Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 366

panied by this frictionally induced, intense updraft it is called an ''end-wall''
vortex, where ''end-wall'' refers to the solid lower boundary.
It is thought that light debris is lofted high in the parent storm from the
corner region's upward-flowing jet and then caught up in the storm's updraft;
subsequently the debris may be deposited downstream from the storm, sometimes
at great distances from its source. There have been many cases in which personal
banking data and family photographs have been carried and dropped as much as
100 km downstream. John Snow and his students at OU have conducted studies of
lofted debris in powerful tornadoes. Personal effects have been found and re-
united with their owners whose houses have been demolished by strong tornadoes.
The equation of motion for radial flow in the corner region from (6.9) is

p 0

2

u @ u =@ r þ w @ u =@ z v = r ¼ 0 @ =@ r ð 6 : 22 Þ

Based on numerical experiments, Tim Wilson and Rich Rotunno, in a 1986 paper,
showed that corner flow is considered inviscid because the turbulent friction term
is not significant in comparison with the other terms in (6.22). This finding is in
accord with Burggraf et al.'s finding that the depth of the friction layer varies as
the radius: in the corner region the depth of the viscous layer ! 0asr ! 0. As the
flow turns upward, as noted earlier, there is less rubbing of the horizontal wind
against the ground. It is ironic that where tornadoes are the most violent (their
wind speeds are the highest), they are characterized by laminar flow.
Doppler radar measurements in the friction and corner regions are di cult to
obtain, owing to ground clutter contamination from trees, utility poles, houses,
etc. and because the friction layer is so shallow that the vertical resolution needed
to discern vertical variations in wind speed is dicult to achieve in practice.
Nevertheless, Bluestein et al. in a paper published in 2007a have provided some
relatively high-resolution Doppler radar measurements that may indicate that the
wind speed can increase by more than 25% in the surface friction layer.
The resulting flow pattern in the vertical plane (i.e., for vertical and radial
motions) of the friction layer, the inertial layer, and the corner region, driven in
this case by friction, is called the ''secondary'' circulation. (Synoptic meteorolo-
gists also refer to circulation in the vertical plane that is an instantaneous
response to and counteracts quasi-geostrophic forcing from the horizontal wind
field acting on the temperature field as the ''secondary circulation''.) Vertical
motions are coupled with horizontal motions via the Boussinesq equation of con-
tinuity (6.15). To understand the behavior of tornado boundary layers better and
to put in perspective what we already know, we first consider what happens to
synoptic-scale vortices in the boundary layer. In synoptic meteorology, one consid-
ers what happens when vortices such as synoptic-scale cyclones and anticyclones
rub against a rotating surface (rotating at the speed of the local rotation rate of
the Earth about its axis projected onto the local horizontal plane). Traditional
analysis of a steady-state boundary layer yields the familiar Ekman spiral hodo-
graph, for which there is convergence and divergence in the boundary layer of
cyclones and anticyclones, respectively. The former produces Ekman pumping out
of the boundary layer into the free atmosphere above and the latter produces