The 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.

Abstracts The history of severe thunderstorm research and forecasting over the past century has been a remarkable story involving interactions between technological development of observational and modeling capabilities, research into physical processes, and the forecasting of phenomena with the goal of reducing loss of life and property. Perhaps more so than any other field of meteorology, the relationship between researchers and forecasters has been particularly close in the severe thunderstorm domain, with both groups depending on improved observational capabilities. The advances that have been made have depended on observing systems that did not exist 100 years ago, particularly radar and upper-air systems. They have allowed scientists to observe storm behavior and structure and the environmental setting in which storms occur. This has led to improved understanding of processes, which in turn has allowed forecasters to use those same observational systems to improve forecasts. Because of the relatively rare and small-scale nature of many severe thunderstorm events, severe thunderstorm researchers have developed mobile instrumentation capabilities that have allowed them to collect high-quality observations in the vicinity of storms. Since much of the world is subject to severe thunderstorm hazards, research has taken place around the world, with the local emphasis dependent on what threats are perceived in that area, subject to the availability of resources to study the threat. Frequently, the topics of interest depend upon a single event, or a small number of events, of a particular kind that aroused public or economic interests in that area. International cooperation has been an important contributor to collecting and disseminating knowledge. As the AMS turns 100, the range of research relating to severe thunderstorms is expanding. The time scale of forecasting or projecting is increasing, with work going on to study forecasts on the seasonal to subseasonal time scales, as well as addressing how climate change may influence severe thunderstorms. With its roots in studying weather that impacts the public, severe thunderstorm research now includes significant work from the social science community, some as standalone research and some in active collaborative efforts with physical scientists. In addition, the traditional emphases of the field continue to grow. Improved radar and numerical modeling capabilities allow meteorologists to see and model details that were unobservable and not understood a half century ago. The long tradition of collecting observations in the field has led to improved quality and quantity of observations, as well as the capability to collect them in locations that were previously inaccessible. Much of that work has been driven by the gaps in understanding identified by theoretical and operational practice.

This work investigates development processes of Baiu frontal depressions (BFDs) using a reanalysis data set in June and July from 2000 to 2007. On the basis of the deepening magnitude, 140 BFDs detected in the analysis period are categorized into developed and non-developed BFDs. Developed BFDs are further classified into W-BFDs and E-BFDs; W-BFDs (E-BFDs) peak in the region west (east) of 140°E.A composite analysis and many case studies reveal that the vertical coupling between lower-and upper-level disturbances and latent heating are key factors for the development of both BFDs. It is also shown that the development processes of BFDs depend on their environmental features. Latent heating plays a more important role in the development process of W-BFDs, whose environment is characterized as a large amount of water vapor. On the other hand, it is suggested that low-level baroclinicity makes a larger contribution to the development of E-BFDs, whose environment is characterized as a larger temperature gradient.

Strong gusty winds in a weak maritime extratropical cyclone (EC) over the Tsushima Strait in the southwestern Sea of Japan capsized several fishing boats on 1 September 2015. A C-band Doppler radar recorded a spiral-shaped reflectivity pattern associated with a convective system and a Doppler velocity pattern of a vortex with a diameter of 30 km [meso-β-scale vortex (MBV)] near the location of the wreck. A high-resolution numerical simulation with horizontal grid interval of 50 m successfully reproduced the spiral-shaped precipitation pattern associated with the MBV and tornado-like strong vortices that had a maximum wind speed exceeding 50 m s−1 and repeatedly developed in the MBV. The simulated MBV had a strong cyclonic circulation comparable to a mesocyclone in a supercell storm. Unlike mesocyclones associated with a supercell storm, however, its vorticity was largest near the surface and decreased monotonically with increasing height. The strong vorticity of the MBV near the surface originated from a horizontal shear line in the EC. The tornado-like vortices developed in a region of strong horizontal shear in the western part of the MBV, suggesting that they were caused by a shear instability.

