Rainfall enhancement downwind of hills due to standing waves on the melting-level and the extreme rainfall of December 2015 in the Lake District of northwest England

Abstract. Flow over orography can be investigated through stationary gravity waves, i.e. those whose speed exactly opposes, and therefore cancels, that of the airstream in which they are embedded. They give rise to persistent zones of ascent and descent, which modulate precipitation patterns and contribute to large accumulations, e.g. through the well-known seeder-feeder mechanism. It is shown here that opposite, stationary waves on the melting-level focus rain, potentially multiplying intensity downwind of hills by a factor of rain fall speed divided by snow fall speed, and that the effect is maximised when the vertical profile near the melting-level is isothermal. A 2D diagnostic model based on linear gravity wave theory is used to investigate the record-breaking rainfall of December 2015 in the Lake District of northwest England. The pattern of vertical velocity is shown to have a good, qualitative fit to that of the Met Office’s operational, high-resolution UKV model averaged over 24 hours, suggesting that orographically excited standing waves were the principal cause of the rain. Precipitation trajectories suggest that a persistent, downstream, elevated wave caused by the Isle of Man maintained a spray of seeding ice particles directed towards the Lake District; that these grew whilst suspended in strong upslope flow before being focussed by the undulating melting-level into intense shafts of rain.