Type 3: External Boundary or Quasi-stationary Frontal Boundary
South or Southwest of Linear MCS.
(1) 4 of the 15 cases studied
revealed this type of storm - mesoscale pattern.
- External boundary was identified by:
a) small convective cells which
may be oriented west-east or
northwest-southeast along the
boundary.
b) reflectivity fine line
c) mesoscale surface analyses
/ visible satellite imagery.
d) In some cases small-weak
transient vortices may be identified
with the small convective cells
near the external boundary.
- 4 warm season events
1 late afternoon - evening
3 early morning
Conceptual model of TYPE 3 reflectivity pattern
- In one the
four cases studied, an external boundary was located only
20 - 60 km south of the linear MCS. In the three remaining linear to
bowing MCSs, the convective system remained well north or northeast
(80 - 120 km) of a quasi-stationary frontal boundary.
- In each of the four cases...a cool layer of air
(varying from 0.5 km -
3 km deep) was present north of the boundary.
- As a first approximation, nearby wind profiler / WSR-88D velocity
wind profile (VWP) data were tools used to determine the
depth of
a stable layer.
An example of Type 3 pattern is
shown in the 25 May 1996 bow echo case
across east-central Missouri and southwest Illinois.
2148 UTC 25 May 1996 reflectivity / storm relative velocity images from KLSX.
(b) Characteristics of convective-scale vortices
- In two of the four cases we examined, 1st and 2nd core
vortices
intensified near the merger between an isolated cell
and
convective line. In one of the two cases, the
isolated cell
did not appear to be anchored along a external
surface boundary.
While in the second case, isolated cells appeared to form
along a warm frontal boundary aloft (e.g. 850 mb).
1. Circulation Trends:
- First convective-scale vortex (Core
#1 often develops along
the cyclonic shear side (northern part) of a linear to slightly
bowing
convective line segment. Early stages of the depth of this
vortex
may often vary from 2-6 km).
- These vortices may exhibit weak - moderate magnitudes of
cyclonic
shears.
Example
of a first core convective-scale vortex associated with
TYPE 3 events.
Rotational Velocity (Vr) trace of
Circulation #1; 25 June 1998.
Magnitudes are in m/s.
(c) Second and subsequent vortices (Cores #2 /
#3) often
form near or as much as 30 km north of the apex of the
linear or bowing segment.
- In two of the four cases sampled, the early stages of
these
circulations often exhibit non-descending
characteristics.
(d) Tornadogenesis:
- Weak tornadoes only occurred in one
of the four cases
documented. In the tornadic case, the old external
boundary was approximately 20 to 60 km south of the
convective line.
Tornadogenesis did not immediately occur during
the
period of vortex deepening
and intensification of
core #2. Rather,
tornadogenesis occurred well into the
mesocyclone's mature stage (10
to 15 minutes after the
circulation reached it's greatest depth)
and where the vortex
exhibited strongest cyclonic shears
(within the lowest 5 km
of the circulation).
- Weak tornadoes associated with the convective line on
May 25, 1996, occurred as far as 50 km north of an old
external boundary. This distance is similar to
observations
recorded by Markowski et al. 1998.
- (NOTE) Forecasters need to be aware that weak transcient
cyclonic shears or couplets may be detected near isolated cells
which are anchored to an old external boundary. In two of the
four cases we examined, two (three) weak vortices were
identified with two (three) discrete isolated cells respectively
along an old external boundary. The weak circulations did not
intensify in either case. See example below.
2213 UTC 25 May 1996 reflectivity / storm-relative velocity imagery from KLSX. 0.5° slice.
Example of a Second (or Third) Core
Convective-scale vortex associated
with TYPE 3 events.
Rotational Velocity (Vr) trace of
Circ #2; 25 May 1996
- This mesocyclone was initiated from a
isolated cell - convective line
merger.
Some characteristics of
Circ #2:
- Compared to Vr traces
associated with TYPE 1 and 2 events, rapid
deepening and/or intensification of this vortex (core #2) was not as
pronounced.
- Strongest cyclonic shears occurred well into the mature stage of the
circulation's lifecycle.
- Tornadogenesis occurred well into the mature stage of
mesocyclone evolution. Not during the later period of the
Organizing stage or very early part of the Mature stage. As
we have observed with the 2nd core vortices of Types 1 and 2.
- Could the lack of an external boundary play have some significance
here? (It's possible; we really don't know).
