Preliminary Analysis of Convective Modes for HSLC Tornadoes

Preliminary analysis of convective modes for the radar climatology portion of the HSLC project is now complete. Over 100 days between 2006 and 2011 were identified by area WFOs as possible HSLC events. Tornadoes that occurred in an environment with surface-based CAPE (SBCAPE) less than 500 J/kg were included in the analysis. SBCAPE was determined from gridded hourly SPC mesoanalysis data. 302 tornadoes on 59 of the days in the HSLC case list met this criteria. As some days may have contained HSLC environments in some areas of the domain but higher CAPE environments in other locations, this helps to ensure that only truly HSLC tornadoes were included.

Convective mode information was obtained for these tornadoes from the Storm Prediction Center’s (SPC’s) convective mode database. Due to initial spatiotemporal filtering applied to the database, convective mode information was only available for 226 of the 302 HSLC tornadoes. Convective mode was manually assigned retrospectively to these tornadoes by SPC forecasters. Storms were classified as either supercells or non-supercells. A storm was classified as a supercell based on the presence of an area of rotation that met mesocyclone criteria, as well as other radar reflectivity signatures present in volumetric radar data. Sub-classifications for supercells included discrete supercell, supercell in cluster, and supercell in line. Non-supercells that met quasi-linear convective system (QLCS) criteria were classified accordingly. Note that tornadoes classified as QLCS tornadoes therefore were not associated with mesocyclones. Tornadoes in a convective line that were associated with a mesocyclone were classified with the “supercell in line” designation. More information on this classification scheme can be found in this conference paper: It was also the subject of a recent webinar.

 The following are some preliminary findings from investigating the convective modes database. Figure 1 shows the distribution of convective modes for the 226 HSLC tornadoes in our dataset. The majority of HSLC tornadoes were associated with supercells, but there was a substantial fraction of HSLC QLCS tornadoes. Discrete supercells made up the smallest percentage of HSLC supercell tornadoes, while supercells in clusters and supercells embedded in convective lines made up the majority of HSLC supercell tornadoes.

 Figure 2 shows the breakdown of HSLC tornadoes by county warning area (CWA) for the cases included in the dataset. The Peachtree City, GA and Huntsville, AL offices had the greatest amount of HSLC tornadoes in their CWAs, according to this figure. When these values are normalized by the area of the CWA, it is found that the Huntsville, AL CWA has the most HSLC tornadoes per 10,000 square kilometers in the dataset.

In figure 3, the percentage of each CWA’s HSLC tornadoes that were supercells is shown in the left panel, and the percentage of each CWA’s HSLC tornadoes that were from QLCSs is shown in the right panel. Interestingly, this plot shows that most of the HSLC tornadoes in the Sterling, VA CWA were associated with QLCSs, while most of the HSLC tornadoes in other CWAs were associated with supercells. Also of interest, the CWAs located closer to the coast tended to have the greatest percentage of supercell HSLC tornadoes. It should be noted, however, that the sample sizes for some of the CWAs were fairly small.

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3 Responses to Preliminary Analysis of Convective Modes for HSLC Tornadoes

  1. Jonathan Blaes @ WFO RAH says:

    I was a little surprised by the large fraction of HSLC tornadoes produced by supercells thinking that there would be a greater number of QLCS (non-mesocyclone) tornadoes. Part of this may be a personal bias of mine that reserves supercell classification to a smaller fraction of storms that show longer lived rotation. Another potential explanation is that the authors use the old mesocyclone classification nomogram that originates back in the mid/late 90s. That nomogram was based on more coarse WSR-88D data before the advent of super-resolution and other enhancements which likely result in larger velocity values that could trigger a larger identification of mesocyclones and supercells.

    The breakdown of HSLC tornadoes by county warning area is interesting. In examining the tornadoes with SBCAPE < 500 J/Kg for each normalized CWA, 4 of the 5 CWA's with the lowest frequency are "interior" or "northern" offices – RNK and ILM (2), RAH (3), GSP and LWX (4). I would have thought that these offices would tend more frequently experience marginal instability compared to the more southern offices. In fact, the 'southern tier" offices – HUN, FFC, CAE, CHS acount for 64% (50/78 total) low CAPE tornadoes. In this data set at least, tornadoes with SBCAPE < 500 are not a Piedmont problem nearly as much a problem across more southern locations.

    • Jason Davis says:

      The large percentage of HSLC tornadoes from the southern offices was a bit surprising. It would be interesting to try to obtain the convective modes database for all tornadoes in order to see if some of these results are similar when looking at other types of tornadoes, and to see what percentage of each CWA’s tornadoes are HSLC.

  2. Matt Parker says:

    Non-comment so I can receive e-mail notifications!

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