Morning Transition of Steep Slope Flows in a Narrow Alpine Valley

Under clear-sky conditions and weak synoptic-scale forcing, thermally-driven flows predominate in mountainous terrain due to the uneven heating of the atmosphere inside the valley. During the day, upslope and upvalley flows are found, while at night winds travel downslope and downvalley. These diurnal mountain winds have been investigated quite extensively over the past decades given their importance in wind energy forecasting and pollution transport, thanks to numerous field experiments and numerical studies. Most of this past work has however been focused on flows over idealized terrain under quasi-steady conditions, and thus less so on transitional flows over realistic topography. This study aims to gain a deeper understanding of the morning transition period, or morning breakup of the nocturnal temperature inversion, guided by unique field observations taken over a steep alpine slope in Val Ferret, a narrow and meandering valley in the Swiss Alps. The slope (20° to 37°) was instrumented throughout summer 2010 with two turbulence flux towers, several temperature measurement stations, and two additional meteorological masts with the purpose of monitoring heat and momentum exchanges during the transition periods of slope flows. A tethered balloon was also deployed to collect mesoscale atmospheric profiles during a few intensive observation periods. The field data revealed a few striking features of the morning transition period, which usually lasted 2 to 3 hours after astronomical sunrise. Tethered balloon data revealed that subsidence warming triggered by conservation of mass in the cross-valley plane played an active role in the erosion of the local surface inversion. Temperature measurements near the surface unveiled a local counter-gradient heat flux typically lasting 30 min long after direct solar radiation reached the measurement site. The change in wind direction from downslope to upslope begins several meters above the ground before reaching the surface where a very shallow nocturnal drainage flow (~ 1 m) was usually found. An analysis of the turbulent kinetic energy, momentum and thermal energy budgets will also be presented. We believe these observations should help us develop better parameterizations for non-stationary, transitional slope flows over complex terrain.