Numerical Simulation of Seeding Extra-Area Effects of Precipitation Using a Three-Dimensional Mesoscale Model

摘要

The Fifth-Generation NCAR/Penn State Mesoscale Model (MM5) has been used to investigate the extra-area effects of silver iodide (AgI) seeding on stratiform clouds performed at the supercooled layer. A bulk two-moment microphysical scheme and the new software package for silver iodide are incorporated in MM5. Extra conservation equations are applied to trace the seeding agent, which is transported along the flow field and interacts with the supercooled cloud fields. In this study, the model was run using three nested grids, with 3.3 km×3.3 km horizontal resolution in the finest grid. The model results showed that seeding with AgI at the -5 to -15°C levels had microphysical effects on the simulated clouds and that the simulation produced a longer-lasting seeding effect because of the transport of the seeding agent by upper-level winds. Most of the AgI particles acted as deposition nuclei, and the deposition nucleation process contributed mostly to additional cloud ice formation in this study. The results showed that more precipitation results from seeded than unseeded case, and the precipitation was redistributed downwind of the target. Augmented precipitation (varying from 5% to 25% downwind) was confined in space to within 250 km of the seeding target and in time to the 3-h period after initial seeding.