The Development of a HSLC Composite Parameter

During this morning’s conference call, I presented some preliminary verification results from a composite parameter being designed to improve detection of significant severe weather within high shear, low CAPE environments. The powerpoint presentation is attached.

Here are some notes regarding the parameter, its benefits, drawbacks, and utility for HSLC (any possibly other) severe events:

  • Last month, I provided a preliminary equation for the Severe Hazards in Environments with Reduced Buoyancy parameter (SHERB) which included the effective shear magnitude. In the most recent version, I have used the 0-3 km shear magnitude instead.
  • Additionally, the normalization of the parameter is such that the optimal (most skillful) value is 1, consistent with the optimal value of the STP. Values above 1 are more likely to be associated with significant severe convection than non-severe convection.
  • The 0-3 km layer appears to be representative for HSLC cases. Some preliminary modeling simulations indicate a typical depth of 3-5 km for storms using HSLC composite soundings.
  • Using the 0-3 km layer rather than the effective layer eliminates any dependency on CAPE, which is subject to error propagation in model output and SPC mesoanalysis fields. A recently published article by Coniglio showed that 25% of SBCAPE readings in SPC mesoanalysis (when compared to observed soundings) were overestimated by 450 J/kg or more, while another 25% were underestimated by at least 150 J/kg. With each derived term–such as the effective shear and effective SRH, which are both used in the STP–there is further risk for propagation of error.
  • Previous studies have indicated that EMLs are an important factor in substantial severe weather outbreaks in the eastern U.S. Other studies have shown that low-level instability is especially relevant in HSLC events. These are represented by the 700-500 mb lapse rate and 0-3 km lapse rate, respectively.
  • By just using a lower threshold for STP or VGP, the false alarm area will increase substantially.
  • Arguably the most important point and the biggest reason for using the SHERB is that it is designed to detect significant tornadoes and significant winds.
  • Many of the drawbacks for the SHERB may not necessarily affect its skill in HSLC environments, but rather would be reasons for concern if the parameter were utilized nationally. For instance, deep and dry boundary layers (leading to near-adiabatic lapse rates) in marginally sheared environments may result in false alarms. In weakly sheared environments (i.e., summertime pulse severe convection), there may be misses of significant microbursts.
  • The True Skill Statistic was used for two reasons: first, it was used by Thompson et al. to determine the optimal value and skill of the STP; second, the TSS rewards correct nulls, which is an important component to testing the performance of parameters (in contrast to warning operations, when correct nulls cannot be accounted for).
  • The “case study” provided at the end of the presentation tracks the hour-by-hour SHERB fields against the severe reports for the following hour (e.g., the SHERB plot for 2200 UTC/10 PM is compared to the reports between 2200 UTC-2300 UTC/10 PM-11 PM). This is the March 4-5, 2008 event.


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2 Responses to The Development of a HSLC Composite Parameter

  1. Jonathan Blaes @ WFO RAH says:

    Nice work Keith. This is very exciting and promising. Once this parameter stabilizes, seeing another case study and/or providing data for inclusion in the WFO lead case studies will be very valuable.

    It appears this will be an interesting parameter to train users about since it is restrictive to HSLC events and once you are out of that situation the applicability becomes very limited and could lead to inappropriate use. If results continue to be favorable, a question for implementation may be its utility outside out of our immediate region. This may limit its acceptance or inclusion in nationally supported analysis fields. A strong set of metrics and example cases could ensure that it gets attention and consideration for the widely recognized issues associated with HSLC convection. -JB

  2. Keith Sherburn says:

    Thanks for the comment. Although the parameter was designed specifically for HSLC events, we have not yet tested its skill across other environments. It may end up being applicable to other environments, as well; however, given that our null database is restricted to HSLC events, we are currently unable to test its skill outside of HSLC environments other than doing some spot-checking of soundings and comparing them to nearby reports (or lack thereof). I plan on testing the POD across all environments once I get some data from SPC, but FAR will be a little bit trickier. I’ll see what I can do about that.

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