This study on cold-air pool formation in the wide Cerdanya Valley in the Pyrenees mountain range was conducted using available observational information from September 2010 to August 2014. Cold-air pools occur during almost 60% of the nights, mainly during winter. Cold pools develop even under significant synoptic pressure gradients. Additionally, drainage currents transporting air down-valley occur most of the nights. In particular one representative cold-air pool event has been analysed in detail by a high-resolution mesoscale simulation, combined with an analysis of data from both ground-based stations and satellites. Radiative processes dominate the evolution of cold-air pools, together with turbulence in the lowest layers, while drainage flows down from the high mountains mainly through the tributary valleys and from the valley sidewall slopes play a key role in bringing air to the pool. Cold pool formation begins approximately 1 hr after sunset, and it extends across most of the valley bottom, with a very strong thermal inversion close to the surface that has a depth of up to 100¿m in the lowest parts of the valley. Wind veers down-valley along the main axis 2–3 hr after sunset and the wind direction is approximately maintained until after sunrise.

Peer Reviewed