Investigating lee wave trapping mechanisms over the UK and Ireland

Although various lee wave trapping mechanisms have been studied theoretically since Lyra (1940), not much is known about the relative occurrence of these trapping mechanisms in the real world. For this purpose, vertical atmospheric profiles associated with trapped lee waves are clustered here using self-organising maps.

Because in-situ observations of trapped lee waves are scarce, these vertical profiles are extracted from the Met Office’s convective-scale UKV model (which encompasses the UK and Ireland). To demonstrate that this model accurately represents the conditions relevant to trapped lee wave generation, the wavelength and orientation of trapped lee waves visible in satellite imagery are compared to those in the model. The model is found to reproduce these observed characteristics well.

Subsequently, we use the trapped lee wave identification model developed by Coney et al. (2023) and a linear Taylor-Goldstein equation solver to determine which vertical profiles are associated with trapped lee wave activity. We confirm that high low-level wind speeds are a necessary condition for the generation of trapped lee waves of substantial amplitude. We find that wind speeds increasing with height contribute to wave trapping in most cases. Temperature inversions are present in roughly one-third of trapped lee wave cases. The implications of these results for the development of a trapped lee wave drag parametrisation scheme are discussed.

 

References:

Lyra, G. (1943) Theorie der stationären Leewellenströmung in freier Atmosphäre. Z. Angew. Math. Mech., 23, 1-28.

Coney, J. et al. (2023) Identifying and characterising trapped lee waves using deep learning techniques. Quarterly Journal of the Royal Meteorological Society, 150, 213–231.