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A Simple Parameterization of Aerosol Emission in RAMS

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December 7, 2012
Ted Letcher
Hosted by Bill Cotton (advisor), Sonia Kreidenweis, Jorge Ramirez (Civil and Environmental Engineering)

Abstract

Throughout the past decade, a high degree of attention has been focused on determining the microphysical impact of anthropogenically enhanced concentrations of Cloud Condensation Nuclei (CCN) on orographic snowfall in the mountains of the western United States. This area has garnered a lot of attention due to the implications relating to local water resource distribution within this region. Recent advances in computing power and the development of advanced microphysical schemes within numerical models have provided an estimation of how sensitive orographic snow is to changes in CCN. However, what is still lacking, is a coupling between these advanced microphysical schemes and a real-world representation of CCN sources. A new CCN emissions parameterization was developed for the Regional Atmospheric Modeling System (RAMS) that incorporates aerosol sources derived from WRF-CHEM model output. The aerosol sources were prescribed by running WRF-CHEM for a one hour simulation, after which the near surface aerosol concentration along with the hygroscopicity parameter (κ) were saved as the aerosol sources for RAMS. A comparison of the emissions parameterization to observations collected at Mesa Verde National Park (MVNP) during the Inhibition of Snowfall from Pollution Aerosol (ISPA-III) field campaign yielded promising results. The model was able to simulate the variability in near surface aerosol concentration with reasonable accuracy. Furthermore, this model compared much better to the observations than did WRF-CHEM, using only a fraction of the computational expense. As a final experiment, this scheme was used to in an orographic snow simulation. The results of this simulation revealed, that the ISPA effect is smaller and more variable than previous sensitivity studies have estimated.