Interest in the use of hygroscopic seeding has been spurred by recent successes in South Africa and Mexico, which have shown that seeding with hygroscopic flares can produce substantial increases in precipitation. The details of the physical mechanisms leading to these increases are poorly understood, especially the longer-term effects typically noted. The purpose of our current research is to apply a new microphysical scheme designed for numerical simulations of seeding with hygroscopic flares in the three-dimensional dynamic cloud model developed by Terry Clark and associates to provide realistic treatment of hydrometeor development as a natural consequence of the nucleation characteristics of the background (and seeding) aerosol population. The objective is to better establish the physical processes involved in the microphysical and dynamical response of northern Great Plains convective storms to hygroscopic (and glaciogenic) seeding.

The new scheme includes the following features: Nucleation of both natural and artificial aerosols is treated directly, with explicit prediction of supersaturation. The warm rain process is treated in sufficient detail that development of drizzle and rain is a function of the cloud droplet distributions produced by activated aerosols. Six hydrometeor classes are used; cloud water and rain define the liquid water spectra, while the ice particle spectra is divided into four classes - ice crystals, snow, graupel and hail. Two moments of the size distribution, number concentration, and mixing ratio are predicted for each hydrometeor class, including the cloud droplet and ice crystal populations, with the particle size distributions generalized as gamma distributions (of which exponential distributions are a special case). Each hydrometeor class interacts with water vapor and the other hydrometeor classes through a series of idealized representations or parameterizations of the physical processes of nucleation of cloud condensation nuclei, condensation/evaporation, collision/coalescence including drop breakup, both homogeneous and heterogeneous nucleation of ice crystals, deposition/sublimation, collision/aggregation, accretion, freezing, melting and shedding, and ice multiplication via the rime splintering mechanism.

In this presentation we will describe simulations with the new scheme and with the widely used Lin et al. (1983) IAS scheme. Cases include historic cases from the North Dakota Thunderstorm Project (28 June 1989) and the North Dakota Tracer Experiment (1 July 1993). We also hope to include additional cases, especially weaker storms from more recent field operations, in the conference presentation. Simulations using the new microphysical scheme are compared to results obtained using the older IAS scheme, and to observations, to evaluate the performance for the new scheme. Additional tests are performed to examine the sensitivity of the new scheme to variations in such properties as cloud base temperature, CCN and IN spectra.