Computation of Particle Scattering Matrices and Polarimetric Radar Variables for Winter Precipitation Using T-Matrix Method, DDA Method, and Higher Order MoM-SIE Method

This paper addresses scattering models of ice hydrometeors and computation of realistic particle scattering matrices and full polarimetric variables for winter precipitation, focusing only on single particle scattering properties. In terms of scattering models and techniques, the T-matrix method and the discrete dipole approximation (DDA) method are the two most frequently used tools in atmospheric particle scattering analysis. The T-matrix method is extremely fast. However, most of the working T-matrix tools are able to calculate scattering properties of rotationally symmetric particles only, and only those with smooth surfaces. The major advantage of the DDA method is that it can be applied to arbitrarily shaped particles. However, the numerical accuracy of the method is relatively low, and improves slowly with increasing the number of dipoles, which makes the DDA computation very time-consuming.

Alternatively, we present scattering models of winter precipitation particles based on a full-wave numerically rigorous computational electromagnetic technique using a higher order method of moments (MoM) for solving surface integral equations (SIEs) (Djordjević and Notaroš 2004). The numerical results demonstrate the capabilities and potential of the higher order MoM-SIE method, and discuss its advantages over both DDA and T-matrix methods in several characteristic examples. DDA and T-matrix computations are obtained using DDSCAT 7.2 code by Draine and Flatau (2012) and T-matrix code by Mishchenko (2014), respectively. It is shown that the higher order MoM-SIE approach is much faster, more accurate and robust than the DDA code, and much more general and versatile than the T-matrix code. In addition, the DDA code does not converge for any reasonable predefined accuracy and number of iteration steps in some cases with high-contrast dielectric materials and large electrical sizes of particles. The T-matrix solution does not converge or exhibits an erratic behavior in some cases with electrically large or geometrically complex particles, namely, those with a large aspect ratio. These observations are presented as clear indication of problems in scattering modeling and computation using these two methods, and not as general conclusions about the limitations of the methods which would require a more exhaustive study. The same is true for the demonstrated advantages in terms of the accuracy and efficiency of the higher order MoM-SIE approach compared to the two conventionally used methods in particle scattering analysis.

Once the higher order MoM-SIE is established as a method of choice, we perform scattering simulations of a very large number of snowflakes and ice particles as a part of NSF MASCRAD (MASC + Radar) project, whose principal goal is to establish a novel approach to characterization of winter precipitation and modeling of associated polarimetric radar observables, with a longer-term goal to significantly improve the radar-based quantitative precipitation estimation in stronger winter events. The newly built and established MASCRAD Field Site at the Easton Valley View Airport, south of Greeley, Colorado, with a 2/3-scaled DFIR double wind fence housing a multi-angle snowflake camera (MASC), 2D-video disdrometer, GPS advanced upper-air system sounding system, PLUVIO snow measuring gauge, and VAISALA weather station, covered by two state-of-the-art S-band polarimetric radars, CSU-CHILL and NCAR-SPOL radars, and supported by excellent geometrical and image processing and scattering modeling and computing capabilities, is one of the currently best instrumented and most sophisticated field sites for winter precipitation measurements and analysis in the nation. This is the first time real (measured) snowflake images are used with realistic scattering calculations, to obtain radar measurable parameters. We are computing “particle-by-particle” scattering matrices to arrive to the radar measurable sets over a larger time interval, which are then compared and analyzed against measurements by highly precise polarimetric radars.