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The results of a recent field experiment focusing on the near-surface pressure and flow fields downstream of the Pennines in northern England are presented. The main aim of the experiment is the improvement of wind forecasts downstream of orography. Trapped lee waves commonly occur in westerly flow in this region, and during the experiment there were numerous instances of apparent flow separation, indicating the formation of lee-wave rotors. The spatial variability of the near-surface flow in these circumstances is closely linked to the positions of lee-wave crests and troughs aloft, and appears to be a response to pressure gradients induced by the lee waves. For large-amplitude waves, it has been possible to demonstrate a correlation between the fractional change of the flow speed across the measurement array (which if large enough may lead to flow separation) and a normalized pressure-perturbation amplitude. For a group of lee-wave cases during which the cross-mountain flow is strong, a rapid decrease in the Scorer parameter within the lower portion of the troposphere appears to be a prerequisite for rotors to form. However, this does not guarantee their occurrence. For a fixed Scorer-parameter profile, idealized two-dimensional simulations indicate that the lee-wave-induced pressure-perturbation amplitude, and hence the occurrence of rotors, is controlled largely by the strength of the wind upstream close to the mountain-top level. It seems that the combination of a favourable Scorer-parameter profile and sufficiently strong low-level winds is required for rotors to develop. © Crown Copyright 2007. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd. |
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