Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 378

Doppler wind data to an idealized, circularly symmetric model of a vortex, so that
the average radial and azimuthal components are estimated and the continuity
equation can be used to estimate vertical motion. The first version of Wen-Chau
Lee's technique was first applied to tropical cyclones and is known as GBVTD
(ground-based velocity track display), and is essentially a VAD (velocity azimuth
display) applied to a vortex whose center is not coincident with the location of the
radar. This technique fits a Fourier series to azimuthal and radial wind fields. It is
a dicult problem to find a closed solution, but it has been solved subject to some
simple restrictions.
Radar-observed tornadoes (and dust devils and other vortices too weak to be
classified as tornadoes) frequently have a central weak-echo ''eye'' or hole (WEH)
( Figure 6.49 ). Ted Fujita discussed early measurements of this radar signature
based on conventional radars, while many others have shown more recent ex-
amples of the WEH with higher resolution, more sophisticated Doppler radars. It
is thought that by centrifuging precipitation particles and other debris radially
outward into sheaths or rings, the center of a tornado/small-scale vortex is left
devoid of large scatterers, leaving only scatterers too small to be detected by
radar, or few if at all. John Snow was one of the first to explain this phenomenon
and David Dowell and collaborators described it more quantitatively based on
numerical experiments. It has been demonstrated more recently, using polarimetric
Doppler radars, that the rings of enhanced radar echoes in tornadoes are indeed
most likely composed of debris ( Figure 6.6 ; see the h panel at the lower left).

The vertical structure of tornadoes observed by radar sometimes exhibits a
bowl-shaped or otherwise closed-off bottom in the radar reflectivity factor and an
open, weak-echo or echo-free eye/hole above ( Figure 6.50 ). The closing off of
radar reflectivity near the ground is evidence of the frictionally induced radial
inflow of scatterers in the friction layer. Above the friction layer, centrifuging
removes the largest pieces of debris and scatterers, while in the friction layer
radial inflow may more than compensate for outward centrifuging.
The weak-echo hole may extend all the way to the storm top as a weak-echo
column (WEC) ( Figure 6.51 ). In this case is it possible that there is subsidence
originating at the storm top? Or, is it possible that there is rising motion, but the
air mass is composed of only a few, small scatterers, which cannot be detected by
radar, because the largest scatterers have already been centrifuged radially
outward beyond the eye down below? Unfortunately, it is dicult to find radar
evidence for either an updraft or downdraft on the scale of the eye (evidenced by
divergence/convergence) because radar reflectivity is too weak to detect Doppler
velocities accurately there. Because whether or not there is a deep updraft or
downdraft has implications for the potential intensity of the tornado, we will
return to this issue later. Does the WEC exist because there is centrifuging of
larger scatterers all the way to the top of the storm? This is unlikely because the
highest azimuthal wind speeds in a tornado are confined to relatively low levels. If
much or even part of a tornado is embedded within an updraft connected to the
parent storm above, then the updraft advects vorticity upwards and it would not
be surprising if tornadoes extended rather high up in the troposphere, especially