QLCS Group 1

Group 1 Cases:

Low-level boundary intersecting the northern part of the larger convective line (Intensifying stage - or early stages of bow echo evolution)  
(Updated info -  December 2009)

(1) Thirteen of the forty-three cases  

Fig 1 - Conceptual Model for Group 1 cases during the 'Intensifying Stage" of QLCS evolution across Mid-Mississippi Valley region (left); and Klimowski's conceptual model for Squall Line - Bow Echo (SLBE) (merger and boundary) (right side).  Klimowski et al. 2000

- Nine cases occurred in the late afternoon or evening hours.
- Four cases occurred during the early to mid morning.

                                                                      From Atkins et al. 2006

a) Reflectivity and Doppler velocity characteristics

A bow echo system moved across parts from eastern Missouri through south-central Illinois during the late afternoon of 10 June 2003.  This convective system occurred during the Bowecho and MCV experiment (BAMEX) covering much of the middle United States. The 10 June 2003 St. Louis Bow Echo shown above (Fig.2) by Atkins et al. (2005) revealed a four stage evolution of the bow echo system.  The early stages shows two clusters of thunderstorms over central Missouri at 2059. One cluster of storms were located southeast of the Columbia Missouri Airport (COU) while the second cluster was south of Jefferson City Missouri (JEF).  Around 2159 (UTC) (all times UTC). isolated cells form along a surface convergence boundary between the northern cluster and a developing bow echo to the south.. These cells rapidly merged, intensifying the system cold pool and formed to a nearly linear convective line at 2300. A few weak intensity mesovortices formed with the northern cluster at 2159 while one non-tornadic and one-tornadic mesovortex formed with the developing bow echo to the south (Bow Echo #1).  

Between 2230 and 2300 we observe the formation of a nearly solid convective line echo (Intensifying Stage).  Two small bowing segments have formed within the larger convective line. A strong low-level reflectivity gradients extended along the leadirng edge of the line indicating a strong updraft-downdraft interface. Aloft, the beginning's of the mesoscale Rear Inflow Jet (RIJ) was revealed by SRM Doppler velocity data from near the back edge of the convective line and developed rearward with time (not shown).  

Between 2235 and 2245, the first three of five mesovortices (MV 6, 7, and 8) formed along the QLCS during this period
from southern St. Louis County and south into Jefferson County Missouri. Two of the three MVs spawned tornadoes over far western part of St. Clair County Illinois (MV 6 - 7 km just south of downtown St. Louis and the second near the Monroe - St. Clair County line). A fourth strong mesovortex formed at 2256 just east of downtown St. Louis, spawned a tornado (high end EF1) near Caseyville Illinois (approx 9 km east of downtown STL). This tornadic vortex was located in the vicinity of a reflectivity inflection point over at the northern part of one of the two smaller bowing segment and a solid convective line extending to the northeast.  Based on witnesses descriptions, the tornado associated with MV 9 was partially rain wrapped. The mesovortices appeared to form just behind the surface gust front in a they traveled from St. Louis and Jefferson Counties into St. Clair county IL.  (All of this was occurring during peak rush hour in the downtown St. Louis area).  MV 6 formed just north of the apex of the small northern bowing segment within the larger line and spawned two tornadoes. MV 9 formed formed near the reflectivity inflection point over the northern part of St. Louis City and spawned a tornado (high-end EF 1) near and west of Caseyville Illinois. Ray Wolf (SOO WFO DVN) and our group have documented non-supercell tornadoes to occurr near these reflectivity inflection points or where a surface boundary intersects the northern part of the convective line.

Recently, Atkins and St. Laurent (2009) have shown a different tornado-genesis mechanism in constrast to work completed by Trapp and Weisman (2003) in mesovortex formation. They showed that the northerm members individually as well as a northern member associated with couplets were cyclonic while the southern member was anticyclonic.  Simulations completed by Trapp and Weisman (2003) showed a reverse pattern with couplet pairs in which the southern member was cyclonic and the northern member was anti-cyclonic.

