1.Nanjing University of Information Science & Technology;2.Department of Atmospheric Sciences, Yonsei University, Seoul, Korea
云滴谱宽度对模式中云的光学厚度的参数化、气溶胶间接效应的评估以及降水形成过程的研究至关重要。本文利用美国POST（Physics of Stratocumulus Top）项目2008年7月19日的飞机观测资料，分析了微物理量和云滴谱的垂直分布及微物理过程。结果表明，该云系云滴谱宽度在云底附近较大，这是由低层核化过程导致的；中层凝结增长过程使得云滴谱宽度随高度增加逐渐减小；云顶附近夹卷混合过程导致云滴谱宽度增大。绝热云中垂直速度的增大会促进云凝结核的活化使云滴数浓度增大，促进凝结增长使云滴尺度增大、云滴谱宽度减小，云滴谱宽度与云滴数浓度、云滴尺度呈现负相关关系；云洞中受夹卷混合过程影响，垂直速度减小，云滴蒸发，云滴数浓度和云滴尺度减小、云滴谱宽度增大，且该效应随绝热程度减小而增强。建议云滴谱宽度的参数化将垂直速度、云滴数浓度、云滴尺度和绝热程度等考虑在内。
The spectral width of cloud droplet spectra is crucial to the parameterization of the cloud optical depth, the assessment of the indirect aerosol effect and the formation of precipitation in numerical simulation. Based on the aircraft observations of cloud microphysics in stratocumulus on July 19, 2008 during the Physics of Stratocumulus Top (POST) field campaign, the vertical distribution of microphysical properties and cloud droplet spectra are analyzed, as well as the cloud microphysical processes. The results show that the spectral width of cloud droplet spectra is basically larger near cloud base which is caused by aerosol nucleation. The spectral width decreases as height increases in the middle layer of the cloud, which is caused by condensation growth. The increase of the spectral width near the cloud top is caused by the entrainment-mixing processes. The increase of vertical velocity in the adiabatic cloud increases the cloud droplet number concentration by promoting the activation of cloud condensation nuclei (CCN), and increases the cloud droplet size and leads to the decrease of the spectral width by promoting the condensation growth, and ultimately leads to the negative correlation between the spectral width and the cloud droplet number concentration as well as the cloud droplet size. The decrease of vertical velocity caused by entrainment-mixing processes in cloud holes decreases the cloud droplet number concentration and the cloud droplet size and increases the spectral width by promoting the evaporation of cloud droplets, which will be enhanced by the decrease of adiabatic fraction. Based on these observations, it is recommended that the parameterization of the spectral width of cloud droplet spectra takes into account vertical velocity, cloud droplet number concentration, cloud droplet size, adiabatic fraction, etc.