AI 中文总结
该研究针对北极边界层的双模态分布，采用漂移-扩散框架分析15个冬季观测数据，发现其源于状态依赖随机波动而非双稳态，揭示了晴空-多云过渡区变异性增强的机制。
AI 中文摘要
冬季北极边界层具有两种不同状态：辐射清晰（无云）和辐射不透明（有云），这在观测中表现为双模态分布。以往研究假设这些状态与系统动力学的潜在双稳态有关。我们应用基于Kramers-Moyal展开前两项的漂移-扩散框架，对挪威Ny-Alesund的15个冬季观测数据进行分析。我们的模型捕捉到了双模态；然而，我们发现确定性漂移只有一个稳定点，这对确定性动力学双稳态的存在提出了质疑。相反，观测到的状态可归因于状态依赖的随机波动，其最大振幅接近晴空-多云过渡区。稳定点的位置取决于气象条件。不过，控制天气尺度变异性并不能消除扩散的状态依赖性。多云状态与西南风、高比湿和上升运动相关，后两者有利于云的形成；而无云状态则出现在较干燥的条件和东北风下。通过一个说明性例子，我们提出了对统计分析的可能过程解释：一旦大气湿度达到饱和，云形成的非线性起始可将湿度的状态独立波动映射为云量的状态依赖波动。
英文摘要
The wintertime Arctic boundary layer is characterized by two distinct states, radiatively clear (cloud-free) and radiatively opaque (cloudy), which is reflected in observations as a bimodal distribution. Previous studies hypothesized that these regimes are related to an underlying bistability of the system's dynamics. We apply a drift-diffusion framework based on the two leading terms of the Kramers-Moyal expansion to observations from 15 winter periods in Ny-Alesund, Norway. Our model captures the bimodality; we find a single stable point for the deterministic drift, however, which questions the existence of bistable deterministic dynamics. Instead, the observed regimes can be attributed to state-dependent stochastic fluctuations with maximum amplitude close to the clear-cloudy transition. The position of the stable point depends on the meteorological conditions. Controlling for synoptic variability does not remove the state-dependence of diffusion, however. The cloudy regime is associated with southwesterly winds, high specific humidities, and uplifting motion, the latter two being conducive for cloud formation, while the cloud-free regime occurs under drier conditions and northeasterly winds. Through an illustrative example, we suggest a possible process-based interpretation of our statistical analysis. The nonlinear onset of cloud formation, once atmospheric humidity reaches saturation, could map state-independent fluctuations in humidity to state-dependent fluctuations in cloudiness.
