Characteristic nature of vertical motions observed in Arctic mixed-phase stratocumulus
Characteristic nature of vertical motions observed in Arctic mixed-phase stratocumulus
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DOI：
10.5194/acp-14-3461-2014
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发表时间：
2013-11
影响因子：
6.3
通讯作者：
J. Sedlar;M. Shupe
中科院分区：
文献类型：
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作者：
J. Sedlar;M. Shupe
摘要。在北冰洋上空，人们对混合相层积云内由云产生的浮力翻转垂直运动知之甚少。这种运动的特征对于理解与垂直运动相关的绝热过程、云层的寿命及其微观和宏观物理特征具有重要意义。在这项研究中，我们在2008年8月长达一周的持续层积云期间，利用一套地面遥感器在高北极海冰上获得云内垂直运动特征。云内垂直速度偏度和方差曲线与低纬度层积云内的观测结果明显不同，这表明这些北极混合相云与大气热力学（云顶延伸到稳定的逆温基础之上）的相互作用不同，地表和云之间的耦合状态也不同。我们发现了云在云底下产生垂直混合的证据，无论地面-云耦合状态如何，尽管解耦的地面-云状态发生得最频繁。详细的案例研究进行了检查，重点放在云层内的三个层次，其中小波和功率谱分析被应用于表征与云产生的垂直运动相关的主要时间和水平尺度。一般来说，我们发现在云内的垂直水平和整个云层深度之间的垂直运动信号呈正相关。相干性取决于其他非云控制的因素，如较大的中尺度天气通道和上方一个或多个云层对低层积云的辐射屏蔽。尽管垂直速度在云中具有一致性，但相对于云中部和云底部，云顶附近的速度变化总是较弱。结合偏度、方差和热力学剖面特征，我们观察到云顶附近的垂直运动与云层内较低的垂直运动表现不同。光谱分析表明，在解耦情况下，相对于耦合情况，峰值云产生的w方差时间尺度仅略微减慢；当从耦合过渡到解耦时，水平波长仅略有增加。尺度上的相似性表明，也许所有情况下的主导强迫都是由云层产生的，而不是地表强迫在云内垂直速度变化的时间和空间尺度上具有特征。这表明，北极混合相云的弹性本质在具有北极特有的热力学机制时将持续存在。
Abstract. Over the Arctic Ocean, little is known on cloud-generated buoyant overturning vertical motions within mixed-phase stratocumulus clouds. Characteristics of such motions are important for understanding the diabatic processes associated with the vertical motions, the lifetime of the cloud layer and its micro- and macrophysical characteristics. In this study, we exploit a suite of surface-based remote sensors over the high-Arctic sea ice during a weeklong period of persistent stratocumulus in August 2008 to derive the in-cloud vertical motion characteristics. In-cloud vertical velocity skewness and variance profiles are found to be strikingly different from observations within lower-latitude stratocumulus, suggesting these Arctic mixed-phase clouds interact differently with the atmospheric thermodynamics (cloud tops extending above a stable temperature inversion base) and with a different coupling state between surface and cloud. We find evidence of cloud-generated vertical mixing below cloud base, regardless of surface–cloud coupling state, although a decoupled surface–cloud state occurred most frequently. Detailed case studies are examined, focusing on three levels within the cloud layer, where wavelet and power spectral analyses are applied to characterize the dominant temporal and horizontal scales associated with cloud-generated vertical motions. In general, we find a positively correlated vertical motion signal amongst vertical levels within the cloud and across the full cloud layer depth. The coherency is dependent upon other non-cloud controlled factors, such as larger, mesoscale weather passages and radiative shielding of low-level stratocumulus by one or more cloud layers above. Despite the coherency in vertical velocity across the cloud, the velocity variances were always weaker near cloud top, relative to cloud middle and base. Taken in combination with the skewness, variance and thermodynamic profile characteristics, we observe vertical motions near cloud top that behave differently than those from lower within the cloud layer. Spectral analysis indicates peak cloud-generated w variance timescales slowed only modestly during decoupled cases relative to coupled; horizontal wavelengths only slightly increased when transitioning from coupling to decoupling. The similarities in scales suggests that perhaps the dominant forcing for all cases is generated from the cloud layer, and it is not the surface forcing that characterizes the time- and space scales of in-cloud vertical velocity variance. This points toward the resilient nature of Arctic mixed-phase clouds to persist when characterized by thermodynamic regimes unique to the Arctic.