Cloud droplet sedimentation, entrainment efficiency, and subtropical stratocumulus albedo
Cloud droplet sedimentation, entrainment efficiency, and subtropical stratocumulus albedo
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DOI：
10.1029/2006gl027648
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发表时间：
2007-02-09
影响因子：
5.2
通讯作者：
Uchida, J.
中科院分区：
文献类型：
--
作者：
Bretherton, C. S.;Blossey, P. N.;Uchida, J.
用大涡模拟（LES）和体微物理方法研究了云滴沉降对夜间无毛毛雨层积云卷吸率和液态水路径的影响。与阿克曼等人（2004）先前的研究一致，发现沉降降低了卷吸率，从而增加了液态水路径。他们认为这是由于边界层湍流的减少。我们的模拟结果显示并非如此。相反，沉降通过从夹带区去除液态水来减少夹带。这抑制了两种促进夹带空气下沉到云层中的机制--蒸发冷却和长波辐射冷却。一项敏感性研究表明，辐射效应不如减少蒸发重要。本文提出了一种可能的混合层模式中沉积作用对卷吸率影响的参数化方法，并进行了检验。由于云滴沉降速率与云滴（可能还有气溶胶）浓度呈负相关，并且几乎不毛毛雨的海洋层积云广泛存在，因此在气溶胶间接效应的气候模式模拟中应考虑沉降对层积云卷吸效率的影响。
The effect of cloud droplet sedimentation on the entrainment rate and liquid water path of a nocturnal nondrizzling stratocumulus layer is examined using large-eddy simulations (LES) with bulk microphysics. In agreement with a prior study by Ackerman et al. (2004), sedimentation is found to decrease entrainment rate and thereby increase liquid water path. They suggested this is due to reduction of boundary-layer turbulence. Our simulations suggest otherwise. Instead, sedimentation reduces entrainment by removing liquid water from the entrainment zone. This inhibits two mechanisms that promote the sinking of entrained air into the cloud layer entrainment-induced evaporative cooling and longwave radiative cooling. A sensitivity study shows that the radiative effect is less important than the reduced evaporation. A possible parameterization of the effect of sedimentation on entrainment rate in a mixed layer model is proposed and tested. Since the droplet sedimentation rate is inversely related to cloud droplet (and presumably aerosol) concentration and nearly nondrizzling marine stratocumulus are widespread, sedimentation impacts on stratocumulus entrainment efficiency should be considered in climate model simulations of the aerosol indirect effect.