Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 345

Figure 6.31. Small-scale vortices along the rear-flank gust front as detected by the U. Mass.
W-band mobile Doppler radar on June 5, 1999 in north central Nebraska. Range rings shown
every 500m. (Left) Radar reflectivity in dBZ e ; (right) Doppler velocity in m s 1 . (Top) Two
spiral echoes (white lines) and associated vortex shear signatures (small circles); large circle
encloses vortex couplet associated with the tornado vortex signature (TVS). (Bottom) Valid
about 3min later showing weak-echo holes (WEHs) for the tornado and gust front vortices
(GFV) 2 and 3; small circles show vortex signatures for GFV1, GFV2, and GFV3, the line
along which they are connected curves along the gust front; no WEH is evident for GFV1.

any size next to hailstones (stronger evaporative cooling in the case of the former)
or along the edge of water-loaded air or pre-existing horizontal vorticity asso-
ciated with boundary-layer vertical shear, which is subsequently tilted onto the
vertical as air parcels pass through the gradient in vertical velocity as they enter
an updraft or pass in between a downdraft and an updraft or as they exit a
downdraft and then enter an updraft ( Figures 4.52-4.54 and 6.32 ) . Polarimetric
radar observations can determine if there are gradients in hydrometeor type. An
additional complication is that the humidity of the air into which hydrometeors
are falling may vary in space, particularly in the case in which the convective
storm is situated just behind a frontal boundary; in some instances it may be drier
farther behind a front, while in others it may actually get moister. Another com-
plication is that the height at which hydrometeors begin their descent can affect
surface temperature: descent from high aloft can yield much higher net cooling
than descent from a much lower altitude. If

e decreases with height as (by
definition) it does in a convectively unstable environment, then low values of

e