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RESEARCH PRODUCT

Observed versus simulated mountain waves over Scandinavia – improvement by enhanced model resolution?

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Abstract. Two mountain wave events, which occured over northern Scandinavia in December 2013 are analysed by means of airborne observations and global and mesoscale numerical simulations with horizontal mesh sizes of 16 km, 7.2 km, 2.4 km and 0.8 km. During both events westerly cross-mountain flow induced upward propagating waves in the troposphere and stratosphere and trapped waves in the lee of the mountains. Despite similar forcing conditions gravity wave breaking occured during the first event at altitudes between 25 km to 30 km due to weak stratospheric background winds, while waves propagated to altitudes above 30 km during the second event. In the lower troposphere trapped lee waves with horizontal wavelengths of 15 km to 40 km, which propagated horizontally up to 300 km in the lee of the mountains were observed. Global and mesoscale simulations with 16 km and 7.2 km grid sizes were not able to simulate the mountain and trapped lee waves properly due to unresolved mountain peaks. In simulations with 2.4 km and 0.8 km horizontal resolution mountain waves were captured, but exhibited too small amplitudes, too strong decay of trapped waves in the lee of the mountains and too high energy and momentum fluxes at flight level. Increased fluxes in simulations are caused by reduced downward propagating waves due to weaker jumps in static stability at the tropopause and reduced gravity wave reflection.