Multi-sensor observations of an elevated rotor during a mountain wave event in the Eastern Pyrenees

The 15 January 2017 a strong northerly synoptic flow lead to the generation of mountain waves and heavy snowfall over the eastern Pyrenees, particularly over the Cerdanya valley near the border between France, Spain and Andorra. Measurements from several instruments deployed during the Cerdanya-2017 field campaign and satellite imagery revealed the presence of mountain waves and the formation of an associated rotor underneath the first mountain wave crest. The evolution and location of the mountain waves were studied using high temporal resolution data from a UHF wind-profiler and a vertically pointing K-band Doppler radar, separated a few kilometres in horizontal distance. A mountain wave with a wavelength about 18 km was detected in the morning and shortened slightly in the afternoon when a transient rotor, elevated approximately 140 m above the ground, was formed, disconnected from the surface flow. A strong turbulence zone was identified at the upper edge of the mountain wave, above the rotor, a feature observed in previous studies. The mountain wave and rotor induced circulation was favoured by the valley shape and the second mountain ridge location, in addition to the weak and variable winds, established during the sunset close to the valley surface.