Recent studies have shown that a diagnostic relation between the water content and the intercept parameter (N0 ) of droplet size distribution (DSD) can be derived from disdrometer measurements. The relation can be found by formulating relations between moment pairs of the DSD. When applied to the output of a supercell storm simulated using a three-moment ice microphysics scheme, the relationships obtained are found to be consistent with those obtained from the disdrometer data. Further, moment-fitting methods can be used to derive the relations between the slope and shape parameters of the gamma-distribution, using either disdrometer data or model output obtained with multi-moment microphysics.

The relationships thus derived will be used to implement lower-moment microphysics schemes that attempt to capture the key DSD characteristics of the higher-moment schemes. For example, the diagnostic-N0 scheme can be used to design a single moment scheme that yields results similar to the two-moment scheme. It is known that the intercept parameter varies widely in nature both spatially and temporally and thus it is hoped that freedom from a fixed value for N0 will allow for more accurate modeling of the DSD. This has the potential to provide improvements to the traditional Marshall-Palmer based single moment scheme. The slope-shape relation can be used to build a two-moment scheme with gamma-DSD that performs close to the three-moment scheme.

The schemes developed will be implemented inside the ARPS model and tested for real data cases where initial conditions are obtained through radar data assimilation. Forecast results using the new schemes will be compared to the results of matching higher-moment schemes and verified against polarimetric radar data and surface disdrometer measurements.