The Arctic climate is changing rapidly, warming at about twice the rate of the planet. Global climate models (GCMs) are invaluable tools both for understanding the drivers of these changes and predicting future Arctic climate evolution. While GCMs are continually improving, there remain difficulties in representing cloud processes which occur on scales smaller than GCM resolution. Since clouds influence the Arctic energy and water cycles, their accurate representation in models is critical for robust future projections. In this work, we examine the representation of Arctic clouds and precipitation in the Community Earth System Model (CESM) with the Community Atmosphere Model (CAM), comparing the newly released version (CESM2 with CAM6) with its predecessor (CESM1 with CAM5). To isolate changes in the Arctic mean state, we compare preindustrial control runs. Arctic cloud ice has decreased slightly, while cloud water has increased dramatically in CESM2. Annual mean liquid‐containing cloud (LCC) frequency has increased from 19% in CESM1 to 51% in CESM2. Since LCCs strongly modulate downwelling radiation at the surface, their increase has led to an increase in mean downwelling longwave (+22 W m⁻²) and corresponding decrease in downwelling shortwave (−23 W m⁻²) radiation. The mean Arctic surface temperature increased from 257 K in CESM1 to 260 K in CESM2, with the largest seasonal difference in winter (+6 K). Annual average snowfall has decreased slightly (−1 mm month⁻¹), while rainfall has increased (+5 mm month⁻¹).

中文翻译：

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社区地球系统模型版本 2 中的北极云和降水


北极气候正在迅速变化，变暖速度约为地球变暖速度的两倍。全球气候模型（GCM）是了解这些变化的驱动因素和预测未来北极气候演变的宝贵工具。虽然 GCM 不断改进，但在表示小于 GCM 分辨率的尺度上发生的云过程仍然存在困难。由于云会影响北极的能源和水循环，因此它们在模型中的准确表示对于未来稳健的预测至关重要。在这项工作中，我们使用社区大气模型 (CAM) 检查了社区地球系统模型 (CESM) 中北极云和降水的表示，并将新发布的版本（CESM2 和 CAM6）与其前身（CESM1 和 CAM5）进行了比较。为了隔离北极平均状态的变化，我们比较了工业化前的控制运行。在CESM2中，北极云冰略有减少，而云水却急剧增加。年平均含液体云 (LCC) 频率从 CESM1 的 19% 增加到 CESM2 的 51%。由于LCC强烈调制地表的下降流辐射，它们的增加导致平均下降长波（+22 W m ^-2 ）增加，下降流短波（ − 23 W m ^-2 ）辐射相应减少。北极平均表面温度从CESM1的257 K增加到CESM2的260 K，其中冬季季节差异最大（+6 K）。年平均降雪量略有减少（ -1毫米月^-1 ），而降雨量增加（+5 毫米月^-1 ）。