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

A fatal light aircraft crash, within the Kareedouw mountains, highlighted the need to equip forecasters with the knowledge of the turbulence produced when wind flow encounters complex small-scale terrain near a cold front. With this in mind, an experiment was designed, according to the 6 W’s (why, who, where, when, what, whereby) of design, to measure and address the unanswered questions regarding blocking, gap flow and mountain waves. Experiments were conducted with party balloons, dronesondes, dropsondes, thethersondes, parasails, simultaneous ascents and smoke grenades within the Kareedouw Pass. The topographical extent of the pass is the smallest where such a field experiment have ever been conducted. Data were collected during six radiosonde field experiments and from an installed Automatic Weather Station network. Numerous parameters were calculated, where features were successfully characterised by the; Burger-, Froude number, Froude derived height scale and thermal wind equation. The upwind blocking region rendered the Bernoulli equation, which governs gap flow, unusable. Gap flow was identified by pressure- and temperature gradients during non-blocking and weak synoptic conditions. The nocturnal inversion positively influenced gap flow, in a Venturi-type effect. Mountain wave trapping criteria and the Scorer parameter were ineffectual, due to the constantly changing and unstable upwind conditions. Turbulence was identified using potential temperature, cumulative normalised ascent rate, Ellrod Turbulence Index, vorticity, Richardson number, standard deviation from the gust and also by subjectively judging the wind directional and speed shear. Blocking jets reached wind speeds up to 26ms-1 producing virtually no turbulence with scales of 600m deep, 80km wide and penetrating to 30km downwind of its exit region. The blocking wind speed measured second strongest wind speed, second to the ridge; (44.7ms-1) due to the compressional effect. The blocking jet changed the synoptic scale surface wind and sea-level pressure, while driving gap flow and altering the mountain wave formation criteria; via a dynamic barrier. Gap flow, weakened the blocking region and measured below average wind speeds, while remaining the most turbulent feature. Phenomena observed at the gap exit included, divergence, hydraulic jump/expansion fan, eddies, maximum updraft and downdraft observations as well as large wind directional and speed shear. Mountain waves, and its associated severe rotors, were found to be the second most turbulence producing feature and measured the most severe ever measured in South Africa, while being the smallest in wavelength and amplitude. Mountain wave formation criteria were updated given that mountain waves were observed during previously defined unfavourable conditions. Erratic-moving and complex-shaped mountain waves formed during unstable conditions with the wind almost perpendicular to the ridge. Severity of these mountain waves was found to be unpredictable; a nomogram or numerical weather prediction product is suggested. The results of this study are widely applicable, from the aviation industry to the renewable energy sector. Pilots are recommended, to keep a ground clearance of at least 1.5-2 times the highest topography; in a 10km upwind radius. It is recommended that forecasters keep in mind the blocking-induced dynamic barrier and to not exclude mountain waves under unstable conditions as well as to use a cross-barrier wind speed ≥7ms-1.Dissertation (MSc)--University of Pretoria, 2018.Geography, Geoinformatics and MeteorologyMScUnrestricte