Progress Toward Addressing the Challenge of Mixed-phase Precipitation for the GPM Combined Algorithms

Available evidence indicates that accurate electromagnetic (EM) single scattering properties (SSPs) obtained from hydrometeors with realistic morphology are crucial for simulated signals to align with radar and radiometer observations (across all frequencies). Melting hydrometeors are of particular interest. Although they are confined to the melting layer, occupying only a few radar range gates, their greatly enhanced reflectivity and extinction obscure the EM signal of rainfall from below when observed from space, increasing uncertainty in surface precipitation estimates.
All recent efforts to enhance the realism of melting hydrometeor models and their SSPs have been constrained by computational costs and uncertainties in the scattering solutions. The Discrete Dipole Approximation (DDA) method, for its versatility with target geometry, has been applied to realistic solid hydrometeors, achieving unprecedented consistency in active (radar) and passive (radiometer) retrievals of snowfall. However, when applied to melting hydrometeors of mixed liquid-solid composition with high refractive contrast, DDA methods reveal their limitations, producing significant and varying uncertainties depending on dipole resolution and liquid mass fraction. 
To tackle these challenges in the relevant microwave spectrum for the full range of hydrometeors, we developed MIDAS, a numerically efficient 3D full-wave model for scattering by complexly shaped scatterers. Its core concept involves devising a direct-solver-based domain decomposition for the Method of Moment based on the volume integral equation to solve the EM scattering of electrically large and arbitrarily shaped scatterers. MIDAS has demonstrated not only a significant computational advantage over DDA-based codes when applied to realistic solid snow particles but also a greater potential to overcome DDA’s limitations concerning melting hydrometeors 
Indeed, promising initial results indicate that MIDAS outperforms the DDA code ADDA in calculating the SSPs of heterogeneous particles. We observe a good agreement, with relative differences below 2%, among MIDAS, ADDA, and Mie solutions for the scattering by heterogeneous (ice and water) 2-layer spheres and melting hydrometeors, provided the dipole size for MIDAS and ADDA is 5 times smaller than required by the normal criterion. However, MIDAS is 30 times faster than ADDA when SSPs are computed for 703 particle orientations. 
Furthermore, as we understand the need to economize further to meet the demands and constraints of melting hydrometeors, we have implemented adaptive mesh in MIDAS. The concept involves using a cell size inversely proportional to the material’s (i.e., water or ice) refractive index and ensuring compliance with the stricter validity criterion for liquid water without over-meshing the solid ice components of the melting hydrometeor. Initial results obtained with a mixed-resolution mesh where the finer mesh's cell size is half that of the coarser mesh are promising. The mere reduction in cell size by a factor of two for the liquid water portion significantly decreases computation costs, shortening the total computing time from 13.75 hours to 6.15 hours for the entire melting process (25 melting stages). The outcomes of this ongoing research will directly enhance the accuracy of SSPs for melting hydrometeors and provide a robust characterization of the uncertainties related to hydrometeor scattering in precipitation retrievals.