A Detailed Microphysical Simulation of Hygroscopic Seeding on the Warm Rain Process

Abstract

The Wisner one-dimensional time-dependent model has been modified to allow the condensed water forms to be represented by 52 logarithmically spaced size categories covering a range of just under 2 ?m radius to slightly less than 5 mm radius. The size distribution of the water drops was allowed to evolve with time as a result of the physical processes of vertical advection, condensation/evaporation, stochastic coalescence, and drop breakup. Salt seeding was simulated by the introduction of a distribution of raindrop embryos at cloud base for a specified period of time. The raindrop embryo distribution was derived from calculations on the diffusional growth of a distribution of salt particles in the unsaturated air below cloud base. This model was applied to the 23 July 1970 salt seeding case reported by Biswas and Dennis. This ?detailed microphysical? study has indicated that salt seeding can be effective in stimulating the warm rain process only if breakup-induced chain reactions result. In order for the chain reaction to develop, high vertical velocities (greater than 10 m s?1) are required. Salt seeding acts mainly as a catalyst in initiating this Langmuir-type chain reaction. Without breakup, salt seeding has little effect other than to allow a few of the big drops to fall out of the model clouds. Breakup acting alone may cause the model clouds to precipitate but much longer periods are required than when seeding and breakup are combined. The effects of salt seeding and breakup-induced chain reactions are also strongly dependent on the dynamics of the model cloud.