                                                                              
                                                                                    From Atkins and St. Laurent 2009

Fig. 2 - (a) Planview reflectivity image (0.5° slice) at 2303 UTC 2 July 1992 from WFO LSX. (b) Planview
reflectivity image (0.5° slice) at 0402 UTC 25 May 2004. Check out 25 May 2004 case below under Case Studies. . (Click on image for a larger image). 

Another example of bow echo - surface boundary intersections occurred outside the WFO LSX County Warning Area (CWA) over central Oklahoma on 5-6 September 1992.  The surface boundary or outflow from the north intersected the northern part of a bow echo where there was a weak reflectivity notch. Stumpf and Burgess (1993) used dual-Doppler analysis to chart the tracks of four misocyclones (M1 through M4) from west of Norman through the Norman OK area (Fig. 4a).  From Fujita (1981) a misoscale vortex would be classified of having a core diameter between 0.04 - near 4 km, while a mesovortex core diameter would be at or greater than 4 km.  Recently Trapp and Atkins (2008) redefine the mesovortex as a vortex which ascends and grows upscale with time. This would include misovortices and mesovortices from Fujita's earlier definition. .

    a.                       b.

Three of the four misocyclone tracks were associated with wind damage. Wind damage was associated with M1, while damage was not reported with M2. Both M3 and M4 traveled through the Norman OK area and produced individual swaths of damage. Figure 3b shows that M3 was located in the  reflectivity notch and the intersection of a southward moving east-west surface boundary and the eastward moving gust front associated with the bow echo. The damage swath of M3 ended about 1 km east of OU Research Park while M4 continued to produce damage through the city of Norman to four miles east of Norman OK. Of the four misovortices, M4 did exhibit some rotational damage characteristics compared to the other three. A ground survey was conducted while no areal survey was completed with case. It is interesting to note that the misovortices in this case extended to a height of around 2.5 km.  Recent studies have shown that moderate to strong mesovortices (Atkins et al 2004; Atkins et al. 2005; Przybylinski et al. 2000); Przybylinski et al. 2006)  can extend to heights of 5 to 6 km if not greater at times.



b) Mesovortex Location and Characteristics

- In our study we found that QLCS tornadoes (F0 - F2 damage) were correlated with mesovortex cores 2 and 3 in one case, core 3 in a second case and mesovortex cores 2, 3 and 4 in two other cases. These mesovorticies formed at low-levels from just north of the apex of the bowing segment to the northern end of the bowing structure.  

You may ask the question what is a mesovortex and how does it differ from mesocyclones in supercells. Mesovortices often form along the leading edge of a convective line at low-levels (0.5 - 2.0 km layer)  mainly during the early stages of a bow echo where crosswise vortex lines are tilted upward by convective-scale downdrafts (Trapp or Weisman 2003 - Part 2). In supercells the origins of mesocyclones forms at mid-level beginnings. Numerical simulations shown by Trapp and Weisman revealed downward tilting of the vortex lines will result in a cyclonic member south of the region of downdraft air and a anticyclonic member north of the region of downdraft air. Nolan Atkins and I have studied Doppler velocity data at ranges within 100 km from a number of cases and was unable to find the anticyclonic member.  Recent numerical simulations completed by Atkins and St. Laurent 2009, have shown that a region of a updraft along the leading edge of the linear to bowing convective line where vortex lines are tilted upward stretched.  In Atkin's simulation parcels may originate from the east of the convective line or even north of a northeast-southwest surface boundary.  In this case streamwise vorticity may play a role north of the surface boundary. The parcels along the boundary are never stretched. Atkin's numerical simulation shows parcels slightly descending towards the cyclonic vortex core which is on the cooler and north side of the region of updraft air.  A weak anticyclonic member may show up on the southside of the region of updraft air. 

 - Moderate Shear ( 15 m s-1 and greater -  within the 0-3 km layer) / high CAPE was present in all four cases.

