The Effect of Pollution Aerosol on Wintertime Orographic Precipitation in the Colorado Rockies Using a Simplified Emissions Scheme to Predict CCN Concentrations

Abstract

he impacts of enhanced CCN concentrations on various cloud and precipitation systems are potentially significant both to the large-scale climate system and local precipitation patterns. Precipitating stable orographic cloud systems are particularly susceptible to increases in CCN as parcel lifetimes within these clouds are typically short compared to clouds of similar depth. As such, even small perturbations to the precipitation efficiency within these clouds can have substantial impacts. In the mountainous regions of the western United States, where water resources are derived primarily from orographic precipitation during the cold season, this effect is of particular interest. The aims of this study are twofold. The first part is focused on the implementation of a simplified aerosol emissions scheme into the Regional Atmospheric Modeling System (RAMS). This scheme uses aerosol output from the Weather Research and Forecast Chemistry model (WRF-Chem) to initialize aerosol sources in RAMS. The second part of this study uses this scheme in the simulation of an orographic snow case that occurred in northwest Colorado during February 2007. The result of this study suggests that atmospheric CCN concentrations can be reasonably simulated using a simplified parameterization of aerosol emissions, despite a lack of explicit secondary aerosol (SA) within the model. Furthermore, the spatial and temporal variations in CCN predicted by this scheme produced a complicated response in the surface distribution of precipitation from the orographic snowstorm, a result not seen in studies where CCN concentrations are set to be horizontally homogenous.