Gravity waves, turbulence and rotors in the lee of mountains

• Main Author: Cardoso, Rita Margarida A. P.
• Other Authors: Mobbs, Stephen D.
• Published: University of Leeds 2005
• Subjects: 910.02143
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.416803

This thesis has focussed on analysing the influence of a temperature inversion in the upstream profile of flow over ridges. Under northerly conditions, a temperature inversion is commonly observed in the Falkland islands radiosonde launches and severe turbulence is usually coupled with it. The turbulence is presumed to be linked to gravity wave activity and is associated with high temporal and spatial variability in wind speed and direction. Recirculation zones are commonly found under the crest of the waves and their three dimensional structure pose a significant hazard to air traffic. The data obtained from a field campaign aimed at observing the flow field downwind of a mountain ridge in the vicinity of Mt. Pleasant airport in the Falkland islands is analysed. From the data analysis, a correlation between the temperature inversion and the difference in temperature between the sea surface and air is established. The seasonal distribution of these phenomena is also explained. Due to logistic constraints, there were no radiosonde launches performed upstream during the campaign. To obviate that problem, a one-dimensional boundary layer model with a 1.5-order closure scheme, radiation and cloud parameterisations was developed. The model was then used to simulate the upwind characteristics of some particular case studies during which strong downwind flow was observed. The influence of temperature inversions on the dynamics of trapped lee-waves was simulated through a series of two--dimensional simulations of flow over idealised ridges. The analysis focussed on the effects of topography composed by two ridges and on the effects of an inversion below the ridge summit. These simulations show that the effects of the second ridge can significantly alter the downstream flow and that a temperature inversion below the summit can also induce strong downwind phenomena. A two-dimensional simulation using the one-dimensional vertical profile from the boundary layer model was performed. The results were in reasonable agreement with the observed data for that day.