Type 2: External Boundary / Convergence Line Intersecting
the Central
or South-central End of a Convective Line.(Intensifying Stage)
(1) 4 of 15 cases studied
supported this type of reflectivity pattern.
- External boundary
(ies) are
identified by:
a) reflectivity fine line or
b) small isolated cells oriented
orthogonal (usually east-west or
southeast-northwest) intersecting
the central or south-central part of
the convective line.
- 4 warm season events
3 late night / early morning.
1 late afternoon.
Conceptual Model of Type II Severe Wind MCSs.
(b) In the four cases
studied, initial bowing of the convective line
and first reports of damaging winds
occurred just south of
the intersection of the external boundary and linear
convective line.
- In each of the four cases, damaging winds were not
reported
north of the intersection.
- The MARC velocity signature was found in each of the four
cases, mainly identified south of the intersection during
this stage of MCS evolution.
- Boundaries we speak of appear not to be
associated with relatively
large areas of precipitation (where the air north of the boundary may
not be saturated). Rather temperature gradients of 5°K or greater
across a boundary may be present. (Markowski et al 1998).
Example of the Type 2 pattern is
shown on the reflectivity image
1101 UTC, 14 June 1998 MCS
case across east-central Missouri.
- The boundary is identified by small
isolated convective cells oriented
orthogonal (southeast-northwest)
intersecting the southern part of the
convective line.
- Passage of the external boundary
over KLSX caused a significant
change in the local low-level wind
shear profile. Magnitudes of 0-2
(0-3) km storm-relative helicity
increased from 340 (500) m2/s2
at 1025 UTC to 670 (993) m2/s2
at 1111 UTC. Rasmussen (personal
communications) stated that old
external boundaries may lose their
thermal properties with time, however
helical flow will persist for an
extended period of time. - Note the intensifying convective cell
just north of the intersection.
Plan view of 0.5° slice WSR-88D reflectivity
image at 1101 UTC from KLSX.
(c) Characteristics of convective-scale
vortices
(Type
2 systems)
- In 2 of the 4 cases
documented, 1st and 2nd core vortices
formed just
after an isolated cell, anchored along an
external boundary, merged with the convective line. In both
cases, non-supercell tornadoes occurred after the merger.
Circulation Trends:
- First convective-scale vortex
(core #1)formed just north of the
intersection between the convective
line and (isolated cell) external
boundary.
This circulation may exhibit either a:
1) nearly a persistent depth or
2) non-descending characteristics.
- Second convective-scale vortex
(core #2) will frequently form just
south of the first vortex. This
circulation becomes the strongest
and most persistent of the group
of vortices identified with the MCS.
- Early stages of this circulation will
reveal 'non-descending'
characteristics. This vortex will
often rapidly intensify and deepen
within the first 20 minutes of the
circulation's lifespan. Yet the
strongest cyclonic shears will
often be detected within the lowest
2 km of the circulation's depth.
Four panel reflectivity / storm
relative velocity
presentation from KLSX at 1101 UTC. Upper
two slices 1.5° / lower two slices 0.5° slice.
(d) Non-supercell
tornadogenesis:
- In three of the four cases studied, weak tornadoes (F0 / F1
intensity)
occurred
in the vicinity of the 2nd core and within 15 km on the
'cool side' of the external
boundary. Isolated cell-convective line
mergers occurred in 2 of these 4 cases.
- Tornadoes occurred during the later part of the
'Organizing Stage' and /
or very early part of the 'Mature Stage' of
mesocyclone evolution
(just preceding the mesocyclone core's
greatest depth). This period
of tornadogenesis is earlier compared to
observations recorded by
(Burgess et al. 1982) for mesocyclones associated
with traditional
supercells.
- bowing of the convective line south or southwest of Circ
2 will occur
resulting in enhanced wind damage just south of the vortex.
- The 2nd vortex appeared to play a role in
transfering momentum from the
storm's lower-mid-level region to the surface.
- Circ 2's overall lifespan may vary from 40
to 80 minutes.
- Subseqent 'non-descending' tornadic and non-tornadic
vortices often form
near or north of the apex of the bowing convective line.
(2) Examples of the First
and Second Core circulations associated with
TYPE 2 events.
Rotational Velocity (Vr) trace of
Circ #1; 14 June 1998.
Vr magnitudes are in m/s.
Rotational Velocity (Vr) trace of
Circulation #2; 14 June 1998.
