Research On The Numerical Computational Models And Application Of The Scattering Properties Of Nonspherical Atmospheric Particles

The scattering properties of atmospheric particles(such as aerosol particles and ice crystal,etc.)are the fundamental parameters for the simulation of atmospheric radiative transfer.Owning to its importance,their scattering parameters have been the essential datasets for the military target identification,climate numerical simulation and the remote sensing of atmosphere and ocean.In the actual atmosphere,since the aerosol and ice crystal particles are not only irregularly shaped and heterogeneously mixed but also with a wide size range,it is a challenging work to simulate their scattering process numerically.Therefore,there still are substantial uncertainties in the light scattering properties of atmospheric particles,which further influence the modeling accuracy of radiative transfer.On this account,obtaining the scattering properties of nonspherical atmospheric particles has become a hotspot in atmospheric science.So far,for particles with their size parameter far smaller and larger than 1,the scattering process can be effectively simulated by Rayleigh Approximation and Geometrical Optics Approximation;while for particles with their size parameter ranging from 0.1 to 100,since particle size is comparable to the light wavelength,the scattering process should be simulated by numerically solving the Maxwell's equations and Helmholtz equation,which is very difficult owning to the restrictions of boundary condition,computational complexity and numerical stability of the calculation process.Owning to this reason,the widely-used scattering models,such as T-Matrix method,Discrete Dipole Approximation(DDA)and Finite Difference Time Domain(FDTD)still can't meet the need for the scattering simulation of atmospheric particles in size parameter range,particle shapes and model complexity.Concerning this issue,our study is carried out from the following five aspects:(1)The improvement and self-development of the parallel PSTD scattering model for atmospheric nonspherical particles.In this model,the ADE-PML(Perfectly Matched Layer with Auxiliary Differential Equation)is derived for PSTD scheme to improve the performance of absorption boundary;the weighted Total Field/Scattered Field(TF/SF)technique that applicable for 3D-PSTD scattering model is established to introduce the incident wave;to meet the need for radiative transfer simulation,the calculation models of scattering parameters are developed based on the near and far electromagnetic field;To improve the computational efficiency,the model is further parallelized by the Open MP technique.The results indicate that,when the refractive index is not large,light scattering by particles with irregular shapes and inhomogeneous compositions can be effectively simulated by the PSTD model;the incident wave can be introduced into the computational domain accurately by TF/SF technique.The absorption performance of ADE-PML is better than traditional BPML,and a 6-layer ADE-PML is thick enough to suppress the reflected electromagnetic wave.Though with a low spatial resolution,reliable results still can be achieved by PSTD;with the increasing of refractive index,smaller spatial grid is needed to guarantee the modeling accuracy.The computational efficiency is notably improved after the model parallelization,compared with the pure scattering field technique;the weighted TF/SF technique can save computational time to some extent.Limit to the principle of PSTD,the scattering properties of the high absorptive particles cannot be calculated by PSTD effectively.(2)The self-development of MRTD light scattering computational model.To cover the shortage of the PSTD scattering model,the MRTD(Multi-Resolution Time-Domain)technique in electromagnetic computational field is introduced into the light scattering simulation.In this model,MRTD technique is applied to calculate the near electromagnetic field,the CPML(Convolutional Perfectly Matched Layer)is established for MRTD to truncate the computational domain;the volume integral method is drived from Helmholtz equation to perform the near-to-far transformation.To achieve higher computational efficiency,the model is further parallelized by MPI non-blocking repeated communication technique.The results found that the scattering parameters obtained by MRTD scattering model show an excellent agreement with those of Lorenz-Mie theory,T-matrix method and DDA;compared with PSTD,MRTD can calculate the scattering parameters of the particles with large refractive index.The performance of the volume integral method is slightly better than surface integral method on the whole.The parallelization scheme based on MPI technique can accelerate model's computational speed notably.(3)The establishing of the Multi-size Synchronous-Computational Scheme(MSCS).To improve the computational efficiency of MRTD model,the concept of transfer function in “Signals and Systems” field is introduced to the scattering simulation,and the Multi-size Synchronous-Computational Scheme(MSCS)is proposed.By using the MSCS scheme,the scattering properties of the particles with different sizes can be simultaneously calculated by MRTD model in one wave-particle interaction simulation.In this scheme,the pulse plane wave with a wide spectrum is taken as the incident light,and the light scattering simulation for particles with different sizes is transformed into the scattering calculation for a size-fixed particle at different wavelengths.To guarantee the stability and precision of the Improved MRTD(IMRTD)model,the method to design model's input parameters(such as the spatial resolution,discrete time interval and pulse width)is proposed;besides,the influence of spatial resolution on the modeling accuracy is analyzed,and the modeling efficiency is discussed as well.The results show that,IMRTD method can calculate the scattering parameters of the particles with different sizes simultaneously and accurately,but the simulation accuracy is reduced with the increasing of particle size.With the increasing of spatial resolution,the simulation accuracy is improved for all particles,and the improvement for large particles is more notable than the small ones.It can also be found that the computational efficiency of IMRTD is much higher than that of the traditional version.(4)The developing of the Invariant Imbedding T-Matrix Method(IIM-T Matrix Method).The Invariant Imbedding T-Matrix Method is a powerful scattering computational model.Once the T Matrix of the scatterer is obtained by this model,not only can the single scattering parameters of the particle be calcualated for arbitrary orientation and incident light(with different propagation directions and polarization states),but also the scattering properties of randomly orientated particles can be computed analytically without orientation-averaging process,which is a tremendous advantage over PSTD and MRTD.The IIM-T Matrix Method is firstly developed by Johnson(1988),and is further improved and coded by Yang P.and Bi L.in USA.Since Yang P.and his team is doing the research for NASA and American Army,this code of this model is regarded as a core technology and is closed to the public.Owning to this reason,A T-Matrix method was developed and realized by ourself.In this model,the invariant imbedding technique is combined with Lorenz-Mie theory and EBCM method to solve the T matrix,and by using this model,light scattering by particles with arbitrary shapes and inhomogeneous compositions can be effectively simulated.Based on the obtained T matrix,the method to analytically calculate the scattering parameters of the randomly-oriented non-rotational symmetric particles is also further derived.The modeling accuracy of IIM-T Matrix method is validated against EBCM-T method,DDA and Aden-Kerker theory,and the influence of the discretization form and the number density of quadrature points on the modeling accuracy is analyzed.The results found that the scattering properties obtained by different models are in good agreement.The precision of Gauss-quadrature discretization is higher than that of the uniform discretization in the integral calculation;with the increasing of quadrature points number,the calculation accuracy of IIM-T Matrix method is improved as well.The scattering properties of randomly-oriented particles can be calculated by the IIM-T Matrix method accurately and efficiently;after orientation averaging,the simulation accuracy of the scattering phase matrix and integral scattering properties is improved notably compared with the single scattering results.(5)The simulation of the scattering characteristics of typical ice crystals.The method to generate the typical shapes of ice crystal is introduced,and scattering parameters of the single particle and randomly oriented particles are calculated by MRTD and IIM-T Matrix method,respectively.It can be found that a good agreement is achieved between the results of MRTD and IIM-T Matrix method,indicating the high precision of the two models.The scattering properties of the ice crystal with different shapes and orientation are different from each other,especially in backward scattering directions.With the increasing of particle size,the forward scattering peak becomes much stronger,and the oscillating characteristic of the phase matrix curves becomes more remarkable.After orientation averaging process,the fluctuation of the phase matrix curves is smoothed,especially for the scattering phase function.