Mixed-Phase Orographic Clouds: Pollution’s Influence to Precipitate Snow
Because much of California’s water source comes from melting mountain snow, understanding the factors contributing to snow precipitation over the Sierra Nevada Mountains has important implications for predicting the water cycle and local climate of the western United States. The researchers employed the most detailed cloud model to understand the impacts of aerosol particles on how clouds form precipitation over the mountains.
This study improves fundamental understanding of aerosol particles’ effects on cloud properties and precipitation in a mountainous area. The research discovered a new mechanism explaining how high concentrations of human-caused pollution particles could seed mixed-phase clouds formed around mountains. This phenomenon dramatically increases snow precipitation, which may have significant implications for mountainous precipitation extremes and water cycles, especially in highly polluted regions such as China and India.
How orographic (mountainous) mixed-phase (both water and ice) clouds respond to the change of aerosol particles that can serve as cloud condensation nuclei (CCN or cloud droplet seeds) and ice nucleating particles (INPs are the seeds for ice formation) is highly uncertain. Researchers at the Department of Energy’s Pacific Northwest National Laboratory studied CCN and INP impacts on supercooled cloud water content, cloud phases, and precipitation for two types of mixed-phase clouds with contrasting winds and moisture. Their study used very high-resolution (0.5 km) simulations with an advanced cloud microphysics model to understand the complex interactions of cloud microphysical processes with CCN and INPs. In the model, they added CCN (such as sea salt or sulfate) and found that the CCN suppresses precipitation with a warm cloud temperature and low INP conditions. However, that phenomenon is strongly reversed with CCN of 1000 cm-3 and higher. The team discovered a new mechanism through which CCN can invigorate mixed-phase clouds over the Sierra Nevada Mountains and drastically intensify snow precipitation when CCN concentrations are 1000 cm-3 or higher. In this situation, more widespread shallow clouds with a greater amount of cloud water forms in the California Central Valley and foothills west of the mountain range. As these clouds form, latent heat increases and in turn strengthens the local transport of moisture to the windward mountain slope. Thus, the mixed-phase clouds are invigorated over the mountains and produce higher amounts of snow precipitation. Increasing INPs (such as dust and biological particles) leads to increased snow precipitation through enhancing deposition in cold-temperature clouds, but hail-forming (riming) in the warm-temperature clouds.