Blocking, gap flow and mountain wave interaction along the coastal escarpment of South Africa
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A fatal light aircraft crash occurred in the complex mountainous terrain along the coast of the South African Southern Cape in December 2015. An investigation into the meteorological conditions on this day revealed the interaction between mountain waves, gap flow and blocking near a cold front. The crash highlighted the need to equip forecasters with knowledge of the turbulence produced under these circumstances. With this in mind, experiments were conducted in the vicinity of the crash site, with automatic weather stations and radiosondes, to answer this question. Turbulent features were successfully characterised by Froude numbers, the Froude-derived height scale and the thermal wind equation. The Bernoulli equation, which classifies gap flow, was not helpful due to the effect of the upwind blocking area. Phenomena in descending order of wind strength produced compressional effect (44.7 ms−1), blocking (26 ms−1) and lastly, gap flow. Gap flow negatively impacted blocking jet strength. Phenomena in descending order of turbulence intensity gap flow, mountain wave/rotors and lastly, blocking. Gap flow produced greater vertical velocities than mountain waves. These mountain waves produced the highest vertical velocities measured to date in South Africa, associated with the shortest wavelength waves. Blocking jets of 600 m deep, 80 km wide and extending 30 km downwind of its exit region was found to modulate mountain wave characteristics significantly. A combination of mountain waves, gap flow and blocking was most likely responsible for the crash, highlighting that these three features cannot be seen as separate processes.
KeywordsTurbulence Blocking Gap flow Mountain wave Small-scale topography Coastal mountain
I, Markus Geldenhuys, would like to express my sincere gratitude to the late Prof J van Heerden for teaching me everything I know about experiment design. I would also like to thank Mr E Engelbrecht for always availing himself during field campaigns and for not complaining when he climbed Africa Peak during 160-km/h winds and − 7 °C apparent temperature. I am also indebted to Mr H van Niekerk and the SAWS Eastern Cape forecasting team. This project would not have been possible without your support.
Thank you to InterMet Africa Systems for sponsoring 20 radiosondes and the use of radiosonde equipment, and Air Liquide for sponsoring the hydrogen gas for the balloons. I would also like to thank the South African Weather Service for the time, use of equipment and study bursary.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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