National Weather Service Weather Forecast Office

Group 1 Cases: Low-level boundary Intersecting the northern part of a larger convective line (Intensifying Stage - MCS)

(1) Ten of the twenty-eight cases studied supported this type of storm conceptual model - reflectivity pattern (e.g. during the stage of linear to early bowing of the convective line.

Fig 1. Conceptual Model for Type 1 QLCS during the Intensifying
Stage of MCS Evolution

a) Reflectivity Characteristics
In nine of the ten events, a line of isolated cells, reflectivity fine line, or surface boundary generated by a downshear supercell is oriented orthogonal and intersects the northern part of the larger quasi-linear convective system. The line of isolated cells reflects the location of the low-level boundary. However, in one case, there was no discernible radar reflectivity feature which suggested the presence of a surface boundary other than surface mesoscale analysis.

- Several cases showed the presence of strong low-level reflectivity gradients along the
leading edge of the larger convective line indicating the location of the convective line's
organized updraft region.

- In all ten cases, the first reports of damaging winds occurred south of the low-level
boundary - convective line intersection (just preceding bowing of the larger convective line).
The MARC velocity signature was identified in the "pre-bowing stage" in nine of the ten cases studied. The MARC velocity signature was detected mainly south of the intersection where the overall convective line showed nearly linear characteristics. This type of reflectivity pattern is similar to observations documented by Przybylinski and DeCaire (1985) and Smith (1989).

Examples:

b) Mesovortex Location and Characteristics

- In 9 out of 10 events, the first and second mesovortices (Cores 1 and 2) formed in the vicinity of just south of the low-level boundary - convective line intersection. In 7 of the 10 events, isolated cells anchored near 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 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 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.

- Tornadoes (F0 - F2 damage) were correlated with Cores 2 and 3 in one case, Core 3 in a second case, and Cores 2, 3, and 4 in two other cases.

- Moderate Shear (0-3 km) / 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.
- Tornadoes (F0 - F2) damage) were documented in only 2 of the 6 cases. Tornadoes were correlated with Core #2 in one case, and Cores 2 and 3 in a second case. In the remaining 4 cases, there was an absence of tornadic activity.

- In nearly all cases, mesovortex core 3, and subsequent cores moved in the direction of the bow echo and did not merge with the larger line-end vortex.

c) Mesovortex Trends

- Core #1 - generally weaker magnitudes of Rotational Velocities (Vr) and shallow depths compared to Core #2 and subsequent cores.
Core #1 often showed descending characteristics. - Core #2 - revealed significantly stronger Vr shears, greater depths, non-descending characteristics, and longer lifetimes compared to Core #1. This core rapid deepens and intensifies within the first 20 to 25 minutes of the circulation's lifetime. The mesovortex may reach heights exceeding 8 km. Core #2's lifetime will often exceed 60 minutes (2 cases -
70 minutes).

- 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 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.

- 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).

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

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: Organizing Stage (OS); Mature Stage (MS); and 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

Mature Stage

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.