A Numerical Simulation of Wintertime, Orographic Precipitation: Part II. Comparison of Natural and AgI-Seeded Conditions

Abstract

The multi-level, microphysical cloud model described in the accompanying article is applied in an orographic situation to simulate the development of precipitation under both natural and seeded conditions. In the case studied, a 1600 m thick cloud deck (base and top temperature of 0 and ?10C) extends well west of a barrier ridge (top 3100 m MSL). Streamlines for flow over the barrier were taken as input to the model. The model predicts a natural precipitation rate of 0.06 gm sec?1 for a 1 cm path width over the barrier. This can be increased 500-fold through cloud-top seeding with AgI at 40 km upwind of the ridge crest and represents a precipitation efficiency of 19.5%. The proper distance upwind for cloud-top seeding may be determined from the streamlines using a fall velocity of 1.2?1.4 m sec?1 starting 5 min after seeding. These findings support previous observations that seeding orographic clouds to increase precipitation is likely to be more successful when the cloud-top temperatures are too warm for significant ice phase nucleation to occur naturally. Cloud-top seeding is suggested to be more efficient than ground-based seeding in targeting the resultant precipitation on the ridge crest, thereby reducing subcloud evaporation losses.