CAM6 simulation of mean and extreme precipitation over Asia: Sensitivity to upgraded physical parameterizations and higher horizontal resolution

The Community Atmosphere Model version 6 (CAM6), released in 2018 as part of the Community Earth System Model version 2 (CESM2), is a major upgrade over the previous CAM5 that has been used in numerous global and regional climate studies. 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 version 6 (CAM6<span classCombining double low line"inline-formula">α</span>), with a default 1<span classCombining double low line"inline-formula">ĝ</span> horizontal resolution (<span classCombining double low line"inline-formula"><math xmlnsCombining double low line"http://www.w3.org/1998/Math/MathML" idCombining double low line"M3" displayCombining double low line"inline" overflowCombining double low line"scroll" dspmathCombining double low line"mathml"><mrow><mn mathvariantCombining double low line"normal">0.9</mn><msup><mi/><mo>ĝ</mo></msup><mo>×</mo><mn mathvariantCombining double low line"normal">1.25</mn><msup><mi/><mo>ĝ</mo></msup></mrow></math><span><svg:svg xmlns:svgCombining double low line"http://www.w3.org/2000/svg" widthCombining double low line"58pt" heightCombining double low line"11pt" classCombining double low line"svg-formula" dspmathCombining double low line"mathimg" md5hashCombining double low line"b0eef53033dbc8d59ab0cb64dd5de83b"><svg:image xmlns:xlinkCombining double low line"http://www.w3.org/1999/xlink" xlink:hrefCombining double low line"gmd-12-3773-2019-ie00001.svg" widthCombining double low line"58pt" heightCombining double low line"11pt" srcCombining double low line"gmd-12-3773-2019-ie00001.png"/></svg:svg></span></span>, CAM6<span classCombining double low line"inline-formula">α</span>-1<span classCombining double low line"inline-formula">ĝ</span>) and a high-resolution configuration (approximately 0.25<span classCombining double low line"inline-formula">ĝ</span>, CAM6<span classCombining double low line"inline-formula">α</span>-0.25<span classCombining double low line"inline-formula">ĝ</span>), against various observational and reanalysis datasets of precipitation over Asia. CAM6<span classCombining double low line"inline-formula">α</span> performance is compared with CAM5 at default 1<span classCombining double low line"inline-formula">ĝ</span> horizontal resolution (CAM5-1<span classCombining double low line"inline-formula">ĝ</span>) and a high-resolution configuration at 0.25<span classCombining double low line"inline-formula">ĝ</span> (CAM5-0.25<span classCombining double low line"inline-formula">ĝ</span>). With the prognostic treatment of precipitation processes and the new microphysics module, CAM6<span classCombining double low line"inline-formula">α</span> is able to better simulate climatological mean and extreme precipitation over Asia, better capture the heaviest precipitation events, better reproduce the diurnal cycle of precipitation rates over most of Asia, and better simulate the probability density distributions of daily precipitation over Tibet, Korea, Japan and northern China. Higher horizontal resolution in CAM6<span classCombining double low line"inline-formula">α</span> improves the simulation of mean and extreme precipitation over northern China, but the performance degrades over the Maritime Continent. Moisture budget diagnosis suggests that the physical processes leading to model improvement are different over different regions. Both upgraded physical parameterizations and higher horizontal resolution affect the simulated precipitation response to the internal variability of the climate system (e.g., Asian monsoon variability, El Niño-Southern Oscillation - ENSO, Pacific Decadal Oscillation - PDO), but the effects vary across different regions. For example, higher horizontal resolution degrades the model performance in simulating precipitation variability over southern China associated with the East Asian summer monsoon. In contrast, precipitation variability associated with ENSO improves with upgraded physical parameterizations and higher horizontal resolution. CAM6<span classCombining double low line"inline-formula">α</span>-0.25<span classCombining double low line"inline-formula">ĝ</span> and CAM6<span classCombining double low line"inline-formula">α</span>-1<span classCombining double low line"inline-formula">ĝ</span> show an opposite response to the PDO over southern China. Basically, the response to increases in horizontal resolution is dependent on the CAM version.