基于数字全息的云中冰晶微物理参数观测方法

Observation Method of Microphysical Parameters of Ice Crystals in Cloud Based on Digital Holography

冰晶是云滴谱的重要组成部分,其对全球辐射收支平衡、全球气候变化、水文循环、人工影响天气作业等具有重要的影响。目前2~100μm的冰晶与液滴混合相态难以区分,且存在难以提供冰晶微物理参数的瓶颈问题。针对这两个问题,本文基于数字全息理论,利用全域数字图像融合方法、局部亮度梯度方差法和旋转卡壳法,结合固液相粒子圆度概念,实现了云中液滴和冰晶的混合相态识别,在特定圆度阈值下,液滴和冰晶的识别率大于93%;再结合光学图像识别技术,获得冰晶粒子的面积、周长、凸包和最小外接矩形数据;最终利用上述数据获得了冰晶微物理参数。通过在低温云室中的观测实验,获取了板状、枝状和六角冰晶的微物理参数,该方法解决了冰晶观测中的瓶颈问题。此外,通过冰晶采样间隔时间和不同时刻的质心三维坐标和等效直径,还可获得冰晶粒子的三维运动速度与轨迹。该方法对提升数值天气预报精确度,以及人工影响天气作业具有重大意义。

Objective As an important component of the cloud droplet spectrum,ice crystals exert important effects on global radiation budget balance,global climate change,hydrological cycle,and weather modification.Due to the limitation of observation means,the understanding about the microphysical characteristics of ice crystal particles is not perfect till now.At present,it is difficult to identify the mixed phase of 2-100μm ice crystals from droplets,and there is a bottleneck to provide microphysical parameters of ice crystals.The lack of sufficient ice crystal detection data can cause large differences in the mean value of ice water paths in different models,especially in mixed-phase clouds.Therefore,we study the microphysical parameters of ice crystals.Methods For these two problems,based on the digital holography theory,we propose to employ the global digital image fusion method,the local Tenengrad variance method,and the rotating caliper method for identifying mixed phase states of droplets and ice crystals in the cloud by combining the roundness concept of solid and liquid particles.Combined with optical image recognition technology,we obtain the area,perimeter,convex hull,and minimum enclosing rectangle data of ice crystals.Finally,the microphysical parameters of ice crystals are acquired by adopting the above data.The microphysical parameters of plate,dendritic and hexagonal ice crystals are obtained by observation experiments in lowtemperature cloud chambers.Results and Discussions Some obvious conclusions can be obtained by adopting the proposed method.1)By leveraging the rotating caliper method and the specific geometric parameter roundness F,the mixed phase identification of droplets and ice crystals in clouds is realized.Under the specific roundness threshold,the recognition rate of droplets and ice crystals is greater than 93%(Fig.8).2)Combined with optical image recognition technology,morphological data(the area,perimeter,convex hull,and minimum enclosing rectangle data)of ice crystal particles are obtained(Table 1).3)The microphysical parameters of ice crystals are acquired by morphological data of ice crystals(Table 2).4)When the digital hologram of ice crystal particles is obtained with the frequency of 30 frame/s,the three-dimensional kinetic velocity of ice crystal particles can also be acquired by this method(Fig.11).Conclusions An ice crystal detection method based on a pulse-modulated laser,high-resolution optical system,and coaxial digital holography(DH)is presented.The local Tenengrad variance method,the rotating caliper method,and the specific geometric parameter roundness F are adopted for phase state identification of particles.To verify the validity of the detection method and identification algorithm,we observe the mixed particles of droplets and ice crystals in the cloud chamber.Additionally,the three-dimensional motion velocity and trajectory of ice crystals can be obtained from the sampling interval time and the three-dimensional coordinates and equivalent diameters of the center of mass at different time.This method solves the bottleneck problem that the existing observation technology cannot identify the phase states and obtain the microphysical parameters of ice crystals.Meanwhile,the method is of significance to improve the accuracy of numerical weather prediction and weather modification operation.