- In the remaining 6 cases studied, two or three mesovortices were documented along the cyclonic shear side of the bow (from the apex northward).  Similar to above, Core #3 formed as much as 5 to 10 minutes after the initial indentification of Core #2. Of these 6 cases tornadoes (EF0 - EF2) occurred in two cases. The tornadoes were correlated with mesovortex core #2 (northern end of the bowing structure) in one case and cores #2 and #3 in a second case. In the remaining four of the six cases there was an absence of any tornadic activity. 

- In nearly all of the cases, mesovortex core 3, and subsequent mesovortex cores moved in the direction of the bow echo and did not merge with the larger northern line-end vortex. This is similar to results shown by Atkins et al. (Mon. Wea. Rev. Vol 132 2004).

In 9 out of 12 cases, the first and second mesovortices (Cores 1 and 2) formed in the vicinity of or just south of the surface boundary - convective line intersection. In 7 of the 10 events, isolated cells were in the vinicity of the surface boundary were present. In a number of these cases Cores 1 and 2 formed at or just south of the intersection just after merger between the isolated cells and larger convective line (preferred region of enhanced low-level convergence - vertical stretching of the updrafts).  In two other cases, Cores 1 and 2 formed near the vicinity of a fineline - convective line intersection (no apparent cells mergers were noted). In the remaining case, the first and second core formed in the vicinity of a warm frontal boundary (no isolated cells or fine lines were documented).

- In 4 of the 10 cases studied, numerous convective-scale (gamma-scale) vortices (4 or more cores) were documented along the cyclonic shear side of the bow (from the apex northward).  Core #3 and subsequent mesovortices formed as much as 5 to 15 minutes after the initial identification of Core #2.  However, there in one case, a cyclonic mesovortex also formed south of the apex of the bow.
 

c) Mesovortex Trend Info

- Non- descending characteristics.

- Core #2 - over the northern part of the convective line (but southeast of Core #1) revealed significantly stronger Vr shears, greater depths, non-descending characteristics, and longer lifetimes compared to Core #1. This core rapid ascends with height while the strongest rotation remains at low-levels (lowest 2 - 3 km) within the first 20 to 25 minutes of the mesovorticies lifetime.  The strength of the low-level rotation often intensifies during this period and Vr magnitudes may exceed 25 m s-1. The mesovortex may reach heights exceeding 8 km. Core #2's lifetime will often exceed 60 minutes (2 cases - 70 minutes). Tornadogenesis frequently occurs during just prior to the mesovortex reaching its greatest height while low-level rotation is strongest during this time.

- Core #3 and subsequent cores - (near or north of the apex - along the cyclonic shear side of the bow) also revealed mainly 'non-descending characterisitcs' and displayed similar Vr magnitudes and nearly equal depths to mesovortex core #2. However, many of these mesovortex cores revealed shorter lifespans compared to Core #2. In two cases, where mesovortex cores #3 and #4 were tornadic, the location of these circulations appeared to be similar to the location of mesovortices shown in numerical simulations by Trapp and Weisman (2000) (QLCS tornadoes).

- Our studies showed that strong mid-level rotation found in supercells was absent during t he early stages of mesovortex cores #2, #3, and #4. In many cases, the strongest rotation was found within the lowest 2 - 3 km during the later part of the organizing stage and into the mature stage of the circulation's lifespan.

- In some cases, our investigations also showed that once Core #2 evolved into the Mature Stage (MS) of mesovortex evolution, the circulation appeared to enhance the flow , around the southern periphery of the vortex, resulting in line acceleration and potential enhanced wind damage just south and southeast of Core #2's track. Similar observations were shown in mesovortex cores #3 and #4.

