University of Pretoria
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Turbulence forecasts from the ensemble unified model case study

posted on 2023-07-20, 09:47 authored by Lauren SmithLauren Smith, Liesl Dyson, Stephanie Landman

The tables; 4.5 to 4.7 are the summaries of the weather category results. This dataset is what was used for the dissertation and the database is the data that the case study were taken from. 

SAT 1 is the  MSG WV062 channel (left) and Airmass (right) RGB of 16 July 2018 at 09h00 UT. A cut-off low is indicated to the west of the country and the low-pressure centre by the bold black ‘L’ (left). On the WV06 image dry mid and upper level air is indicated by the black shades, west of the low and on the airmass RGB the dry descending air from the stratosphere is indicated by the light brown shades. Copyright (2020) EUMETSAT 

SAT 2 is the MSG DNC RGB (left) and IR10.8 single channel (right) of 16 July 2018 at 09h00 UT and 16h00 UT, respectively. The yellow stars indicate the locations of all three waypoints, from left to right it is EGNOM, KYV and then BEBAS. The red dots indicate the location of the two skew-t diagrams, FAUP (left) and FABL (right). The very cold cloud top temperatures over South Africa on the IR10.8 image is indicative of convective cloud. Copyright (2020) EUMETSAT. FABL skew-T item is the skew-t diagram at Bloemfontein Airport at 12h00 UT. The air temperature is represented by the black line on the right of each graph and the dew point temperature is the black line on the left. Wind strength, indicated in knots, and direction are indicated on the extreme right column of each graph. The short line on the wind barb indicates 5 kt, a full line is equal to 10 kt and a triangle equals 50 kt. The diagram was obtained from the University of Wyoming ( but was done by the South African Weather Service.

Low level jet stream item is the conceptual model of a low-level jet associated with a cold front. As the cold front propagates towards the warm air region, a temperature gradient is generated. The dense dry cold air is pushed below the warm moist air. The colliding air masses may result in wind directional change at the surface ahead of the cold front, thus developing a wind speed increase due to rotary circulation (Adopted from World Meteorology Organization, 2007). Cut off low SH item is the schematic diagram of CAT associated with the formation of a COL. (1) CAT is more likely to develop on the east side of the trough line to the north of the lowest pressure, where the maximum cyclonic curvature occurs. Moderate to severe CAT may be found after maximum amplification of the upper-air trough. (2) The trough will amplify and regions of confluence and diffluence become stronger resulting in CAT development in those regions. (3+4) The low pressure is now ‘cut-off’ and light to moderate CAT may be associated with the region just north of that low pressure. Moderate to severe CAT may occur around the ‘throat’ which is the zone between the ‘cut-off’ low and mean westerly flow (adopted from Holton, 2004, and redrawn by L. Smith).

Jet stream southern hemisphere item is the schematic diagram of the subtropical jet stream over South Africa. The figure shows the convergence (CONV) and divergence (DIV) zones of the upper-air jet stream at the (rear and front) entry and exit, respectively. The jet stream follows a natural curvature (dashed arrows) along the pressure gradients (upper-level geopotential heights, solid arrows). The cross-section (AB) depicts the vertical motion at the polar- and equator-sides of the jet stream as a result of convergence and divergence. Kelvin-Helmholtz item is the schematic of Kelvin-Helmholtz wave formation. The difference in airspeed of the warm air and cold air results in vertical shear. (B) A photo of Kelvin-Helmholtz cloud waves (Stull, 2018; Photo by Dr May Wong, Boulder, Colorado). 

Synoptic chart item is the surface synoptic chart on 16 July 2018 valid at 12h00 UT. A surface tough was over the western parts of country and the low pressure over Namibia (marked by the bold ‘L’). The South Atlantic high pressure (marked by the bold ‘H’) was located to the south of the country.Way Points item is the way points where turbulence was recorded on 16 July 2018 (yellow). The black symbols indicate light and moderate clear air turbulence.Ensemble model schematic (ens) item is the A schematic explaining how uncertainty is captured by ensemble models. 



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