Impact of High Contrast in Indices of Refraction on Scattering Calculations with Discrete Dipole Approximation
Abstract
The radar-radiometer combined algorithm of NASA Global Precipitation Measurement (GPM) mission has made significant advancement by implementing a self-consistent methodology, between active and passive instruments, for snowfall retrieval with the use of scattering properties derived from an extensive database of realistic irregular solid hydrometeors. The next, highest priority objective for the GPM combined algorithm is to address the challenges presented by mixed-phase hydrometeors in the melting layer to the physics-based retrieval of precipitation profile. The strategy of our approach to address these challenges is similar to that adopted for mono-phase rainfall or snowfall retrievals. We will first numerically simulate the melting of solid hydrometeors in our collection to obtain and output snapshots of the melting hydrometeors at prescribed intervals. These synthetic mixed-phase hydrometeors in turn become input to a discrete dipole approximation (DDA) method for electromagnetic scattering calculations. The scattering properties so derived form the foundation for forward modeling of instrument responses, which are then used as a basis for retrievals. However, in executing this strategy, we have encountered unforeseen issues at various stages, including the preservation of mass conservation and consistency in relative liquid-solid fraction. This issue is especially acute if we wish to maximize computation efficiency by using fewer but larger dipoles, i.e. coarser dipole resolution, close to the maximum of published DDA criterion. A number of finer dipoles, which may be of different phases and thus different indices of refraction, must be merged into one coarser dipole. We find that 1) a naive merging not only modifies the spatial liquid-solid distribution of the melting particle but also violates mass conservation and relative fraction consistency and 2) for lower frequencies, e.g. 14 & 35 GHz, where the contrast in index of refraction between liquid water and solid ice is high, the accepted criterion is no longer valid and a more stringent one must be used. We present an innovative procedure for achieving mass and relative fraction consistency. We then describe our attempt in characterizing the impact of high contrast of indices of refraction to the dipole resolution for DDA calculations.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMH206...08K
- Keywords:
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- 3354 Precipitation;
- ATMOSPHERIC PROCESSES;
- 3360 Remote sensing;
- ATMOSPHERIC PROCESSES;
- 1854 Precipitation;
- HYDROLOGY;
- 1855 Remote sensing;
- HYDROLOGY