| Airborne cloud particle observation is an important means of cloud physics research.The relevant airborne detection instruments can have been developed to observe and record the ice crystal particles in the cold cloud,and they can obtain the counting and shape information of the particles.However,the micro-physical processes and mechanisms in the cloud are unknown.Therefore,the research on the detection of small ice crystals in clouds(especially cold clouds)can be based on principles,methods,and instruments to establish an airborne cloud particle measurement system to solve the current problems of understanding ice crystals in cloud radiation and cloud physics.Whether the calibration of cloud particle detection system is accurate or not also determines the validity of the measurement data obtained and the applicability in the research of cloud precipitation physics and artificial weather.This paper will introduce a set of polarization-based airborne cloud particle measurement system,combined with T-matrix and discrete dipole approximation method for system calibration and actual measurement analysis of the simulation of non-spherical particle scattering characteristics.It can provide a reference for solving the current problems of identifying ice crystals in cloud physics and cloud radiation.T-matrix and discrete dipole approximation methods are used to simulate typical non-spherical particle scattering characteristics.T-Matrix method is used to calculate the scattering phase functions,the scattering cross section and the depolarization of non-spherical particles with different types and shapes,the variation law between shape factors and equivalent Mie scattering error of scattering phase functions is also discussed in this paper.Results show that the shape of particles has great effects on the space distribution of scattering energy.The scattering properties of particles are sensitive to aspect ratio.The aspect ratio can influence the oscillation frequencies and amplitudes of the scattering parameters.The equivalent Mie scattering error of particles with the equivalent radius is not a simple monotonic relationship.The depolarization values of elliptical,cylindrical andspherical particles are quite different.The depolarization value of spherical particles is close to zero,and it can distinguish between spherical and non-spherical particles.The phase state information of the cloud particles is helpful to improve the measurement accuracy of the scattering cloud particle detector.Based on polarization detection,a cloud particle measurement system that can be used to detect the size and phase state of cloud particles for particle diameters of less than 50 μm,was developed by detecting the energy of the forward scattering(4～14 degrees)and the depolarization of back-scattered light(176～146degrees).A fixed aperture of 30μm is used to detect the depth of field of the system to further determine the sensitive area.The system is calibrated using standard particles with fixed sizes of 2μm,20μm and 40μm,respectively,to obtain the response curve of the system to standard particles.The response curve of the system to the cloud particles was obtained by calculating the relationship between the standard particles and the cloud droplets.Finally,the liquid droplets and typical non-spherical particles were measured,and the results compared with a simulation.The comparison results indicated that the system could detect spherical and non-spherical cloud particles and could discriminate between non-spherical cloud particles and liquid droplets. |
