Thermally Driven Winds Across Gaps in a Linear Mountain Chain

Granite Mountain, an isolated northwest-southeast-oriented mountain range in northwestern Utah, separates a moist playa area on the west side and a dry sagebrush area on the east side. To the south of Granite Mountain are two other mountains: the small Sapphire Mountain and the much larger Dugway Range, resulting in a small-scale gap between Granite and Sapphire Mountain, and a larger scale gap between Granite Mountain and Dugway Range. Because of the high soil moisture on the playa, the air over the playa has a reduced diurnal temperature range relative to the sagebrush area. If only the spatial differences in surface cover are considered, a near-surface flow would be expected from the Playa to the Sagebrush area during daytime and a reversed flow during nighttime. The observed regional flow patterns in the area however, are characterized by northerly flows during daytime and southerly flow during nighttime. It is unclear how flows in the small and large gap respond to the various forcing mechanisms, i.e., the diurnally varying regional flows and associated pressure gradients, and the horizontal temperature differences caused by differences in boundary layer evolution over the Playa and Sagebrush area. We hypothesize that flows through the gaps on clear, undisturbed days are produced primarily by temperature differences that develop between the east and west sides of Granite Mountain as diurnal boundary layers form and decay. These temperature differences will lead to pressure differences that drive the winds in one direction or the other. To test this hypothesis, we analyze data collected by a line of temperature data loggers running across the southern part of Granite Mountain, and meteorological data collected at various sites in the gap area, and over the Playa and Sagebrush areas. We show that there are diurnal changes in the flow across the gaps caused by the differences in the depth and heat content of the nocturnal and daytime boundary layers on the two sides of the ridgeline. However, this feature is occasionally modified by the regional scale flows. These regional-scale flows also induce local terrain flows that can modify or even reverse the horizontal temperature gradients across the mountain range and can induce subtle differences of flow characteristics through the small and the large gap.