Stratospheric mountain waves trailing across northern Europe

Planetary waves disturbed the hitherto stable Arctic stratospheric polar vortex mid of January 2016 in such a way that unique tropospheric and stratospheric flow conditions for vertically and horizontally propagating mountain waves developed. Co-existing strong low-level westerly winds across almost all European mountain ranges plus the almost zonally-aligned polar front jet created these favorable conditions for deeply propagating gravity waves. Furthermore, the northward displacement of the polar night jet resulted in a wide-spread coverage of stratospheric mountain waves trailling across northern Europe. This paper describes the particular meteorological setting by analyzing the tropospheric and stratospheric flows based on the ERA5 data. The potential of the flow for exciting internal gravity waves from non-orographic sources is evaluated across all altitudes by considering various instability indices as δ , Ro, Ro ζ , Ro⊥ , and Δ NBE. The analyzed gravity waves are described and characterized in terms of commonly used parameters. The main finding of this case study is the exceptionally vast extension of the mountain waves trailing to high latitudes originating from the flow across the mountainous sources that are located at about 45 N. As a useful addition to the case study, tracks for potential research flights are proposed that sample the waves by a vertically pointing airborne remote-sensing instrument. Benefits and drawbacks of the different approaches to observe the meridional focussing of the mountain waves into the polar night jet are discussed.