Assessing the effect of soil moisture on katabatic flow dynamics over a shallow slope during the MATERHORN field program

Data collected during the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program are used to evaluate the impact of increased soil moisture on katabatic flow formation and evolution. 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 5-7°. Incoming solar radiation and 50-mm soil moisture data were collected at five locations throughout the slope and an extensive network of weather stations is used to observe 2-m wind velocities and temperatures throughout the region.

The effect of soil moisture is evaluated by considering evening transitions with weak synoptic forcing. The high moisture transitions are observed following rain events. Because katabatic flow development is highly sensitive to larger scale circulations, significant attention is given to the valley and regional circulations. We observe that increased soil moisture has a retarding impact on the development and magnitude of katabatic flow. This is because increased soil moisture lessens the diurnal temperature range of the surface and also dictates the partitioning of the surface energy budget. The phenomenon is discussed in terms of a wall-normal coordinate system where the wall-normal and wall-parallel turbulent fluxes are discussed. Finally, a method for predicting the timing of katabatic flow formation is presented.