The surface radiative imbalance has large impacts on the long-term trends and year-to-year variability of Arctic sea ice. Clouds are believed to be a key factor in regulating this radiative imbalance, whose underlying processes and mechanisms, however, are not well understood. Compared with observations, the Community Earth System Model version 1 (CESM1) is known to underestimate Arctic cloud liquid water. Here, the following hypothesis is proposed and tested: this underestimation is caused by an overactive Wegener–Bergeron–Findeisen (WBF) process in model as too many supercooled liquid droplets are scavenged by ice crystals via deposition. In this study, the efficiency of the WBF process in CESM1 was reduced to investigate the Arctic climate response, and differentiate the responses induced by atmosphere–ocean–sea ice coupling and global warming. By weakening the WBF process, CESM1 simulated liquid cloud fractions increased, especially in winter and spring. The cloud response resulted in increased downwelling longwave flux and decreased shortwave flux at the surface. Arctic clouds and radiation in simulations with reduced WBF efficiency show a better agreement with satellite retrievals. In addition, both coupling and global warming amplify the cloud response to a less efficient WBF process, due to increased relative humidity and enhanced evaporation, respectively. As a response, the sea ice tends to melt over the North Atlantic Ocean, most likely caused by a positive feedback process between clouds, radiation and sea ice during non-summer months. These results improve our understanding of large-scale effects of the WBF process and the role of cloud liquid water in the Arctic climate system.

表面辐射失衡对北极海冰的长期趋势和逐年变化具有重大影响。人们认为云是调节这种辐射失衡的关键因素，但是其潜在的过程和机制尚未得到很好的理解。与观察结果相比，已知社区地球系统模型版本1（CESM1）低估了北极云液态水。在这里，提出并检验了以下假设：这种低估是由于模型中过度活跃的Wegener-Bergeron-Findeisen（WBF）过程引起的，因为过多的过冷液滴被冰晶通过沉积清除。在这项研究中，降低了CESM1的WBF过程的效率以调查北极的气候响应，并区分了大气-海洋-海冰耦合和全球变暖引起的响应。通过减弱WBF过程，CESM1模拟的液态云分数增加，尤其是在冬季和春季。云的响应导致上升的下波长波通量和地表的短波通量减少。WBF效率降低的模拟中的北极云和辐射与卫星检索显示出更好的一致性。另外，由于相对湿度增加和蒸发增加，耦合和全球变暖都放大了对效率较低的WBF过程的云响应。作为响应，海冰倾向于在北大西洋上空融化，这很可能是由于非夏季月份云，辐射和海冰之间的正反馈过程所致。