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proximity to the approach path for Runway 08. This is a 430 meter hill, (indicted by the
arrow on Figure 23), approximately 10 km to the West-North-West of the airport.
Orographic wind effects from this terrain feature, by themselves, or in combination with
the larger scale vortices could also be the source. A brief discussion of these two
mechanisms is presented below, along with a discussion of measurement devices that
could be used to investigate the phenomenon – or even be used as part of an operational
warning system.
Figure 23. Topographic map of the island of Tenerife.
A brief discussion of the synoptic trade wind patterns over the Canary Islands can
be found in Varela et al.1 (see also the references sited in that paper). Figure 24 from that
reference illustrates a typical synoptic wind pattern: cooler maritime air flowing from the
North-East at the surface, with cooler dryer air from the North-West aloft. A thermal
inversion layer forms between 1000-1500 meters.
Figure 24. Synoptic trade wind behavior in the Canary Islands. (from Varela et al.)
von Karman vortex street
It is well-known from fluid mechanics that as a fluid flows around an obstacle,
such as a cylinder, the boundary layers separate from each side of the cylinder surface
and form two shear layers that trail aft in the flow and bound the wake. Since the
innermost portion of the shear layers, which is in contact with the cylinder, moves much
more slowly than the outermost portion of the shear layers, which is in contact with the
free flow, the shear layers roll into the near wake, where they fold on each other and
coalesce into discrete swirling vortices. A regular pattern of vortices, called a von
Karman vortex street, trails aft in the wake. Figure 25 illustrates this phenomenon. For a
vertically aligned obstacle, in this case a cylinder, the vortices are aligned vertically.
Figure 25. A view from above of a von Karman vortex street forming behind a cylinder.
1 A.M. Varela, et al. 2004: Non-correlation between atmospheric extinction coefficient and TOMS aerosol
index at the Canarian Observatories. Remote Sensing of Clouds and the Atmosphere IX, ed. Schafer et al.,
Proceedings of the SPIE Vol 5571, pp. 105-115.
This vortex shedding behavior is often observed with isolated mountains. A
satellite view of this phenomenon associated with the Canary Islands is shown in Figure
26. Tenerife is indicated by the arrow. The lack of clouds just downwind of Tenerife is
due to subsidence of the air mass as it flows down the slopes of the Teide. This does not
mean that the vortices are absent, rather there are no clouds there to mark them.
Figure 26. von Karman street vortices formed downstream of the Canary Islands. The island
of Tenerife is indicated by the arrow.
Figure 27 and Figure 28 illustrate results from a numerical modeling simulation of
the wind field in the wake of the Hawaiian of Kauai, performed by NCAR for NASA. As
can be seen from Figure 27, the island of Kauai is very similar in structure to Tenerife.
Figure 28 illustrates the simulated wind field at 1000 m, with the left-hand image
showing contours of horizontal wind velocities (turbulence is indicated by the blue
shading), and the right-hand image showing the horizontal wind vectors. In this case, the
flow pattern was reasonably consistent in the vertical, so that the winds at 1000 feet are
probably not too different than those shown here. Notice the sharp gradients in horizontal
velocities as the air flows around the island (left-hand image). These simulations also
captured von Karman vortices in the downstream flow. This flow pattern close to the
island, as well as downstream is probably similar to patterns that would be encountered at
Tenerife.
The persistent trade wind field and the flow around and downstream of Tenerife,
as indicated above, is certainly a potential source of the vertical wind shear that is
experienced by pilots approaching Tenerife-Sur from the West.
Figure 27. Model domain used for wind flow simulation around the Hawaiian island of
Kauai. The contour lines indicate the terrain.
Figure 28. Fine-scale model winds at 1000 meter altitude from Kauai simulation. Left is a contour of
the wind field with turbulence indicated with the blue shading. On the right are the wind vectors.
Smaller-scale orographic effects.
As mentioned above, (Figure 23), there is a 430m terrain feature approximately
10 km to the West-North-West of the airport. Localized flow around and over this hill
could also affect aircraft as they approach the airport from the West. Figure 29 shows
another situation of von Karman vortices formed downstream of the Canary Islands.
Figure 30 is a blow-up of the region surrounding Tenerife. The box in this figure is
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