High-resolution Simulations of Downslope Flows Over Complex Terrain Using WRF-IBM

In this work, the evening transition to katabatic (downslope) flows on a mountain slope with complex terrain is examined. The case study is Granite Mountain, Utah, which was the site of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) project. Due to the complex topography of Granite Mountain, an immersed boundary method (IBM) is used. IBM was implemented into the Weather Research and Forecasting (WRF) model by Lundquist et al. (2010, 2012) and removes the restrictions on terrain slope that are associated with WRF's traditional terrain-following vertical coordinate. Flow over Granite Mountain is simulated with 50 m horizontal resolution and approximately 10 m vertical resolution near the ground with no-slip bottom boundary conditions. The primary forcing in the model is incoming solar radiation, including slope and topographic shading effects, as well as realistic land use and soil data. The model is used to examine the location and timing of katabatic flow development relative to local sunset. The spatial and temporal evolution of katabatic flows is also explored. Model results are compared to field data from Intensive Observation Period (IOP) 4, part of the Spring 2013 MATERHORN field campaign. This IOP was classified as “quiescent” to denote weak synoptic forcing, allowing katabatic flows to develop with minimal influence from regional scale effects (Lehner et al. 2015).