Geoscience Reference
In-Depth Information
Figure 6.30. Radial profiles of azimuthally averaged azimuthal velocity (m s
1
), radial velocity
(m s
1
), vertical vorticity (
10 s
1
), divergence (
100 s
1
), circulation (
10
3
m
2
s
1
), and
radar reflectivity factor (dBZ
e
), from analyses of data collected at low elevation angle in a
tornado in northern Kansas on May 15, 1999 by the U. Mass. mobile W-band Doppler radar.
Circulation increases radially outward until about just a little more than 100m, beyond which it
levels off at about 35,000m
2
s
1
(from Tanamachi et al., 2007).
radius—(to be discussed later); not necessarily the radius of maximum azimuthal
wind, but usually close to it (the radius beyond which the angular momentum of
the vortex does not vary with radius)—of 100m, then the circulation of the core is
2
(100m) (50m s
1
)
¼
10
4
m
2
s
1
(
30,000m
2
s
1
). This magnitude of circula-
tion seems to be, based on mobile Doppler radar measurements, characteristic of
the near environment of tornadoes (
Figure 6.30
). Since vorticity is circulation/
area, vorticity
10
4
m
2
s
1
/
¼
1s
1
.
Background vorticity associated with a mesocyclone is
10
3
s
1
, since
radial shear of the azimuthal wind
10m s
1
/5 km. The mesocyclone may be a
''low-level mesocyclone'' that was produced by the parent supercell or a pre-
existing one along a surface boundary, perhaps enhanced by shearing instability.
In the case of the latter, the pre-existing vortex may also be called a
''miso-cyclone'' which, according to Ted Fujita's classification scheme, has a
shorter (average) space scale than a mesocyclone (
40m-4 km in diameter for a
miso-cyclone, as compared with
2-10 km for a mesocyclone). As in the previous
paragraph, another way of estimating vorticity in a mesocyclone is to use circula-
tion. Since circulation is conserved in a frictionless, barotropic atmosphere, the
mesoscale circulation from which the tornado develops (neglecting friction and
baroclinic effects) must have vorticity
10
4
m
2
s
1
/area of the mesocyclone;
for a mesocyclone of radius 2.5 km, vorticity is
1.5
10
3
s
1
.
(100m)
2
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