Applicability of MIDAS with Adaptive Discretization to Melting Hydrometeors at Microwave Frequencies

In this work, we assess the applicability of the fast electromagnetic (EM) scattering solver, MIDAS [1], (short for MoM Integral-equation Decomposition for Arbitrarily-shaped Scatterers), by comparing its performance to a solver based on discrete dipole approximation (DDA), the famed ADDA method, in solving for the scattering properties of simulated melting hydrometeors [2]. At microwave frequencies of Ka-band (~35GHz) and below, the refractive-index contrast between liquid water and solid ice becomes increasingly large, causing significant uncertainties in ADDA solutions dependent upon its computation setup, e.g., dipole size (even while it strictly satisfies the accuracy criterion, inversely related to the refractive index, specified by the method) and the choice of dipole polarizability or interaction schemes. This challenge motivates us to test MIDAS on the same melting hydrometeors, assess the range of uncertainty in its solutions, and compare against that of the ADDA solutions. Since there is no (numericized) analytical EM solution for these complexly irregular scatterers with heterogeneous compositions, no truly accurate solution exists, against which we may assess the absolute uncertainties of MIDAS and ADDA solutions. We thus use the relative range of solution uncertainty to evaluate their applicability.