海洋层积云中的云滴谱宽度及其影响因子

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。

Spectral Width of Cloud Droplet Spectra and Its Impact Factors in Marine Stratocumulus

Spectral width of cloud droplet spectra is crucial to the parameterization of cloud optical depth, assessment of indirect aerosol effect, and the formation of precipitation in numerical simulation. Based on aircraft observations of cloud microphysics in stratocumulus on 19 July 2008, during the Physics of Stratocumulus Top (POST) field campaign, the vertical distribution of microphysical properties and cloud droplet spectra were analyzed, as were the cloud microphysical processes. Results showed that spectral width of cloud droplet spectra was basically larger near the cloud base, which is due to aerosol nucleation. Spectral width decreased as height increased in the middle layer of the cloud, which was caused by condensational growth. The increase in spectral width near the cloud top was caused by entrainment-mixing processes. The increase in vertical velocity in the adiabatic cloud increased cloud droplet number concentration by promoting the activation of cloud condensation nuclei (CCN), increased cloud droplet size, and led to the decrease in spectral width by promoting condensational growth. This ultimately led to the negative correlation between spectral width and cloud droplet number concentration as well as cloud droplet size. The decrease in vertical velocity caused by entrainment-mixing processes in cloud holes decreased cloud droplet number concentration and cloud droplet size and increased spectral width by promoting the evaporation of cloud droplets. This was enhanced by the decrease in the 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.