Observations of lee waves and rotors downwind of the Pennines

A field experiment aimed at observing lee waves and rotors downwind of the Pennines, northern England, UK, is described. The objectives were to understand the causes of turbulence and flow variability, in order to improve forecasting of low-level winds. The experiment operated for roughly 1.5 years and the primary instrumentation consisted of a network of automatic weather stations (AWS) and turbulence masts. The AWSs consisted of a 2 m mast on which wind, temperature and relative humidity were measured and a sensitive microbarograph which recorded surface pressure. Four 20 m turbulence masts were employed. The majority of the instrumentation was located in the Vale of York, downwind of the Pennines in westerly flow, close to Leeming airfield. The experiment was supported by daily radiosonde ascents from Lancaster, to the west of the Pennines, and timelapse skycam imagery at Leeming. On two occasions additional measurements of the lee-wave motion were obtained with the FAAM BAe-146 research aircraft.

Throughout the experiment several episodes of reversed near-surface flow were observed at Leeming, indicating the occurrence of flow separation and the presence of rotors. These episodes are clearly associated with cases of strong lee-wave activity when the wave crests are located immediately above Leeming. The AWS data indicate that the formation of rotors is linked to the presence of large lee-wave induced pressure perturbations which drive the flow near the surface. Upstream information obtained from the radiosonde profiles is used to construct a flow regime diagram which illustrates the necessary conditions for the formation of lee-wave rotors. These findings are supported by the results of a series of high-resolution numerical simulations.