Investigating the Effects Surface Characteristics Have on Mountainous Island Wind Regimes and the Resulting Aerosol Transport

Coastal winds are a product of many physical factors: the land-sea breeze interactions, large-scale flow, potential terrain effects such as blocking or redirection, and the shape of the coastline. These winds are important to understand because they transport aerosols and pollutants, which affect visibility, clouds and precipitation, and can create potential health hazards. Additionally, waves and ship navigation are directly influenced by these wind patterns. Therefore, it is necessary to characterize and quantify the effects that mountains and sea breeze interactions have on the coastal wind environment of an island. While many of these effects have been previously studied using highly idealized models, few accurately represent the surface features of the terrain (e.g., prescribing a single surface roughness length over the entire surface) which results in unrealistic boundary conditions. Boundary layer physics plays a critical role in turbulence generation, entrainment, small-scale winds and the deposition or lofting of aerosols. Direct emissions of pollen and volatile organic compounds from trees and other vegetation can act as potential cloud condensation nuclei. To assess the influence that vegetation has on smaller scale winds and aerosol transport on a mountainous island, we employ the Regional Atmospheric Modeling System, (RAMS 6.2.5) to perform a sensitivity study. The control simulation has a semi-desert surface classification covering the entire island, which is characterized by a roughness length of .07 m and sparse vegetation. Such conditions are representative of an island one might observe in the Caribbean Sea, such as Aruba. Our sensitivity tests are comprised of a suite of simulations, which we run for 48 hours, thus allowing for two full diurnal cycles to occur. In these simulations, we systematically vary the surface classification type and vegetation location on the island. Beginning with very simple changes, we work up to a realistic ecological representation of vegetation on an island. For example, ecologically, surface roughness length tends to decrease with increasing height in mountainous regions as forested regions give way to shrubs, which give way to alpine vegetation. We then investigate how the near-surface flow characteristics vary as a function of surface vegetation type. By examining a combination of boundary layer turbulence, the three-dimensional wind field, the thermodynamics, and the resultant aerosol field, we further explain how the surface classification type influences aerosol spatial distribution on a mountainous island.