Question: What
possible causes may delay or even prevent
tornadogenesis from occuring?
Presence of a
cool-stable layer of air.
e) Depth of the cool layer near the surface:
- Depth of the cool layer near the surface north
of the boundary is
critically important in 2 ways:
- will
tornadogenesis occur
- will
convective-scale downdrafts penetrate this stable layer
dependent
upon:
depth of the stable layer
degree of instability.
- Our preliminary results showed that when the cool - stable
layer's depth
is greater than 2 km...severe
convective-scale downdrafts may not
reach the surface.
- In two of the four events we
examined, the stable layer detected was
relatively shallow (at or less than 1 km):
- tornadogenesis
occurred 50 km north of the boundary in one
of
the two cases.
- in
both cases, convective-scale downdrafts were able to
penetrate
the shallow stable layer producing wind damage.
- In the other 2 cases we examined, the stable
layer's depth was
greater than 2 km...severe winds did not occur
at the surface.
(Caveat - In one asymmetric case...the severe winds
were associated with
the 55 dBZ reflectivity core - southwest most convective
cell of the line).
- Returning back to the 25 May 1996 case. A 3rd convective-scale
vortex (Circ #3) was 18 km north-northeast of Circ #2. This core was
located near the intersection of two strong convective storms (far northern
end of the line). Below is the rotational velocity trace of Circ #3.
Rotational Velocity (Vr) trace of Circ #3; 25 May 1996.
Characteristics of Circ #3:
- Strongest cyclonic shears remain aloft 3 - 7 km layer.
There was only two instances where Vr magnitudes reached 16 m/s
(32 kts) at the lowest elevation slice (0.5°) 2212 UTC and 2236 UTC.
- No severe wind or hail were reported in the vicinity of
this vortex.
(2) (Table 5a) Characteristics of 1st, 2nd, and 3rd core circulations
associated
with TYPE 3 events (Organizing Stage)
ORGANIZING STAGE
| Organizing Stage | OS (Vr) m/s Low |
OS Dia (km) Low |
OS Vr (m/s) | OS Dia (km) | Tornado Occurrence |
| 1st core | 13 m/s * | 5 km | 11 m/s | 6.0 km | N |
| 2nd core | 14.5 m/s * | 3.8 km | 14.5 m/s | 5.1 km | N |
| 3rd core | 13.5 m/s | 3.5 km | 12.5 m/s | 6.2 km | N |
- Due to the distance of the 1st
and 2nd core circulations from the RDA site,
vortices in three of the four cases, during the 'Organizing Stage' were
sampled.
(Table 5b) Characteristics of
1st, 2nd and 3rd core circulations associated with
TYPE 3 events (Mature Stage)
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 | 14.5 m/s | 6.0 - 11.0 | 14.2 m/s | 6.0-16.0 km | 7.0 km | N | 50 min |
| 2nd core | 16.0 m/s | 4.5 - 9.5 | 15.5 m/s | 5.1-10.0 km | 8.0 km | Y 1/4 F1 | 54 min |
| 3rd core | 13.5 m/s | 4.3 - 8.0 | 13.9 m/s | 4.8 - 7.0 km | 7.5 km | N | 47 min |
(3) Preliminary Findings
- Comparing the first three core circulations during the 'Organizing
Stage' in
Type #3 convective systems, the 2nd core vortices exhibited slightly stronger
Vr shears at both low-levels (0.5° slice) and throughout the vortex depth
compared to the 1st and 3rd cores.
- The depth of the 2nd core vortex also exhibited a slightly greater
depth
compared to 1st and 3rd core vortices.
- Overall life span of the first three cores are quite similar (45 - 55
minutes).
The second core again exhibited the longest life span of the three.
- One tornado (F1 damage) occurred with the 2nd core in one of the
four cases.
- The magnitude of Vr shears of 1st and 2nd cores associated with Type #3
MCSs were generally weaker compared the first two cores of Type #1 and
Type #2 MCSs during the 'Organizing' and 'Mature' stages. The weaker
Vr shear magnitudes were quite noticable at low-levels during both stages.
- Additionally, the magnitude of Vr shears of Type #3 1st and 2nd cores
were
also weaker compared to mesocyclones associated with traditional
supercells.
- In contrast, the average depth of the first two cores with Type #3
systems
during the 'Mature Stage' were slightly higher compared to both Types #1
and #2 and mesocyclones associated with traditional supercells.
