Civil Engineering Reference
In-Depth Information
(3.13)
where
r
is the radial coordinate from the centre of the downburst;
R
the characteristic
radius of the downburst 'shaft';
z
the height above the ground;
z
* a characteristic height
out of the boundary layer;
ε
a characteristic height in the boundary layer; and λ a scaling
factor, with dimensions of [time]
−1
.
The velocity profile at the radius of maximum winds (
r
=1.121
R
) is shown in Figure
3.3. The profile clearly shows a maximum at the height of the boundary layer on the
ground surface. Radar observations have shown that this height is 50-100 m in actual
downbursts.
3.2.7
Wind profiles in tornadoes
There have been many studies of the wind structure in tornadoes based on full-scale
studies using photogrammetry and portable Doppler radars (see also Section 1.3.4),
laboratory studies of tornado-like vortices and theoretical analyses.
The simplest model of horizontal wind profile in a tornado is based on the Rankine, or
combined, vortex (Figure 3.4). This consists of an inner 'core' with solid body rotation,
in which the product of the tangential wind velocity component,
U
θ
,
and the radius from
the centreline of the tornado is a constant. In the outer region
(r>R),
the tangential
velocity component is inversely proportional to the radius,
r
. This satisfies the equation
of angular momentum (Lewellen, 1976), except the discontinuity at
r
equal to
R
.
Figure 3.3
Profile of horizontal velocity near
the ground during a stationary downburst.
