Convective Storm Evolution Following Mergers Between Squall Lines and Isolated Supercell Thunderstorms

Adam French who completed his Ph.D. at NC State in July, recently shared some of his research results with WFO Raleigh in his presentation: Convective Storm Evolution Following Mergers Between Squall Lines and Isolated Supercell Thunderstorms.

In the first part, 21 merger cases were examined using analyses from the Rapid Update Cycle forecast model and WSR-88D radar data. Two primary background environments were identified, along with common radar reflectivity and velocity patterns. Additionally, an analysis of storm report data identified an evolving severe weather threat as the merger progresses. In the second part of this study, idealized model simulations were used to isolate the impact that the merger has on squall line evolution, and to identify the storm-scale processes that drive the observed behaviors.

A 45 minute recorded presentation is available along with the power point and a PDF of the presentation.

Observations of squall line-supercell mergers indicated two general environments:
1) “Weakly forced”: weak synoptic forcing, typically with a low-amplitude upper trough, weak surface cyclone
2) “Strongly forced”: strong synoptic forcing typically a high-amplitude upper trough, strong surface cyclone/cold front

Examining storm reports for both environments indicated that:
* Tornado reports maximized approximately at merger time
* Wind reports maximized post-merger
* Hail reports maximized pre-merger
* Strongly forced events: more tornado
* Weakly forced events: more overall
* An evolving severe weather threat was noted (hail to tornado to wind)

Examination of radar signatures for the 21 cases revealed 3 common evolution patterns based on radar reflectivity, all of which produced bow echoes following the merger:
* System-scale bowing (SSB) – WF environment (8 cases), entire system evolves into a large bow echo post-merger, generally single supercell merger, merged supercell becomes north end of line
* Embedded bowing (EMB) – SF environment (4 cases), small-scale bow along larger line, line typically forced by cold front, often multiple supercell mergers, line-echo wave pattern
* Hybrid evolution – Mainly SF environment (8 cases), includes 2 WF cases, small-scale bow at north end of line, elements of SSB and EMB

Idealized model simulations were produced for the System-scale bowing (SSB) pattern and some of the results from these simulations are noted below.

Simulated impacts of merger on the surface fields:
* Enhanced surface winds, consistent with observed increase in severe wind reports
* Large vertical vorticity at lowest model level
* Enhanced surface rainfall

Mergers toward the north end of the line:
* Enhanced bowing
* Strong surface winds/low-level vorticity
* May benefit from more favorable vorticity environment (cyclonic line-end vortex, convergence of  f)
* Consistent with observations of strong meso-vorticies north of bow echo apex.

Mergers toward the south end of the line:
* More localized impacts/less bowing
* Localized/short-lived increases in surface winds/low-level vorticity
* Less favorable vorticity environment (anticyclonic line-end vortex)

Simulations identify merger as a location of potentially severe weather:
* Strong surface winds
* Enhanced low-level vertical vorticity (rotation)
* May be sensitive to merger location (north end= stronger)

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2 Responses to Convective Storm Evolution Following Mergers Between Squall Lines and Isolated Supercell Thunderstorms

  1. Pingback: Update on the Observations of mergers between Squall Lines and Isolated Supercell Thunderstorms | CIMMSE

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