Diurnal cycle of the semi-direct effect from a persistent absorbing aerosol layer over marine stratocumulus in large-eddy simulations
Diurnal cycle of the semi-direct effect from a persistent absorbing aerosol layer over marine stratocumulus in large-eddy simulations
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DOI：
10.5194/acp-20-1317-2020
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发表时间：
2020-02-05
影响因子：
6.3
通讯作者：
Hill, Adrian A.
中科院分区：
文献类型：
--
作者：
Herbert, Ross J.;Bellouin, Nicolas;Hill, Adrian A.
海洋层积云对升高的吸收性气溶胶层的快速调整或半直接影响可能会增强或减弱气溶胶辐射相互作用的辐射效应。在这里，我们使用大涡模拟层积云的敏感性，位于逆温层上方的吸收气溶胶层的属性，重点是辐射热扰动的位置，时间和强度。日平均半直接效应的符号取决于气溶胶层的性质和持续时间、边界层的性质和模式设置。我们的研究结果表明，每日平均半直接影响比以前评估的更难以捉摸。我们发现，日平均半直接效应是由云和吸收气溶胶层之间的距离。在最初的24小时内，半直接效应是积极的，但保持在2 W m（-2）以下，除非气溶胶层直接位于云的上方。对于较长的持续时间，每日平均半直接效应始终是负的，但减弱了30%，60%和95%时，云和气溶胶层之间的距离分别为100，250和500米。云的响应和半直接效应增加的吸收气溶胶层较薄和较密。云响应的日变化相当大，这意味着瞬时半直接效应不能代表日平均值，观测研究可能会低估或高估半直接效应，这取决于一天中的观测时间。云的响应对边界层的混合状态特别敏感：混合良好的边界层通常会导致每日平均半直接效应为负，而混合不良的边界层则会导致每日平均半直接效应为正。边界层和模式设置的特性，特别是海面温度、降水和从自由对流层夹带的空气的特性，也会影响半直接效应的大小和调整的时间尺度。这些结果表明，由粗分辨率模式模拟的半直接效应可能是错误的，因为云的响应是敏感的小尺度过程，特别是源和汇的浮力。
The rapid adjustment, or semi-direct effect, of marine stratocumulus clouds to elevated layers of absorbing aerosols may enhance or dampen the radiative effect of aerosolradiation interactions. Here we use large-eddy simulations to investigate the sensitivity of stratocumulus clouds to the properties of an absorbing aerosol layer located above the inversion layer, with a focus on the location, timing, and strength of the radiative heat perturbation. The sign of the daily mean semi-direct effect depends on the properties and duration of the aerosol layer, the properties of the boundary layer, and the model setup. Our results suggest that the daily mean semi-direct effect is more elusive than previously assessed. We find that the daily mean semi-direct effect is dominated by the distance between the cloud and absorbing aerosol layer. Within the first 24 h the semi-direct effect is positive but remains under 2 W m(-2) unless the aerosol layer is directly above the cloud. For longer durations, the daily mean semi-direct effect is consistently negative but weakens by 30 %, 60 %, and 95 % when the distance between the cloud and aerosol layer is 100, 250, and 500 m, respectively. Both the cloud response and semi-direct effect increase for thinner and denser layers of absorbing aerosol. Considerable diurnal variations in the cloud response mean that an instantaneous semi-direct effect is unrepresentative of the daily mean and that observational studies may underestimate or overestimate semi-direct effects depending on the observed time of day. The cloud response is particularly sensitive to the mixing state of the boundary layer: well-mixed boundary layers generally result in a negative daily mean semi-direct effect, and poorly mixed boundary layers result in a positive daily mean semi-direct effect. The properties of the boundary layer and model setup, particularly the sea surface temperature, precipitation, and properties of the air entrained from the free troposphere, also impact the magnitude of the semi-direct effect and the timescale of adjustment. These results suggest that the semi-direct effect simulated by coarse-resolution models may be erroneous because the cloud response is sensitive to small-scale processes, especially the sources and sinks of buoyancy.