Observations of evening and morning transition in valleys and slopes

Thermally-driven circulation is one of the distinctive features of airflow circulation in mountainous terrain under low synoptic forcing and it is typically characterized by large variations in spatial and temporal scales from large plain circulation to slope and drainage flows. Occurence of unsteady flows during the day-to-night and night-to-day transitions plays an important role in defining the overall structure of such a circulation as well as of the boundary-layer development. In this study, we report on characteristics of thermally-driven circulation in a high elevated valley-basin surrounded by isolated mountains focusing on transition periods. Observations were taken as part of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) field program at Dugway Proving Ground in Utah. We selected data from IOP4 in mid-May 2013 when tethered-balloon measurements both on slopes and valley are availabe in addition to other MATERHORN field measurements. The focus is on day-to-night and night-to-day transition in which mechanisms of formation on the valley and slopes are analysed as well as their interaction. The study builds on a recent analysis by Lehner et al. 2015 showing that the evening transition from daytime upslope winds to nighttime downslope winds was governed by the propagation of the shadow front. The mechanism for evening transition on the valley occur later again in agreement with the shadow front formation. Morning transition in both slopes and valleys using MATERHORN data has not been investigated in great depth so far. Here we limit to investigate the hypothesis firstly proposed by Princevac and Fernando (2008) that intrusions shaving off the upslope flow may entrain into the growing the convective boundary layer growth, thus providing a possible breakup mechanism.