cycle of precipitation rates over most of Asia, and to better simulate the probability density distributions of daily precipitation over Tibet, Korea, Japan and Northern China. Higher horizontal resolution in CAM6 improves simulations of mean and extreme precipitation over mountainous Sichuan and Northern China, but the performance degrades over the Maritime continent. Further diagnosis on moisture budget suggests that the physical processes leading to model improvement are different over different regions. Both upgraded physical parameterizations and higher horizontal resolution affect the precipitation response to internal variability of ocean and atmosphere (e.g. Asian monsoon index, ENSO, PDO), but the effects vary across different regions. Higher horizontal resolution degrades the model performance in simulating precipitation variability associated with the East Asian summer monsoon in the middle and lower reaches of the Yangtze River in China. The precipitation variability associated with ENSO gets better with upgraded physical parameterizations and higher horizontal resolution. Higher horizontal resolution, however, induces an opposite response to PDO in CAM6 over Southern China.
