A case study of lee waves over the Lake District in northern England

The structure and intensity of stationary, orographically‐forced over the Lake District and northern Pennines on he afternoon of 26 November 1991 is determined through a combination of field observations and high‐resolution numerical modelling. Observations were provided by the Meteorological Office's C130 aircraft at four different levels and by five radisondes (of differing ascent rates) released in rapid succession from an observing station located on the Cumbrian coast. Vertical velocity was inferred from both of these data sources and, with the help of satellite imagery, it was possible to build up a comprehensive picture of the lee wave structure. In addition, a non‐hydrostatic primitive equation model was run with a 2‐km grid, covering a 180‐km square region of the northern Pennines, in an attempt to simulate the gravity wave motion observed. The model was initialized with an upstream profile formed by merging a descent profile obtained from the aircraft with one from the sondes. The model orography was defined using a terrain height data‐set with 500‐m resolution and with some smoothing to the model grid. After about four hours of integration the model achieves a quasi‐steady state with strong lower‐tropospheric lee waves. Verification on the model response was sought by comparing the vertical velocity in the model along the paths of the aircraft and sondes with that actually deduced from the observations. The overall agreement of the simulated amplitude and phase of the lee waves with that observed is very encouraging—as found in a different case study (Shutts 1992). The success of the model in predicting lee wave motion gives strong support for the use of these models for the development of gravity‐wave‐drag parametrization schemes. The domain‐averaged surface frictional stress was found to be much larger than the wave drag, and consistent with the high roughness lengths likely in the case of mountainous terrain.