Structural and Environmental Characteristics of Extratropical Cyclones that Cause Tornado Outbreaks in the Warm Sector: A Composite Study

Abstract

he structural and environmental characteristics of extratropical cyclones that cause tornado outbreaks [outbreak cyclones (OCs)] and that do not [nonoutbreak cyclones (NOCs)] are examined using the Japanese 55-year Reanalysis dataset (JRA-55). Composite analyses show differences between OCs and NOCs: for OCs, storm relative environmental helicity (SREH) and convective available potential energy (CAPE) are notably larger, and the areas in which these parameters have significant values are wider in the warm sector than they are for NOCs. The larger CAPE in OCs is due to larger amounts of low-level water vapor, while the greater SREH is due to stronger low-level southerly wind.The composite analyses for environmental fields defined by 20-day means suggest that environmental meridional flows have the potential to advect large amounts of warm and moist air northward, creating atmospheric instability in the troposphere that contributes to the occurrence of a tornado outbreak. A piecewise potential vorticity (PV) diagnosis shows that low- to midlevel PV anomalies are the main contributor to the difference in the low-level winds between OCs and NOCs, whereas upper-level PV anomalies make only a minor contribution.An examination of the structures of the extratropical cyclones and the upper-level jet stream suggests that the difference in the low-level winds between OCs and NOCs is due to differences in the structure of the jet stream. The OCs develop when the jet stream displays larger anticyclonic shear. This causes a more meridionally elongated structure of OCs, resulting in stronger low-level winds in the southeastern quadrant of the cyclones.