Type 4: Outflow Boundary or Convergence Line intersecting the
Updraft region of a High-Precipitation (HP) Supercell
(1) One case supported
this type of storm - mesoscale pattern.
- External
boundary
generated by convection to
the northeast of KLSX was
identified by:
a) reflectivity fine line intersecting
the inflow region (eastern flank)
of a High-Precipitation (HP)
Supercell.
- warm season events
1 evening.
Conceptual model of Type 4 severe wind MCS.
- This type of storm reflectivity evolution is similar to those
documented
by Przybylinski and DeCaire (1985) (Type IV)
pattern, and
Klimowski et al (2000) - northern High Plains.
Additionally, Moller et al. (1990) has also documented this type
of storm evolution across northern and central Texas.
- An example
of TYPE 4 pattern is shown in the 18 June 1998 event which
occurred across eastern Missouri and parts of central and southern
Illinois.
0200 UTC 19 June 1998
reflectivity from KLSX. Image (left) is 1.5° slice. Image on (right) is
0.5° slice...zoomed 2x.
(a) The leading edge of
the outflow generated by the convective line to
the northeast is revealed by a reflectivity fine line. This fine line
intersected the HP storm's updraft (storm's forward flank).
- Local vorticity residing within the external boundary appeared to be
tapped by the HP storm's updraft, resulting in the
intensification of
the storm's mesocyclone.
- Four-panel reflectivity / storm-relative velocity presentations of the
HP storm is shown below.
Four panel reflectivity /
storm-relative velocity Four panel reflectivity
/ storm-relative velocity
presentation at 0200 UTC 18 June 1998
presentation at 0205 UTC 18 June 1998
(Upper two slices 1.5° / Lower two slices 0.5°)
(Upper two slices 1.5° / Lower two
slices 0.5°)
(b) First (or Second)
convective-scale circulation (mesocyclone
core #1 or #2): is often located along the HP storm's forward
flank
(storm's WER).
- Core #1 or #2 will often originate from 'mid-level
beginnings'
where updraft stretching causes the greatest cyclonic spin
(vortex
having 'descending characteristics.' This is a
key evolution
in classifying this storm as a supercell (per discussions with
Burgess 1999).
- The storm-relative velocity imagery of the June 18, 1998 HP storm
from 0200 and 0205 UTC showed an unbalanced velocity couplet
(Circulation #2).
The mesocyclone is located within the weak reflectivity notch
(HP storm's eastern (updraft) flank).
Rotational Velocity (Vr) trace of Mesocyclone core #2.
Rotational Velocity (Vr)
Time-Height trace of the 2th circulation
18 June 1998. Magnitudes of Vr are in m/s.
- After Circ #2 reached
it greatest height and strongest cyclonic
shears at 0154UTC...reflectivity magnitudes along the line weakened
as the line segment began to accelerate. Rear Inflow notches,
signifying entrainment of lower theta-e air were identified along
the trailing flank of the northern part of the convective line.
- First reports of
damaging winds occurred across northern Washington
county Missouri after 0205 UTC (south
of the HP storm).
- It is common for swaths of damaging winds to occur immediately
south of the HP storm and its associated mesocyclone (along the
leading edge of the weaker reflectivity line segment.
- Tornadoes producing (F0 - F2 damage) may be associated
with core #1 (or #2) or even subsequent cores.
- Vr magnitudes of Circ #2 intensified a second time within the
1 and 3 km layer between 0210 and 0220 UTC (Vr = 18 - 20 m/s
2-3 km layer) as the vortex continued to collapse. Such
intensification may enhance the downward transport of momentum
from the storm's mid-level region to near the surface.
0215 UTC 18 June 1998 reflectivity (base velocity (right) from KLSX (0.5° slice).
- Bowing of the convective
line continued well through 0315 UTC
(75 to 120 km) southeast of KLSX. Magnitudes of base velocity
exceeded 40 m/s over parts of southeast Monroe...southern St. Clair
...northern Randolph and western Washington counties in southwest
Illinois. Circ #2 weakened considerably at 0225 UTC...yet
damaging winds continued through 0330 UTC (over 1 hour after
the demise of the vortex).
0321 UTC 18 June 1998 reflectivity (base
velocity (right)) from KLSX (0.5° slice).
0321 UTC 18 June 1998 reflectivity
(base velocity) cross-sections from 0321 UTC 18 June 1998
from 120°/15 nm - 120°/72 nm.
