Zhao Bin, Liou Kuo-Nan, Gu Yu, Jiang Jonathan H, Li Qinbin, Fu Rong, Huang Lei, Liu Xiaohong, Shi Xiangjun, Su Hui, He Cenlin
Joint Institute for Regional Earth System Science and Engineering and Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California 90095, USA.
Jet propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA.
Atmos Chem Phys. 2018;18:1065-1078. doi: 10.5194/acp-18-1065-2018.
The interactions between aerosols and ice clouds represent one of the largest uncertainties in global radiative forcing from pre-industrial time to the present. In particular, the impact of aerosols on ice crystal effective radius ( ), which is a key parameter determining ice clouds' net radiative effect, is highly uncertain due to limited and conflicting observational evidence. Here we investigate the effects of aerosols on under different meteorological conditions using 9-year satellite observations. We find that the responses of to aerosol loadings are modulated by water vapor amount in conjunction with several other meteorological parameters. While there is a significant negative correlation between and aerosol loading in moist conditions, consistent with the "Twomey effect" for liquid clouds, a strong positive correlation between the two occurs in dry conditions. Simulations based on a cloud parcel model suggest that water vapor modulates the relative importance of different ice nucleation modes, leading to the opposite aerosol impacts between moist and dry conditions. When ice clouds are decomposed into those generated from deep convection and formed in situ, the water vapor modulation remains in effect for both ice cloud types, although the sensitivities of to aerosols differ noticeably between them due to distinct formation mechanisms. The water vapor modulation can largely explain the difference in the responses of to aerosol loadings in various seasons. A proper representation of the water vapor modulation is essential for an accurate estimate of aerosol-cloud radiative forcing produced by ice clouds.
从工业化前时代到现在，气溶胶与冰云之间的相互作用是全球辐射强迫中最大的不确定性之一。特别是，气溶胶对冰晶有效半径（这是决定冰云净辐射效应的关键参数）的影响，由于观测证据有限且相互矛盾，具有高度不确定性。在这里，我们利用9年的卫星观测数据，研究了不同气象条件下气溶胶对冰晶有效半径的影响。我们发现，冰晶有效半径对气溶胶负荷的响应受到水汽含量以及其他几个气象参数的调节。在潮湿条件下，冰晶有效半径与气溶胶负荷之间存在显著的负相关，这与液云的“Twomey效应”一致；而在干燥条件下，两者之间则呈现出强正相关。基于云块模型的模拟表明，水汽调节了不同冰核化模式的相对重要性，导致了潮湿和干燥条件下气溶胶的相反影响。当将冰云分解为由深对流产生的冰云和原位形成的冰云时，水汽调节对这两种冰云类型均有效，尽管由于形成机制不同，冰晶有效半径对气溶胶的敏感性在它们之间存在明显差异。水汽调节在很大程度上可以解释不同季节中冰晶有效半径对气溶胶负荷响应的差异。准确表示水汽调节对于精确估算冰云产生的气溶胶 - 云辐射强迫至关重要。
