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

Show simple item record Geldenhuys, Markus Dyson, Liesl L. Van der Mescht, Deon 2020-05-11T12:58:34Z 2020-02
dc.description.abstract 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. en_ZA
dc.description.department Geography, Geoinformatics and Meteorology en_ZA
dc.description.embargo 2020-12-04
dc.description.librarian hj2020 en_ZA
dc.description.sponsorship The South African Weather Service en_ZA
dc.description.uri en_ZA
dc.identifier.citation Geldenhuys, M., Dyson, L.L. & van der Mescht, D. Blocking, gap flow and mountain wave interaction along the coastal escarpment of South Africa. Theoretical and Applied Climatology volume 139, 1291–1303 (2020). https://doi-org/10.1007/s00704-019-03030-4. en_ZA
dc.identifier.issn 0177-798X (print)
dc.identifier.issn 1434-4483 (online)
dc.identifier.other 10.1007/s00704-019-03030-4
dc.language.iso en en_ZA
dc.publisher Springer en_ZA
dc.rights © Springer-Verlag GmbH Austria, part of Springer Nature 2019. The original publication is available at : http://link.springer.comjournal/704. en_ZA
dc.subject Turbulence en_ZA
dc.subject Blocking en_ZA
dc.subject Gap flow en_ZA
dc.subject Mountain wave en_ZA
dc.subject Small-scale topography en_ZA
dc.subject Coastal mountain en_ZA
dc.title Blocking, gap flow and mountain wave interaction along the coastal escarpment of South Africa en_ZA
dc.type Postprint Article en_ZA

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