Cold-air pool evolution in a wide Pyrenean valley

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.