- In two of the four cases documented when numerous cores were present, tornadoes (F0 - F2) occurred during the very early stages of mesovortex evolution (mainly with Cores 3 and 4), and during the later part of the mesovortex core's Organizing Stage (OS) (just preceding the core's greatest depth Cores #2, 3 and 4)). In the other two cases tornadoes (F0 - F1) occurred during the later part of the Organizing Stage (OS) and or early part of the Mature Stage (MS).  Core #1 - northern most vortex generally showed weaker magnitudes of Rotational Velocities (Vr) and shallow depths compared to Core #2 which was located east or southeast of the first core.  Core #1 often showed

Case Examples (click on dates for summary of case):

2 July 1992 Bow Echo
29 June 1998 Derecho
29 June 1998 Line-end Vortex Evolution
24 May 2004 QLCS (WES case)

Table 1 - Group 1 QLCS events
 

Comparison of mesovortex characteristics for 10 QLCS cases (Group 1 events).  D signifies descending mesovortex, ND represents Non-descending mesovortex, and T = Tornado occurrence.
 

Table 1 shows that all of the tornadic bow echo cases occurred in moderate to strong shear environments with ML CAPE values generally exceeding 3000 J/Kg. Case #9 is an event which occurred in April while all of the other cases occurred from the later part of May through August. Case #3 is the only case where tornadoes were absent in the presence of moderate shear and high CAPE values.
 

Fig 3 -Traditional supercell mesocyclone characteristics averaged for height and stage of evolution: a) Organizing Stage (OS); b) Mature Stage (MS); and c) Dissipating Stage (DS).  Characteristics of rotational velocities (V) in m s-1, core diameter (D) in km and shear (S) in 10-3 s-1 units.Circled values are number of cases used in each average (from Burgess et al. 1982).(Click on image for a larger image)
 

Tables similar to Burgess's et al. (1995) study, were constructed.  Comparisons between the two data sets were completed and are shown below.
 

Organizing Stage

Table 2 - Above - Characteristics of 1st and 2nd core mesovortices (10 cases) associated with Group 1 QLCS events. Vr represents the magitude of Rotational Velocities.
 

Table 3 - above - Results from Burgess's study (Including Traditional / Mini Supercells) (Burgess et al. 1995).

Table 4 -Characteristics of 1st and 2nd core mesovortices (10 cases) associated with Group 1 QLCS events. (MS Low (L) - represent 0.5° slice). L represents 0.5° slice.
 

Table 5 - Results from Burgess's et al study (Including Traditional / Mini Supercells).
 (Burgess et al. 1995)
 

e) Preliminary Findings

a. When comparing Group 1 - 1st and 2nd core mesovortices during the OS, the 2nd cores revealed stronger rotational velocities (Vr) at low-levels and throughout the depth of the vortex (see Table 2a). Second core circulations exhibited slightly larger diameters compared to first core diameters.

b. In comparing 2nd core Vr magnitudes during the OS between our data set and Burgess's results, 2nd core circulations with convective lines exhibited overall weaker Vr values compared to Vr magnitudes associated with traditional supercells. 2nd core diameters associated with convective lines were slightly larger compared to traditional supercell core diameters. Remember the strongest rotation associated with 2nd core mesovortices with convective lines nearly remains below 3 km and the vortex deepens with time. Tornadogenesis often occurrs just prior to the time of greatest vortex depth and strongest rotation at low-levels.

c. When comparing Group 1 - 1st and 2nd core mesovortices during the Mature Stage (MS), 2nd core circulations revealed stronger Vr (at low-levels and overall depth) compared to 1st cores. This is why tornadogenesis would have a higher probability of occurrence with second and successive cores compared to 1st core circulations. During the early part of the MS, 1st and 2nd core diameters were quite similar. However, in many cases, 2nd core mesovortices often grew upscale and obtained larger diameters compared to their first core counterparts.

d. When comparing 2nd core mesovortices associated with convective lines to traditional supercell mesocyclones, 2nd cores Vrs were weaker at low-levels and overall depth. Additionally, convective line 2nd core diameters were larger compared to traditional supercell mesocyclone core diameters.

e. When comparing overall core depth, traditional supercell cores exhibited slightly higher depths compared to convective line 2nd core mesovortices.