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storm reaching Canary Islands was analog with the third phase of the Shapiro-Keyser
conceptual model (Shapiro and Keyser
1990
) for cyclone evolution, with a frontal
T-bone and bent-back warm front (MartÃn et al.
2006
). The bent-back front encloses
a pool of warmer air at 850hPa, and contributes to reinforcing the winds in the
southwest region of the cyclone.
Figure
3
c and d shows the results of the model at 1km resolution. The surface
wind speeds before 12:00UTCof November 28 remain below50km/h over thewhole
Canary Islands with well-defined wake areas. The wind speed starts to reinforce at
12:00 UTC November 28, while the Delta storm approaches the archipelago with a
counterclockwise veering of the synoptic flow to south-western direction. The north-
western region of the domain presents the most intense flows, where the maximum
wind speed is produced on the lee-side of La Palma, with the wind reaching a speed
of 72km/h. The development of trapped-lee waves starts at 14:00 UTC in the lee-
side of La Palma. The vertical structure of the flow is reflected at surface level with
regularly spaced regions of intense wind speed above 72km/h. The synoptic veering
of the flow towards south-westerly directions coincides with the flow intensification.
The intense westerly warm core of the Delta storm affects the Canary Islands from
20:00 UTC November 28 to 02:00 UTC 29 November. This period is characterized
by the development of local strong winds lee-ward of La Palma and Tenerife islands.
The maximum surface wind speed is reached at 23:00 UTC November 28 over the
lee-side of La Palma, with an intensity of 144km/h at 10magl (Above ground level).
In the southeast coast of Tenerife an intense core flow of high wind speeds develops,
affecting the sea and part of the coast at 137km/h at 10magl.
A vertical cross section is performed in order to understand the physical mecha-
nisms that lead to the intense wind flows observed and simulated at surface levels for
La Palma and Tenerife islands (Fig.
4
). At 12:00 UTC the main flow affecting Tener-
ife presents a marked westerly direction and important vertical wind shear, and an
elevated inversion is present around 780hPa which delimits two different statically
stable layers. Under these conditions an internal gravity wave develops aloft Tenerife
island as it is shown in the cross section. The wind speed at the lee of the moun-
tain intensifies and the downslope flow enhances. The maximum velocities of the
downslope jet flow are of the order of 130km/h at 100magl. The surface wind speed
remains lower than 115km/h. At 24:00 UTC the windstorm has extended downslope
and its jet core presents a maximum wind speed of 162km/h at 100magl.
Figure
4
shows the cross section along La Palma and Tenerife islands at different
horizontal resolutions (9, 3 and 1km). The cross section along La Palma shows how
the trapped-lee waves do not develop in the simulation at 9km. The simulation results
at 3 and 1km present good performance in developing the trapped-lee waves. As was
noted by Durran (
1986
), the trapped-lee is not taken into account by the hydrosta-
tic hypothesis, and these fine-scale features will not be captured by the hydrostatic
models. The simulation results at 9km point out the impact of the orography repre-
sentation in the non-hydrostatic model used; the increase of the horizontal resolution
provides a better representation of the orography that lead to the development of the
fine-scale features.
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