The Sensitivity of Katabatic Flow Dynamics to External Influences Through the Evening Eransition

Data collected during the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program are used to investigate the sensitivity of the onset and structure of katabatic flow to the local meteorological and physiographic conditions through the evening transition. The MATERHORN field program consisted of two, month-long field campaigns conducted at the US Army Dugway Proving Ground in Utah's West Desert. The first campaign ran from 26 Sept - 7 Nov 2012, with an emphasis on weak synoptic forcing, and the second campaign ran from 1 May - 7 Jun 2013, with an emphasis on high synoptic forcing. The slope is an arid, eastern facing slope spanning roughly 6 km from the ridgeline of Granite Peak to the valley floor, with an elevation drop of ≈ 550 m. Here, we use data collected from four towers that form a transect along the fall line, spanning 1.5 – 4. 5 km down the slope, with a weakly variable pitch of 2-4°.

The external influences are evaluated by considering fifteen evening transitions (11 in the fall and 4 in the spring) with weak synoptic forcing and well-defined katabatic flow through the evening transition (defined here as 1 hr before to 3 hr after local sunset). Ambient winds, soil moisture, shadow front velocity and near-surface turbulent mixing are found to be of significant importance in the timing and structure of the katabatic flow. The slope-aligned budgets of momentum and potential temperature are used to investigate the influence of the significant meteorological and physiographic variables.