Analysis and characterization of the morning and the evening transition over a west-facing slope: case-studies from the MATERHORN Program

The inhomogeneous heating in mountainous areas generates a small-scale circulation called thermally-driven circulation. In between, two transitional periods are observed around sunrise and sunset, called morning transition and evening transition. The scope of this thesis is to characterize the morning and evening transitions of a west-facing slope and adjacent valley, by means of experimental data collected within 4 case studies during the Mountain Terrain Atmospheric Modeling and Observations field campaign (MATERHORN) that took place in Utah between 2012 and 2013. In doing so, this thesis hopes to close some existing gaps concerning the development of thermally-driven circulations on west sides of isolated mountains, where very few in-field studies have been proposed in the past. The analysis addresses 4 case studies from the same number 4 IOPs (Intense Observation Period),characterized by weak synoptic conditions that allows the undisturbed development of thermal circulation. The results on this investigation has shown that during the morning transition, two main phenomena take place: the erosion of the temperature inversion and the rotation (i.e. transition) of the wind. The two phenomena do not necessarily occur in order, depending on the warming process which in turn depends on the nocturnal boundary layer development. When the heating process is triggered from the ground, the transition starts from the flattest part of the west slope before local sunrise. When the heating process is due to mixing with overlying air, the morning transition is delayed near the valley and is faster near the steepest slope. The convective boundary layer characteristic determines the evening transition developing. A weakly developed CBL leads to an earlier transition onset, but very slow and gradual, starting from the steepest gentle slope. A well developed CBL leads to delayed but faster transition starting from the less steep part of the slope.