Investigation of Black Carbon Effects on Precipitation and Surface Hydrology over the Western United States

The current Exceptional Drought (US Drought Monitor) over the western United States warrants an in-depth investigation of possible causes of decreased precipitation and surface hydrology. Black carbon (BC), being the most radiatively-absorptive of any aerosol species, has the potential to semi-directly influence atmospheric physics and dynamics. Aloft, BC can exacerbate the aridity in some areas while increasing precipitation in other locations. On the surface, BC can also alter surface hydrology parameters such as surface runoff and snow water equivalent. In this study, we examine the role of BC and its possible effect on spatial precipitation redistribution and surface hydrology west of and over the Rocky Mountains from an online and coupled meteorological and chemical perspective. In particular, we utilize the Weather Research and Forecasting-Chemistry (WRF-Chem) model at the horizontal resolution of 30 km, employing the Fu-Liou-Gu plane-parallel radiation scheme and a three-dimensional radiation parameterization over mountainous areas to account for BC feedback with clouds, radiation, local circulation, and precipitation. Preliminary results of a January 2005 low pressure system show the inclusion of BC increases (decreases) precipitation on the windward (leeward) side of the Transverse and Peninsular Ranges, and the Sierra Nevada. Results also show BC contributes to an increase in surface runoff on the windward side of the Transverse and Peninsular Ranges, the Sierra Nevada, and Rocky Mountains, but a decrease in snow water equivalent over Sierra Nevada and Rocky Mountains.