Validation of WRF and WRF LES Simulations of the Dispersal of Ground-generated AgI Nuclei

The ability of the WRF model to accurately disperse passive tracers over complex terrain is tested by comparison with airborne measurements. Airborne measurements of ice nuclei (IN) were made by an updated NCAR acoustic IN counter over the Wyoming Medicine Bow Range on 16 February 2011, a day devoid of low-level orographic clouds. WRF simulations were conducted at 500 m and 2.5 km along with a 100-m and 500-m resolution Large Eddy Simulations (LES). On this day IN were detected up to ~1.0 km above the ground. IN concentrations were rather high, and the IN counter generates ice crystals in supersaturated conditions at ~-20°C only, so we can be confident that the measured ice was initiated by AgI nuclei. WRF can reasonably simulate the dispersion and the diffusion of AgI particles at flight level, especially at higher resolutions and especially when using the LES framework. But even the 100 m resolution LES has discrepancies, underestimating the AgI concentration especially above 600m AGL. This is attributed mainly to differences in stability upwind of the mountain. The WRF LES simulated a more stable condition than the radiosondes indicated, leading higher stratification of the WRF LES atmosphere in the lowest few 100 m AGL. Thus, the model simulation underestimated the AgI concentration especially higher than 600m AGL. This paper presents frequency-by-altitude plots of AgI concentrations for different simulations and for the observations. It also explores the boundary-layer structure over complex terrain, mainly by means of velocity power spectra.