California Institute for Energy and Environment (CIEE)

This research achieved insights into the documented impacts of urban and industrial air pollution on orographic clouds and precipitation (and the resulting stream flows) over the California Sierra Nevada, downwind of the pollution sources. The pollution aerosols are submicron cloud condensation nuclei (CCN) that are incorporated into the orographic clouds, thereby slowing the coalescence of raindrops and riming on ice precipitation and delaying the conversion of cloud water into precipitation. The net rain‐volume losses in the polluted central and southern California Sierra Nevada are estimated at 4 x 109 cubic meters (m3) of water per year.

This project’s accomplishments are threefold. First, it demonstrated through hydrological analyses that the negative effect of pollutants on mountain precipitation also results in stream flow losses. Second, quantitative analyses of cloud structure using multi‐spectral satellite imagery showed that the cloud precipitation‐forming processes are being suppressed in the same Sierra Nevada regions that are experiencing losses in precipitation and stream flows. Third, the results of numerical modeling simulations showed good agreement with precipitation and stream flow analyses based on observations that submicron CCN pollution can alter Sierra precipitation.

This study’s finding that aerosols have been suppressing precipitation in high‐elevation areas in California for many years carries significant implications for the state’s water supply.