A study showing impacts of aerosols on clouds and precpitation associated with a large winter cyclone (Invited Presentation)

The parameterization of cloud physics and resulting partitioning of water species into liquid or ice may influence mesoscale characteristics within both large and small-scale storm systems. Furthermore, aerosols increasingly are considered to play a potential role in cloud and precipitation development due to droplet and ice crystal nucleation with numerous complex yet uncertain interactions and feedbacks between them. Whereas liquid-only clouds tend to be somewhat simpler, mixed-phase clouds must consider aerosols changing the population and/or size of droplets that may alter the freezing and riming processes. Some recent research, usually on very small spatial and temporal scales indicates that aerosols play a pivotal role while other research, usually of larger spatial/temporal scales contradicts these findings and reveals a less active role of aerosols.

To address a very complex and uncertain research problem that affects storms from convective to climate scales, the Thompson et al (2008) bulk microphysics scheme was recently updated to incorporate aerosols explicitly. The scheme explicitly nucleates water and ice from their dominant respective nuclei and fully tracks and predicts the number of available aerosols. Using the Weather Research and Forecasting (WRF) model, the scheme was tested in a very high resolution (4-km spacing) simulation of a three-day winter storm event over the entire contiguous U.S. A control simulation was run with climatological aerosol conditions and then two sensitivity experiments with very clean and very polluted conditions were used to evaluate the magnitude of aerosol-cloud-precipitation interactions.

Analysis of these sensitivity experiments clearly shows the expected change to water droplet populations, including both large and small droplets. Less well documented changes to liquid water content and mean size as a function of temperature occur as well, which has interesting implications to frequency and severity of aircraft icing that go beyond the usual findings associated with surface precipitation impacts. Furthermore, we found that aerosols impacted the relative fraction of liquid versus ice phase of the surface precipitation which has implications for the extent and severity of a severe surface ice storm.