Magnitudes of Vr are in m/s.
(3) (TABLE #3a) Characteristics of 1st and 2nd core circulations
associated
with TYPE 2 events (Organizing Stage).
ORGANIZING STAGE
| OS (Vr) m/s Low |
OS Dia (km) Low | OS Vr (km) | OS Dia (km) | Tornado Occurrence | |
| 1st CORE | 17 m/s | 3.9 km | 17.5 m/s | 4.25 km | No |
| 2nd CORE | 19 m/s | 6.0 km | 18.5 m/s | 6.1 km | Y 1/4-(F0) |
(Table #3b) Results from
Burgess's Study (Mini / Traditional Supercells
(Burgess et al. 1995) (Organizing Stage). In their study, vortices were
sampled up to 150 km for traditional supercells and 98 km for
mini-supercells.
| Organizing Stage | OS Vr (m/s) | OS Dia (km) |
| Mini | 13 m/s | 3.9 km |
| Traditional | 20 m/s | 5.4 km |
(4) (TABLE #4a) Characteristics of 1st and 2nd core circulations
associated
with TYPE 2 events (Mature Stage)
MATURE STAGE
| MS (Vr) m/s Low |
MS Dia (km) Low | MS (Vr) m/s | MS Dia (km) | Depth (km) | Tornado Occurrence | Circulation Lifespan | |
| 1st CORE | 18 m/s | 4.2 - 5.5 km | 17.5 m/s | 4.2 - 6.0 km | 5.6 km | No | 30 min |
| 2nd CORE | 21 m/s | 5.8 - 6.6 km | 20.5 m/s | 5.5 - 7.3 km | 7.1 km | Y 3/4 F0/F1 | 55 min |
(Table 4b) Results from Burgess's Study (Mini / Traditional Supercells
(Burgess et al. 1998)
| Mature Stage | MS (Vr) m/s Low | MS Dia (km) Low | MS (Vr) m/s | MS Dia (km) | Depth (km) |
| Mini | 15 m/s | 3.5 km | 17 m/s | 3.7 km | 4.5 km |
| Traditional | 23 m/s | 5.4 km | 25 m/s | 6.0 km | 9.2 km |
Preliminary Findings:
- Similar to the Table
#1a, Doppler observations of vortices during the
'Organizing Stage' showed that the 2nd core (core #2)
exhibited stronger
Vr shears at both low-levels and overall vortex depth compared to the first
core (core #1) (Table 3a).
- Additionally, the 2nd core frequently showed a greater depth and
significantly
longer lifetime compared to core #1.
- Tornadoes were documented in the vicinity of the 2nd core in 3 of the 4
cases
studied. Tornadoes occurred during the later part of the 'Organizing
Stage
and very early part of the 'Mature Stage' of mesocylone evolution.
- In comparison to Burgess's results (Tables 3a and 3b), the magnitude of
bow echo 2nd core vortices during the 'Organizing Stage' exhibited overall
slightly weaker Vr shears compared to traditional supercell vortices. The
overall diameters of the bow echo 2nd cores were slightly larger compared
to core diameters associated with traditional supercell vortices.
- During the mature stage (Tables 4a and 4b), the magnitude of bow
echo
2nd core vortices (low-levels) exhibited slightly weaker magnitudes of Vr
shears compared to traditional supercell vortices. The low-level diameters
were slightly larger compared to traditional supercell core diameters.
When comparing the Vr magnitudes of the overall vortex depth, the bow echo
2nd core vortices were weaker (20.5 m/s) compared to (25 m/s) for traditional
supercell vortices. In contrast, the core diameters of both 2nd core
vortices
and traditional supercell vortices were relatively similar.
In comparing the overall vortex depth between the two groups, bow echo
2nd core vortices exhibited a slightly smaller depth (7.1 km) to
traditional supercell vortices (9.2 km).
- In most of our Type 1 and Type 2 cases, the 'external
boundary appeared
to serve as a source of local vorticity to aid in mesocyclone
intensification.
This vorticity is often tapped by the storm's updraft resulting in vertical
stretching
within that part of the convective updraft structure. End result is genesis and
rapid intensification of (non-tornadic or tornadic) vortices in the vicinity of the
intersection between the external boundary and convective line.
- Subsequent (tornadic or
non-tornadic) vortices which form near or north of the
apex of the bowing structure appeared to result from shearing instabilities /
strong horizontal shears along the leading edge of an advancing gust front /
bowing convective line.
