The University of Pecs and NCAR Bin (UPNB) microphysical scheme was implemented into the Weather Research and Forecast (WRF) mesoscale model which was used to study the impact of AgI seeding on the precipitation formation in the winter orographic clouds. Four different experimental units were chosen from the Wyoming Weather Modification Pilot Project to simulate the seeding effect.
The results of the numerical experiments show that: (i) Comparisons with the soundings, snow gauges and micro-wave radiometer (MWR) data indicate that the three dimensional simulations with detailed microphysics reasonably represent both the dynamics and the microphysics of the real clouds. (ii) The dispersion of the AgI particles from the simulated ground-based seeding was effective due to the turbulent mixing. (iii) In the investigated cases (surface temperature is less than 0°C) the surface precipitation and the precipitation efficiency show low susceptibility to the concentrations of Cloud Condensation Nuclei (CCN) and natural Ice Nucleating Particles (INP). (iv) If the available liquid water content promotes the enhancement of the amount of snowflakes by diffusional growth, the surface precipitation can be increased by more than 5%. A novel parameter relevant to orographic clouds, the horizontally integrated liquid water path, was evaluated to find the relation between the seeding efficiency and the liquid water content. The impact of seeding is negligible if the horizontal LWP is less than 0.1 mm, while it is apparent if the horizontal LWP is larger than 1 mm based on the cases investigated in this study.