Blocking, gap flow and mountain wave interaction along the coastal escarpment of South Africa

• Published in: Theoretical and applied climatology 2020-02, Vol.139 (3-4), p.1291-1303
• Main Authors: Geldenhuys, Markus, Dyson, Liesl L., van der Mescht, Deon
• Format: Article
• Language: English
• Subjects: Aerodynamics; Aircraft accidents; Aquatic Pollution; Atmospheric Protection/Air Quality Control/Air Pollution; Atmospheric Sciences; Automatic weather stations; Climate science; Climatology; Cold front; Cold fronts; Earth and Environmental Science; Earth Sciences; Escarpments; Flow; Fluid dynamics; Jet aircraft; Lee waves; Light aircraft; Meteorological conditions; Mountain waves; Mountains; Original Paper; Radiosondes; Rotors; Thermal winds; Turbulence; Turbulence intensity; Turbulent flow; Vertical velocities; Waste Water Technology; Water Management; Water Pollution Control; Wave interaction; Wave rotors; Wavelength; Waves; Weather; Wind; Wind effects; Wind speed; Winds
• ISSN: 0177-798X; 1434-4483
• DOI: 10.1007/s00704-019-03030-4

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.