MEASUREMENTS OF FLOW OVER AN ELONGATED RIDGE AND ITS THERMAL-STABILITY DEPENDENCE - THE MEAN-FIELD

Measurements of mean wind flow and turbulence parameters have been mad e over Cooper's Ridge, a 115 m high elongated ridge with low surface r oughness. This paper describes measurements of the streamwise and vert ical variations in the mean field for a variety of atmospheric stabili ty conditions. In near-neutral conditions, the normalised speedup over the ridge compares well with measurements from Askervein (Mickle et a l.. 1988). The near-neutral results are also compared to an analytical flow model based on that of Hunt et al. (1988a). Measured streamwise variations show less deceleration at the foot of the hill and slightly more acceleration at the crest of the hill than does the model. In no n-neutral conditions, the speedup over the ridge reduces slightly in u nstable conditions and increases by up to a factor of two in stable co nditions. The model is modified to allow boundary-layer stability to c hange the upwind wind profile and the depths of the inner and middle l ayers. Such a modification is shown to describe the observations of sp eedup well in unstable and weakly stable conditions but to overestimat e the speedup in moderate to strongly stable conditions. This disagree ment can be traced to the model's overestimation of the upstream scali ng velocity at the height of the middle layer through its use of a sta ble wind profile form which has greater shear than that of the observe d profiles, in possible combination with the three-dimensionality of t he ridge which would allow enhanced flow around, rather than over. the feature in more stable conditions.