The Turbulent Structure of the Arctic Summer Boundary Layer During The Arctic Summer Cloud-Ocean Study
The Turbulent Structure of the Arctic Summer Boundary Layer During The Arctic Summer Cloud-Ocean Study
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DOI：
10.1002/2017jd027234
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发表时间：
2017-09-27
影响因子：
4.4
通讯作者：
Brooks, Barbara J.
中科院分区：
文献类型：
--
作者：
Brooks, Ian M.;Tjernstrom, Michael;Brooks, Barbara J.
北冰洋大部分被冰覆盖，主要是低层液态云或混合相云。层积云内的湍流主要是由云顶冷却引起的对流不稳定所驱动的。利用一套现场和遥感仪器，我们描述了北极平流层积云中的湍流混合，并首次在时间(10min)和高度(10m)的较高分辨率下估计了梯度Richardson数的廓线。结果发现，如预期的那样，混合既发生在云内，也通过地面附近的风切变不稳定发生。大约75%的时间，这两层在100-200米的高度被稳定的层结逆温层分开。例外情况是云层基数较低，使得云层驱动的湍流能够到达地表。结果表明，北冰洋表层和云层之间的湍流耦合是零星的或间歇性的。简而言之，夏季北冰洋低层大气通常由两个混合良好的层组成--一个表层混合层和一个云混合层--这两个层在海平面以上约100-300米处被一个弱的解耦层隔开。在这些情况下，云不能与曲面直接交互。大型预报和气候模型始终无法再现这种观测到的结构，因此可能无法正确地再现云的性质以及作为太阳和红外辐射被表面吸收或从表面发射的能量。这导致在再现海冰浓度随时间变化的过程中出现误差。在这里，我们使用在北极中部进行的测量来研究控制云是否耦合到地表的过程。地面风的影响并不是一个控制因素。云混合层的深度很关键，但影响它的多个过程不能用这里提供的数据分开。然而，通过红外辐射在云顶冷却是关键，云进入逆温的延伸也是关键--这是北极云的一个独特特征。
The mostly ice covered Arctic Ocean is dominated by low-level liquid-or mixed-phase clouds. Turbulence within stratocumulus is primarily driven by cloud top cooling that induces convective instability. Using a suite of in situ and remote sensing instruments we characterize turbulent mixing in Arctic stratocumulus, and for the first time we estimate profiles of the gradient Richardson number at relatively high resolution in both time (10 min) and altitude (10 m). It is found that the mixing occurs both within the cloud, as expected, and by wind shear instability near the surface. About 75% of the time these two layers are separated by a stably stratified inversion at 100-200 m altitude. Exceptions are associated with low cloud bases that allow the cloud-driven turbulence to reach the surface. The results imply that turbulent coupling between the surface and the cloud is sporadic or intermittent.Plain Language Summary The lower atmosphere over the summertime Arctic Ocean often consists of two well-mixed layers-a surface mixed layer and a cloud mixed layer-that are separated by a weak decoupling layer at about 100 to 300 m above the surface. In these cases, the cloud cannot interact directly with the surface. Large-scale forecast and climate models consistently fail to reproduce this observed structure and may thus fail to correctly reproduce the cloud properties and the amount of energy absorbed by or emitted from the surface as solar and infrared radiation. This contributes to errors in reproducing changes in sea ice concentration over time. Here we use measurements made in the central Arctic to study the processes controlling whether or not the cloud is coupled to the surface. The effect of wind at the surface is found not to be a controlling factor. The depth of the cloud mixed layer is critical, but the multiple processes influencing it cannot be separated using the data available here. However, cooling at cloud top by infrared radiation is key, as is the extension of cloud into the temperature inversion-a unique feature of Arctic clouds.