This is a collaborative project to study the effect of air pollution on snowfall rates and amounts. Previous work by two of the investigators (Borys and Lowenthal) developed evidence suggesting that the particles in polluted air decrease snowfall rates and amounts. The mechanism, not fully documented but clearly suggested by earlier work, is that the higher particle concentrations associated with air pollution contain increased numbers of cloud condensation nuclei (CCN), the particles on which cloud droplets form. Pollution thus increases the concentration of cloud droplets but decreases their average size, and so decreases the efficiency with which falling ice crystals grow by collision with the supercooled droplets. Pollution therefore reduces precipitation by reducing the amount of supercooled water collected from clouds and deposited in snowfall. The study will take additional steps toward determining how snowfall from winter orographic clouds, observed at the Desert Research Institute's Storm Peak Laboratory, is reduced by the high concentration of aerosol particles associated with pollution. This extension of previous work will include direct measurement of the CCN, continued observations of snowfall and precipitation, and modeling to establish the physical links between pollution and snowfall amount. The project will use a mesoscale model that has demonstrated skill in predicting precipitation rates and amounts. Key objectives are these: * To determine the relationship between pollution-aerosol loading and snowfall rate; * To determine the relative importance of riming, snow crystal size, crystal habit, and large-scale meteorological features in determining the snowfall rate; * To conduct sensitivity studies of effects including the above using the RAMS mesoscale model with bin-emulating microphysics; and * To estimate the quantitative effect of pollution aerosol on snowfall rate and amount in the study region through use of the mesoscale model.
The anticipated result is a new ability to predict how air pollution affects snowfall in pristine mountainous areas and so changes the watershed in areas where water resources are critically important. The project should also lead to improved understanding of how to represent the effects of air pollution in models of weather and climate, especially as they influence precipitation.
