Case #1:  July 2nd, 1992 Bow Echo event 
over central and eastern Missouri


The initial cluster of storms formed southeast of Kansas City and traversed eastward along
an old surface boundary generated by a previous multicell convective cluster which moved
across parts of east-central Missouri and southwest Illinois.  At 2100 UTC, the surface boundary

At 0000 UTC 3 July 1992 the magnitude of ML CAPE at Monett Missouri (UMN - southwest of
Springfield MO) exceeded 4500 J/Kg while values of 0-3 km Bulk Shear were 14 m s-1
(Moderate Shear category).  Further north, the Lathrop profiler at 1900 UTC revealed 0-3 km
Bulk shear magnitudes of 15 m s-1.



Fig 1. Skew-T Log-P diagram of Monett MO (UMN) at 0000 UTC 3 July 1992
from the SHARP program. (Click on image for a larger image)

 

 

 
Fig. 2. Plan view reflectivity (0.5° slice - left) and storm-relative velocity (0.5° slice - right) from
KLSX 2321 UTC 2 July 1992 QLCS over central Missouri.  Mesovortex cores 1 and 2 formed
in the vicinity of isolated cells - convective line mergers near the northern end of the developing
bow echo and in the vicinity of the old surface boundary.  Mesocyclone Core #2 spawned several 
non-supercell tornadoes between 2321 and 2338 UTC over northwest through north-central and central Boone
county Missouri. (Click on images for a larger image)
 

 

Fig 3. Graph of the wind speed at WSO Columbia MO on 2 July 1992. 


The above graph of the wind speed shows the initial surge occurring at 2337 UTC (22 m s-1) 
followed by a close secondary surge occurring at approximately 2343 UTC. 
Studies conducted by 
Goff 1976 and Klingle (1985) have documented the presence of secondary surges behind the 
leading gust front associated with convective lines.  Secondary surges often exhibit similar 
characteristics associated with the passage of the initial gust front (i.e. abrupt changes in wind 
speed and direction, temperature drops etc). Little is known about these disturbances, beyond the 
fact that they do exist. The schematic diagram below shows a conceptual model of the initial and
secondary surges associated with convective line. 

 

Fig 4. Schematic diagram of the vertical structure of a thunderstorm outflow
      and gust front. Motion of the gust front is relative (from Goff 1976).
(Click on image for a larger image) 

 


Fig 5.
Plan-view reflectivity (1.5° slice - left and storm-relative velocity (1.5° slice - right)
from KLSX at 0013 UTC 3 July 1992 QLCS over east-central Missouri.


At 0013 UTC the plan-view reflectivity image (left) shows a mature bow echo with a well defined
rear-inflow notch located along the trailing flank of the bow. The higher reflectivities (50 DBz +) 
observed at 2321 UTC were not apparent at this time within the lower elevation slices.  This was
likely due to the system cool-pool over- whelming the ambient shear downshear from the bow echo.
The storm-relative velocity image (right) shows the breadth of the mesoscale Rear Inflow and a well
defined book-end vortex at the northern end of the bow echo. The bookend vortex grew upscale with 
time.  The greatest degree of damaging winds were located along the southern periphery of the path 
of the bookend vortex.  Wind measuring equipment at Mexico Missouri (central Audrain County MO) 
reported gusts to 84 mph.  Other mesovorticies were located along the cyclonic shear side of the
bow during this stage.

 


Fig. 6
Reflectivity cross-section (left) velocity cross-section (right) along the 295° radial
at 0019 UTC 03 July 1992 from KLSX radar. (Click on image for a larger image).


The reflectivity cross-section (left) in Figure 6 shows classic multicell evolution with new convective 
towers developing along the system's downshear (east) and older convective towers towards the 
rear of the system. The storm-relative velocity cross-section (right) shows a mature descending 
mesoscale rear inflow with an opposing mesoscale front-to-rear flow.  Note the local outbound 
velocity maximum embedded with the front-to-rear flow representing a local strong updraft velocity 
maximum.

 


Fig. 7
Rotational Velocity time-height traces of Mesovortices #1 and #2.
 Magnitudes of Vr are in knots.
 


The rotational velocity (Vr) time-height trace of Circulation 1 showed a general descending 
characteristic with the highest magnitudes of Vr between 4 and 8 km around 2315 UTC.  In contrast,
Circulation 2 showed a rapid deeping ('non-descending') structure with the strongest Vr observed 
between 4 and 7 km at 2321 UTC.  Tornadogenesis occurred during and after the period of rapid
non-descending growth.  This is one of our first Vr traces that we constructed to understand 
mesovortex evolution associated with QLCSs.

 


Fig. 8
Damage map (over Missouri) of the 2 July 1992 bow echo event.
(Click on image for a larger image)


References:

Goff, R.C., (1976): Observations of thunderstorm induced low-level wind variations. Preprints,
9th Fluid and Plasma Dynamics Conference. Amer. Institute of Aeronautics and Astronautics,
San Diego CA.

Klingle D.L. (1985) A gust front case studies Handbook. Lincoln Laboratories. Massachusetts
Institute of Technology. Lexington Massachusetts. 108pp.

Return to Group 1 webpage.

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