Numerical Simulation of Dispersal of Inert Seeding Material in Israel Using a Three-Dimensional Mesoscale Model

Abstract

A mesoscale model RAMS (the Regional Atmospheric Modeling System) was used to investigate the effectiveness of the broadcast static seeding method for dispersing particles into clouds, as it is used in Israel. The model was run using three nested grids, with 500 m ? 500 m horizontal resolution in the finest grid. In this paper, the particles were assumed to be inert; namely, only the wind field controlled the dispersal of the tracer particles, and no interaction with cloud or precipitation particles was considered. Although the resolution of the model is good for mesoscale studies, it could not resolve individual plumes. The results, therefore, present average values of the concentrations at each level. The simulations showed that seeding particles reach altitudes at which they could become effective as ice nuclei. These cases were primarily the ones in which the updrafts developed over the seeding lines when the seeding plane was just passing underneath. In these cases only, seeding at about 1-km level (?4°C) with 500 g h?1 of inert material (simulating AgI particles) resulted in about 1 ? 103?2 ? 103 L?1 being lifted to the ?10°C level. Based on previous laboratory studies of the seeding agent used in Israel, out of these total concentrations, only 1?2 L?1 could form ice at ?10°C. The simulations also suggest that in most other cases the horizontal advection diluted the particles in the air and only very low concentrations (<10?3 L?1, active at ?10°C) reached the ?10°C level. Most other released particles were transported horizontally with the winds and were later on forced down by downdrafts. Although these simulations await some experimental verification, they suggest that the broadcast seeding method used in Israel is not so effective for widespread rain enhancement operations.