Valley wind dynamics and large-scale airflow interaction within the Cerdanya valley (eastern Pyrenees)

The Cerdanya Cold Pool Experiment 2015 (CCP15) aimed to study the air thermal structure and wind circulation within the Cerdanya valley (eastern Pyrenees, Catalonia) under fair-weather conditions. The field campaign took place between 6 and 15 October 2015, focusing on the vertical column of the first hundred meters above the valley together with the surface energy balance at its centre. The Cerdanya valley, east-west oriented, is the largest of the Pyrenees mountain range with a wide bottom at 1000 m above sea level (asl) and bounded by high mountain ranges both to the north and south (peaks above 2500 m asl). Its topographical configuration favours the development of a diurnal valley wind system with the formation of a cold-air pool at night. During the campaign, four of a total of six intensive observation periods (IOPs) were affected by the intrusion of a strong wind channelised down valley during the afternoon and evening periods, followed by the development of a strong surface-based thermal inversion at night. The analysis of the observations leads to the following findings, also supported by high-resolution numerical simulations:

• The wind channelling observed in the valley centre requires the presence of a large pressure gradient in the north-south direction, i.e. perpendicular to the mountain range (orographic dipole).
• The wind channelling has a diurnal evolution, occurring during the afternoon and evening periods. The exact onset time and endurance depends on the particular synoptical configuration and on the valley wind dynamics.
• The presence of the wind channelling affects the evolution of the thermodynamic variables at the surface layer and the exchange fluxes at the surface interface, modifying the initial conditions of the nocturnal boundary layer within the valley.
• A strong surface-based thermal inversion develops over the valley under both fair-weather conditions or the presence of such channelled wind. The latter fosters the vertical growth of such inversion and dumps its intensity. The thermal inversion separates the lowest levels close to the ground from the wind channelling flow influence.