Evening transition characteristics on a slope in an arid environment

We present results characterizing the evening transition on an east-facing moderate (3-8%) slope in an arid environment with sparse vegetation. Data are utilized from the MATERHORN-X (Mountain Terrain Atmospheric Modeling and Observation Program) field campaigns conducted in Dugway, UT, USA. Specifically, observations made along the East Slope of Granite Peak during the Autumn 2012 quiescent Intensive Observation Periods (IOPs) are considered. The measurements include five multi-sonic anemometer eddy covariance towers, eleven small weather stations, fifteen automated weather stations, a distributed temperature sensing (DTS) system, hot-film anemometry, infrared camera surface temperature observations, and Doppler LIDARs. The observations indicate the wind shifts during the transition period are not tightly coupled with the passing shadow front as observed over steep terrain. However, two types of wind shifts were evident during the IOPs that appear to behave as either a local cooling "slab" transition or a passing "front" with origins that are non-local (steeper slopes furthered up the mountain). In this presentation, a description is given of the two transition phenomena in terms of mean wind and thermodynamic variables, as well as turbulence quantities. An attempt is made to understand the conditions leading to the two phenomena.