Understanding the Varied Influence of Midlatitude Jet Position on Clouds and Cloud Radiative Effects in Observations and Global Climate Models
Understanding the Varied Influence of Midlatitude Jet Position on Clouds and Cloud Radiative Effects in Observations and Global Climate Models
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DOI：
10.1175/jcli-d-16-0295.1
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发表时间：
2016-12-01
影响因子：
4.9
通讯作者：
Medeiros, Brian
中科院分区：
文献类型：
--
作者：
Grise, Kevin M.;Medeiros, Brian
本研究探讨了中纬度云和云辐射效应（CRE）的变化，发生在北大西洋，北太平洋和南大洋的急流的纬向变化的动力学机制。当中纬度急流向极地移动时，热带气旋及其相关的向上垂直速度异常紧随其后。因此，极向喷流转移有助于在高顶风暴轨迹云和其相关的长波CRE极向转移。然而，当急流向极地移动时，向下的垂直速度异常增加赤道方向的急流，有助于增强边界层估计的逆温强度（EIS）和低云量的增加。由于短波CRE依赖于各层云对太阳辐射的反射，因此中纬度云的短波冷却效应随上升垂直速度异常和正EIS异常而增强。在中纬度海洋上，极向急流的移动有助于正的EIS异常，但向下的垂直速度异常，这两种影响抵消，短波CRE的净观测变化很小。全球气候模式通常捕获与中纬度急流移动相关的观测异常。然而，在短波CRE异常中有很大的模型间传播，一部分模型显示中纬度海洋上空有一个大的短波云辐射变暖，并有一个向极的急流。在这些模式中，中纬度短波CRE是敏感的垂直速度扰动，但所观察到的敏感性EIS扰动被低估。因此，这些模式可能会错误地估计未来中纬度地区的云反馈，在垂直速度和EIS的预测明显的变化。
This study examines the dynamical mechanisms responsible for changes in midlatitude clouds and cloud radiative effects (CRE) that occur in conjunction with meridional shifts in the jet streams over the North Atlantic, North Pacific, and Southern Oceans. When the midlatitude jet shifts poleward, extratropical cyclones and their associated upward vertical velocity anomalies closely follow. As a result, a poleward jet shift contributes to a poleward shift in high-topped storm-track clouds and their associated longwave CRE. However, when the jet shifts poleward, downward vertical velocity anomalies increase equatorward of the jet, contributing to an enhancement of the boundary layer estimated inversion strength (EIS) and an increase in low cloud amount there. Because shortwave CRE depends on the reflection of solar radiation by clouds in all layers, the shortwave cooling effects of midlatitude clouds increase with both upward vertical velocity anomalies and positive EIS anomalies. Over midlatitude oceans where a poleward jet shift contributes to positive EIS anomalies but downward vertical velocity anomalies, the two effects cancel, and net observed changes in shortwave CRE are small.Global climate models generally capture the observed anomalies associated with midlatitude jet shifts. However, there is large intermodel spread in the shortwave CRE anomalies, with a subset of models showing a large shortwave cloud radiative warming over midlatitude oceans with a poleward jet shift. In these models, midlatitude shortwave CRE is sensitive to vertical velocity perturbations, but the observed sensitivity to EIS perturbations is underestimated. Consequently, these models might incorrectly estimate future midlatitude cloud feedbacks in regions where appreciable changes in both vertical velocity and EIS are projected.