Improvements to the Two-Habit Model Single-Scattering Database: Irregular Hexagonal Column Ensemble, New Size Characterization, and Improved Backscattering

The Two-Habit Model (THM) is a cloud ice particle single-scattering database that utilizes two different particle shapes to best represent ice clouds in radiative transfer simulations. The previous THM consists of a severely roughened hexagonal column primarily representing small particle sizes and an ensemble of 20 aggregate particles primarily representing large particle sizes. The size characterization of the previous THM was maximum dimension, defined as the maximum distance between two vertices of a particle. Using this size characterization for irregular hexagonal columns results in physical and optical inconsistencies when compared to a roughened hexagonal column due to the change in volume and projected area caused by particle distortion. An improved THM database is based on a hypothesis that the optical properties of roughened hexagonal ice crystals can be represented by the ensemble-averaged counterparts of randomly distorted ice crystals. We have thoroughly validated this hypothesis. A large portion of the previous THM database was produced by the Improved Geometric Optics Model (IGOM), mainly for moderate and large particle sizes. IGOM performs poorly for calculating the backscattering portion of the scattering phase function. We also use the Physical Geometric Optics Model (PGOM) for the backscattering portion of the phase function to produce more accurate backscattering calculations.

In this study, three size characterizations based on equivalent sphere diameter are considered for replacing maximum dimension: Surface-, volume-, and volume-surface equivalent sphere diameter. Among these size characterizations, the volume-surface equivalent sphere diameter is shown to be consistent for all single-scattering properties between the roughened hexagonal columns and the ensemble of irregular hexagonal columns. For the improved THM backscattering calculations, to reduce computation time we use a PGOM-based backscattering enhancement correction estimation to improve the backscattering of the scattering phase function instead of performing PGOM calculations for all wavelengths and particle sizes. The results show more pronounced backscattering, particularly for the P11 component of the phase function, when compared to the previous IGOM-produced scattering phase function.