The sensitivity of thermally-driven mountain flows to land cover change.

Like many urban areas in the western United States, the Salt Lake Valley is undergoing rapid population growth, which is driving dramatic changes in land use. Land use changes such as irrigation and urbanization alter the thermal, radiative, and aerodynamic properties of the ground surface. Summertime air quality and local temperatures are often modulated by the magnitude of local thermally-driven flows such as mountain and lake breezes.Understanding the sensitivity of these flows to land cover type will help to inform policy decisions regarding air quality and local climate in arid, mountainous regions.

The sensitivity of slope and valley flows to land-surface characteristics is examined using a series of idealized, three-dimensional simulations by the Regional Atmospheric Modeling System (RAMS). The topography specified was a combination of two, two-dimensional configurations: a plain connected to a plateau, and a flat-bottomed valley within the plateau. The valley bottom was at the same elevation as the plain, so there was no slope-flow contribution to the along-valley flow. Both daytime and nighttime flows were examined using a composite afternoon July sounding from Salt Lake City for thermodynamic initialization. The model runs represent a dry, weakly-forced scenario in which thermally-driven flows dominate.

The land cover of the control run was specified as native grassland over the entire domain, to represent a pre-settlement, "natural" land cover for the Salt Lake Valley. In subsequent runs, the land cover in the domain was altered in three distinct areas: the valley floor and walls, the plain, and the plateau top. Various configurations of grassland, irrigated cropland, and urban surfaces were compared to evaluate the effects of their thermal, radiative, and aerodynamic differences on slope and valley wind systems.