Radar Overview - 25 May 1996
25 May 1996 MCS event - a case of multiple isolated cell - convective line mergers.
Circulations tracks across
east-central Missouri, for 25 May 1996 MCS. Time
of initial circulation detection is depicted in UTC and denoted by marker specific
to each circulation track every 6 minutes.
During the period when the MCS
moved across east-central Missouri , a number of vortices
(many non-tornadic) evolved all along the leading edge of the line. Except for Circ 7, all
of the circulations exhibited cyclonic shears. In this presentation we will key upon
the evolutions of
Circulations 1, 3, and 5.
Plan view of KLSX WSR-88D reflectivity image at 0.5° slice for 2131 UTC.
- The first merger between an
isolated cell and the convective line occurred 15 nm
northeast of Columbia (COU) Missouri between 2142 and 2148 UTC. Preceding merger,
storm-relative velocity data showed a weak vortex (Circ #1) located along the cyclonic
shear side of the line segment. Circ #1 exhibited magnitudes of 11 to 13 m s-1 prior to
merger.
Plan views of KLSX WSR-88D
reflectivity / storm-relative velocity images at 0.5° slice
for 2148 UTC. (Just after isolated cell - convective line merger).
- Circulation 1 deepened and intensified during and just after merger, and evolved into
a vortex having a broad diameter.
Four panel presentation of WSR-88D
reflectivity / storm-relative velocity image from KLSX
at 2148 UTC. Panel 1 and 2 are plan views at 1.5° slice while panel 3 and 4 are at
0.5° slice.
After Circulation 1 deepened and
intensified, the SRM images at 0.5° and 1.5° slices show
that the strongest cyclonic shears remain in the lowest elevation slice.
Time-height series plot of rotational velocities (Vr; in m/s) of Circulation #1.
Time-height series plot of
Circulation's #1 diameter versus time .
- Just after isolated cell-convective line merger, Circulation #1 intensified but the
core diameter broadened to a diameter greater than 10 km.
- Magnitudes of Vr intensified to 18 m/s first at low-levels (aob 3 km) then at
mid-levels (4 - 6.5 km) during the subsequent volume scans.
- Circulation #1 appeared to play a
role in enhancing the downward transport
of momentum from mid to low-levels.
- The convective line segment south of Circ #1 accelerated from 15 m/s to
19 m/s after merger.
Plan views of KLSX WSR-88D
reflectivity images at 0.5° slice for 2206 and 2218 UTC
respectively.
- A second isolated cell formed
over southeast Audrain County between 2155 and 2200 UTC
downwind from the accelerated convective line segment. Both images above show the
merger between the convective line and isolated cell (300° radial 50 nm).
- Other convective cell mergers
occurred over northern Montgomery County MO (south of
the isolated cell - line merger.
Magnified view of the the 2218 UTC
WSR-88D reflectivity / storm-relative velocity images
(0.5° slice).
- The storm-relative velocity image
shows the growth of two new vortices (Circulations #3
and #5) near and just north of the isolated cell - convective line merger (eastern Audrain
county Missouri). Circulation 3 formed just north of the merger at 2206 UTC while
Circulation 5 developed at 2218 UTC along the southeast flank of the isolated cell.
- Circulation #3 rapidly deepened
after 2218 UTC and evolved into a strong mesocyclone.
This vortex formed in a region of strong horizontal shears near the merger and was
responsible for the production of damaging winds at this stage of its lifecycle over
eastern
Audrain county.
- Circulation #5 reached
mesocyclone criteria, however it exhibited weak cyclonic shears
throughout much of its lifecycle. No wind damage was reported throughout the lifetime
of this vortex.
Plan views of
reflectivity / storm-relative velocity images at the 0.5° slice at 2242 UTC from
KLSX.
- Circulation #3
reached its maximum cyclonic shears (Vr values 23 m s-1) between 2236
and 2248 UTC. Storm spotters reported a tornado at 2240 UTC over southwest Pike county
Missouri (associated with Circ #3). Do
you see anything interesting with the storm-relative
velocity pattern at this time?
Six panel view of
reflectivity / storm-relative velocity images for the lowest three
elevation slices (0.5° / 1.5° / 2.4°) at 2242 UTC.
- The above
reflectivity / storm-relative velocity images show Circ #3 deforming
the reflectivity pattern (redistributing the precipitation field) near the center
portion of the convective line.
- Is this storm a supercell? (contact me for the answer)
Time-height series
plot of rotational velocities (Vr: in m/s) of Circulation #3.
W signifies time of wind damage. T represents tornado occurrence.
- Note the rapid
deepening of Circ #3 after 2218 UTC, yet the strongest cyclonic
shears remain within the lowest elevation at this time. The strongest cyclonic
shears are detected during the period of 2236 to 2248 UTC (23 m s-1) within the
lowest 6 km of the vortex.
Time-height series
plot of Circulation #3's diameter versus time. Throughout much of
Circulation #3's lifespan the core diameter remained below 6 km. During the time of
tornado touchdown the overall vortex core dropped to 5km or less. Note the
differences in Circ #3's core diameter compared to
Circ #1.
Plan views of WSR-88D
reflectivity / storm-relative velocity images at 0.5° slice for
2259 UTC from KLSX.
- Circulation #3
continues to reveal strong cyclonic shears (21 m s-1) at this time
within the lowest 3 km. The tornado lifted between 2253 and 2259 UTC. Note the
hook-shaped reflectivity pattern (55 dBZ) associated with this vortex. Damaging
winds continued to occur with Circ #3 well through 2310 UTC.
- Circulation #8
observed 10 km south of Circ #3 initially formed at 2248 UTC. This
vortex exhibited an imbalanced velocity structure throughout much of its lifecycle.
One wind damage report was linked to this vortex at this time (2259 UTC)
Summary:
- We observed two
isolated cell - convective line mergers with this severe weather event.
a) the first merger resulted in the intensification of Circ #1 along with an increase in
the
core diameter. This vortex appeared to aid in accelerating the convective line segment.
b) the second isolated cell - convective line merger resulted in the growth of a strong
mesocyclone (Circ #3) (near the northern end of the merger) spawning a weak tornado
(causing F1 damage). It is interesting to note that Circulation #3 remained
near the apex
of the convective line segment through 2300 UTC.
There is a range of
possibilities that can lead into tornado formation along the leading edge
of convective lines and line segments. We have shown one such scenario. This is one of
three cases we have documented since 1992 where tornadic and non-tornadic vortices
formed in response to isolated cell - convective line mergers.
Reference:
Schmocker G.K., R. W.
Przybylinski, Y.J. Lin, 1998: A Doppler radar analysis of the 25 May 1996
squall line event across east-central Missouri and southwest Illinois. Preprints, 16th
Conf. on
Weather Analysis and Forecasting. Phoenix AZ, Amer. Meteor. Soc. 236-239.
