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outside the updraft and fall as a ''rain curtain''. In many instances the rain curtain
may be very narrow (
Figure 6.35
); I have named these narrow curtains ''umbilical
cords''. Narrow curtains of rain should produce narrow corridors of evaporative
cooling, which lead to a region where horizontal vorticity is generated baroclini-
cally. This horizontal vorticity can be tilted onto the vertical by nearby updrafts or
downdrafts. Why narrow rain curtains are observed in supercells is not well
understood, but horizontal deformation and sharp updraft gradients are possible
relevant processes.
There is some observational evidence that hook echoes can also form as
precipitation falls, so that advection is not the only mechanism responsible for the
hook shape. Davies-Jones postulated that precipitation-induced drag just outside
the center of a strong updraft drives a downdraft that can transport high angular
momentum downward, as first suggested by Ted Fujita in the 1970s, and can
increase surface convergence, owing to surface friction. It is not the baroclinically
generated azimuthal vorticity (due to the radial gradient in precipitation loading
or evaporative cooling) that becomes the tornado, but rather the radially inward-
directed frictionally generated vorticity that is tilted upward and stretched.
Localized, transient areas of convergence may act to spin up tornadoes. When
the RFD descends to the ground, it spreads out and wraps around a low-level
mesocyclone. As it hits the leading edge of the rear-flank gust front, it can
produce localized strong convergence (
Figure 6.36
). This localized convergence
may act to intensify vorticity. Microbursts might therefore be able to trigger tor-
nadoes. Some researchers such as Erik Rasmussen have considered ''descending
reflectivity cores'' (DRC) in supercells as being associated with wet microbursts
Figure 6.35. Radar reflectivity in dBZ
e
(left) and Doppler velocity in m s
1
(right) for a
tornado in Kansas on May 15, 1999, as detected by the U. Mass. W-band, mobile Doppler
radar. The tornado (vortex signature circled at the right) is marked by a weak-echo hole
surrounded by a ring, and connects the hook echo to the bulge in the rear-flank gust front
(RFGF) at an inflection point in the reflectivity field. The hook echo is connected to the ring by
a very thin line of reflectivity marked as an ''umbilical cord'', which appears to connect the
tornado to the parent storm. Range rings shown every 1 km.
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