Observations of mountain waves with interference generated by coastal mountains in South Africa

摘要

>A fatal light‐aircraft crash in December 2015 highlighted the severity of mountain waves occurring north of the Tsitsikamma Mountains along the southern Cape coast of South Africa. EUMETSAT visible‐channel imagery was used to identify three features of the wave event. The waves occurred to the north of the coastal mountains; they displayed evidence of lee‐side interference and they appeared to be generated by terrain features south of the east–west‐orientated main Tsitsikamma Ridge. These three characteristics were used to identify 20 additional events over the period 2008–2016. It was found that the mountain waves were formed in the west to southwest low‐level winds, which are associated with passing cold fronts, upper‐air cut‐off lows and ridging high‐pressure systems. Three geographical areas were identified where severe turbulence may occur during such events. The first was an area of wave interference where waves generated by the Karatara Ridge, situated in an adjacent mountain range, intersected waves generated by Formosa, the highest point in the Tsitsikamma Mountains. The other two areas of potential severe turbulence were identified owing to the presence of waves propagating in different directions, and crossing at different altitudes, in the lee of the isolated peaks Formosa and Witelskop. Formosa also appeared to generate two distinct primary wave shapes, each with a unique range of summit wind directions. The duration of wave events was estimated to be between 3 and 5?hr. Finally, events were observed where clear slots of suppressed mountain waves appeared among visible mountain waves in the lee of Witelskop.