Aerosol impacts on wintertime orographic precipitation

As part of an NSF-sponsored collaborative grant “Inhibition of Snowfall by Pollution Aerosols”, we have been performing simulations and observations to investigate the impacts of varying aerosol pollution amounts on precipitation. Sixty-day “seasonal” simulations were performed from January 1 – March 1 for the years of 2005, 2006, 2007, and 2008 in which CCN concentrations were varied from 100 and 1500 cm-3 to represent “clean” and “polluted” environments, respectively. The results show a significant response to varying aerosol amounts. Specifically, precipitation is found to decrease on the windward slopes and increase on the leeward slopes. This effect, is most evident in the southern and western region of the San Juan Range of the Rocky Mountains where wet storms are more prevalent. The aerosol-induced downwind precipitation shift over the Park Range is also present in each simulated season, but with lower amplitudes and slightly varying magnitudes among seasons. The results are consistent with the findings of Saleeby et al. (2009) wherein higher CCN concentrations reduce ice particle riming rates and thus alter the seeder-feeder process.

Those simulations were performed assuming aerosol are of a uniform chemical composition and size. Ward et al.,(2010) modified the droplet activation look-up table in RAMS to include variable aerosol chemical composition using the parameter “ê” as a proxi for aerosol composition. The WRF/Chem output is used to infer ê as well as aerosol size and concentration. This provides an objective method of predicting spatial and time varying amounts of CCN. This method of CCN prediction is evaluated against surface and airborne measurements of CCN along the western slopes of Colorado. The impact of these inferred CCN amounts on orographic precipitation is simulated for a few case studies.