- A large swath of
damaging winds occurred along the southern periphery
of Circ #2 across northeastern Washington through the southern half of
Jefferson county (south of STL) from 0204 through 0230 UTC and points
further east and south across southwest Illinois. (the greatest degree of
damage occurred southeast of STL over southern St. Clair...northern
Randolph and western Washington counties in southwest Illinois - estimated
winds of 70 - 80 kts).
(2) SUMMARY:
- Of the 19 severe wind
MCS events we surveyed, we were able to
categorized 15 of them into four types (categories). There were
4 severe wind MCS events we placed in the 'OTHER' category.
- External boundaries or
quasi-stationary frontal boundaries intersected
a convective line in three of the four TYPES (categories)
(Type 1- external boundary intersecting the northern end of the line
segment; Type 2 - external boundary intersecting the central or south
-central part of the line segment; Type 4 - external boundary
intersecting the updraft flank of an HP supercell).
- These boundaries appeared to have a role in the development of the
first two convective-scale vortices associated with the system.
Our sample showed that the 2nd convective-scale vortex revealed
stronger shears (Vr magnitudes) and greater depths compared to
the first circulation. The external boundaries appeared to serve as a
source of local horizontal vorticity which aided in the development
of the first two circulations.
- In 7 of the 15 cases surveyed (47%), weak F0 / F1 tornadoes
were associated with the 2nd core.
- 3rd and subsequent circulations formed near the apex of the
bowing structure south of the first and second cores.
- There were four events where the MCS did not intersect an external
boundary or quasi-stationary frontal boundary. Weak tornadoes
occurred in two of the four events (due to an isolated cell - convective
line mergers). In the two tornadic cases, the depth of the cool layer
of air appeared to be quite shallow (less than 0.5 km deep).
- We were able to develop
prelimiary conceptual models based on
the cases studied.
(3) REFERENCES
Burgess, D.W., R. R. Lee, S.S.
Parker, D.L. Floyd, and D.L. Andra Jr., 1995:
A study of mini-supercells observed by WSR-88D radars.
Preprints,
27th Conf. on Radar Meteorology. Vail CO. Amer.
Meteor. Soc. 4-6.
Houze, R.A., S.A. Rutledge, M.I. Biggerstaff, B.F. Smull, 1989: Interpretation
of Doppler weather radar displays of mid-latitiude
mesoscale convective
systems. Bull. Amer. Meteor. Soc., 6, 608-618.
Johns, R.H., and W.D. Hirt, 1987: Derechos: Widespread convectively induced
windstorms. Wea. Forecasting, 2, 32-49.
Maddox, R.A., L.R. Hoxit, C.F. Chappell, 1980: A study of tornadic thunderstorm
interactions with thermal boundaries. Mon. Wea. Rev., 108,
322 - 336.
Markowski, P.M., E.N. Rasmussen, J.M. Straka, 1989: The occurrence of
tornadoes in supercells interacting with boundaries during
VORTEX-95.
Wea Forecasting, 13, 852 - 859.
Przybylinski, R.W., T.J. Shea, D.L. Ferry, E.H. Goetsch, R.R. Czys, and
N.E. Wescott, 1993: Doppler radar observations of
High-Precipitation
Supercells over the Mid-Mississippi Valley Region. Preprints,
17th Conf.
on Severe Local Storms. St. Louis, MO. Amer. Meteor. Soc.,
158 - 163.
Rasmussen, E.N., and S.A. Rutledge, 1993: Squall line evolution. Part 1:
Kinematic and reflectivity structure. J. Atmos. Sci., 50,
2584-2606.
__________, J.M. Straka, R.P. Davies-Jones, C.A. Doswell III, F.H. Carr
M.D. Eilts, and D.R. MacGorman, 1994: The
Verifications of the Origins
of Rotation in Tornadoes Experiment: VORTEX. Bull.
Amer. Meteor.
Soc., 75, 997-1006.
Weaver, J.F., S.P. Nelson, 1982: Multiscale aspects of thunderstorm gust
fronts and their effects on subsequent storm development.
Mon. Wea.
Rev., 110, 707-718.
__________, and J.F.W. Purdom, 1995: An interesting mesoscale storm-
environmental interaction observed just prior to changes in
severe
storm behavior. Wea Forecasting, 10,
449-453.
Weisman, M.L., 1993: The genesis of severe, long-lived bow echoes.
J. Atmos. Sci., 50, 645-670.
