Structures and Environment of Extratropical Cyclones Cause a Tornado Outbreak

Typical springtime tornado outbreaks in the United States often occur in a warm sector of extratropical cyclones (hereafter, ECs). However, how the structures and environment of ECs that cause tornado outbreaks is different from those that does not is not well understood. In this paper, we investigate, by means of composite analyses of reanalysis data (JRA-55; Ebita et al., 2011) and idealized numerical simulations, differences in the structure and environment of these ECs and physical mechanisms for the differences.

In the composite analyses, we have categorized ECs which developed in the United States in April and May between 1995 and 2011 into two groups: ECs accompanied by 15 or more tornadoes (hereafter, referred to TOCs (tornado outbreak cyclones)) and ECs accompanied by 5 or less tornadoes (NOC (non-tornado outbreak cyclones)).

The composite analyses show that significant differences in convective environmental parameters are found between TOCs and NOCs. For TOCs, convective available potential energy (CAPE) and storm relative environmental helicity (SREH) are larger and the areas in which these parameters have significant values are wider in the warm sector(Fig.1). The larger CAPE in TOCs is due to larger amount of low-level water vapor, while the larger SREH in TOCs due to stronger wind at low levels. A piecewise potential vorticity (PV) diagnostics (Davis and Emanuel, 1991) indicates that low- to mid-level PV anomalies mainly contribute to the difference in the low-level winds between TOCs and NOCs. On the other hand, the low-level winds associated with upper level PV anomalies have little contribution to the difference.

In the idealized numerical experiments, ECs are developed in a basic states which are obtained by 5-day average of composite environment for TOCs and NOCs, and their structures are examined. Weather Research and Forecasting (WRF) ver. 3.4 is used for the numerical experiments. The size of the calculation domain is 12000 km, 6000 km, and 25 km in the zonal, meridional and vertical directions, respectively. The horizontal grid size is 20 km. The boundary conditions are cyclic at the zonal boundaries, symmetric at the meridional boundaries. The lower boundary is assumed to be grassland. Kain-Fritsch scheme is used for the cumulus parameterization. WRF Single Moment 6 class (WSM6) for cloud physics, and Yonsei University Scheme for the boundary layer are used.

The results of the numerical experiments for the composite environments for TOCs and NOCs (hereafter, referred to as EX-TOC and EX-NOC, respectively) show that the low-level wind fields and SREH of the simulated ECs in EX-TOC and EX-NOC are similar to those for TOCs and NOCs, respectively. In EX-TOC, SREH in the east-southeast region of the EC center is larger than that in EX-NOC. These results suggest that low-level wind fields and SREH in ECs are associated with the structures of jetstream. The structure of jetstream in EX-TOC has larger anticyclonic shear to southern part of jet axis than that in EX-NOC. As shown in Wernli et al. (1998), larger anticyclonic shear of the jet stream causes stronger cold frontogenesis in the south-southeast region of the EC center in EX-TOC, resulting in stronger low-level winds.