This study used the JRA-55 reanalysis dataset for analyzing the structure and environment of extratropical cyclones (ECs) that spawned tornadoes (tornadic ECs: TECs) between 1961 and 2011 in Japan. Composite analysis findings indicated that the differences between the structure and environment of TECs, and those of ECs that did not spawn tornadoes (non-tornadic ECs: NTECs), vary with the seasons. In spring (March-May), TECs are associated with stronger upper-level potential vorticity and colder mid-level temperature than NTECs. The colder air at the mid level contributes to the increase in convective available potential energy (CAPE) of TECs. TECs in winter (December-February: DJF) and those northward of 40°N in autumn (September-November: SON) are accompanied with larger CAPE than are NTECs. The larger CAPE for TECs in DJF is caused by larger moisture and warmer temperature at low levels and that for TECs northward of 40°N in SON (NSON) is caused by the colder mid-level temperature associated with an upper-level trough. The distribution of the energy helicity index also shows significant differences between TECs and NTECs for DJF and NSON. On the contrary, the distribution of the 0-1 km storm-relative environmental helicity (SREH) showed no significant differences between TECs and NTECs in most seasons except DJF. A comparison of TECs between Japan and the United States (US) shows that SREH and CAPE are noticeably larger in the US. These differences possibly occur because TECs in the US (Japan) develop over land (ocean), which exerts more (less) surface friction and diurnal heating.

To clarify the effects of the horizontal shear of the jet stream on the structure and environment of extratropical cyclones that are accompanied by tornado outbreaks (OCs) and those that are not (NOCs), two idealized numerical experiments are performed. The experiments (OC-CTL and NOC-CTL) adopt the basic states taken from the corresponding composites of reanalysis data (JRA-55), except that the humidity field in both cases is taken from the OC composite. The simulated cyclone in OC-CTL exhibits a more meridionally elongated structure and stronger low-level wind in the southeast quadrant of the cyclone center, resulting in larger values of storm relative environmental helicity (SREH) than those in NOC-CTL. These results are consistent with the characteristics of the cyclones found for OCs and NOCs in the authors’ composite study. The distributions of surface-based convective available potential energy (SBCAPE) show no notable differences between OC-CTL and NOC-CTL, while those of CAPE based on the most unstable air parcel (MUCAPE) show some differences. A sensitivity experiment without moist processes such as condensation heating and evaporative cooling shows that the differences in the cyclone structure and environmental parameters between OCs and NOCs can be qualitatively explained by the dry dynamics. However, inclusion of moist processes results in notably larger differences.

Convective available potential energy (CAPE) is known to lack skill in discussing the environments of tornadic and nontornadic storms, or those of tornado outbreaks and nonoutbreaks. In this paper, a composite analysis of extratropical cyclones that caused 15 or more tornadoes [outbreak cyclones (OCs)] and 5 or fewer tornadoes [nonoutbreak cyclones (NOCs)] in the United States in April and May between 1995 and 2012 shows that entraining-CAPE (E-CAPE), which considers the effects of the entrainment of environmental air, is useful in the analysis of the environments of OCs and NOCs. E-CAPE in the warm sector of OCs is larger than that in the warm sector of NOCs (statistically significant at the 95%–99% level). Moreover, the regions with large E-CAPE for both OCs and NOCs are more closely correlated with the locations of tornadoes than those with large CAPE. The larger E-CAPE near the center in the warm sector of OCs is due to greater moisture at low and midlevels that results from advection by strong southerly winds and synoptic-scale ascent, respectively. The composite analysis also shows that E-EHI, E-SCP, and E-STP, for which traditional CAPE used in the energy helicity index (EHI), supercell composite parameter (SCP), and significant tornado parameter (STP) is substituted by E-CAPE, are more strongly correlated with tornado locations than are the original EHI, SCP, and STP, respectively.

The frontal structures of extratropical cyclones developing in the Northwestern Pacific storm track are relatively poorly understood compared with those in Europe and the Atlantic Ocean, for which representative conceptual models have been developed. In this paper, the structures of cyclones and their associated fronts in the Northwestern Pacific (NP), as well as in the Okhotsk Sea and Sea of Japan (OJ), are examined at their developing and mature stages using Japanese 55-year reanalysis dataset. Furthermore, the frontal structures in the NP are compared with those in the Northwestern Atlantic (NA). At the time of maximum deepening rate, cyclones in the NP are accompanied by strong warm and cold fronts, whereas cyclones in the OJ are more frequently accompanied by cold fronts than by warm fronts and tend to have stronger cold fronts than warm fronts. The weaker warm fronts than cold fronts to the east and northeast of cyclones in the OJ is likely due to the cyclones developing to the north and away from the region where the horizontal gradient of environmental potential temperature is strong. A comparison between mature cyclones in the NP and NA shows that the warm fronts in the NA tend to extend northeastward, whereas those in the NP extend more southeastward. These differences in warm fronts between NP and NA are suggested to be due to the difference in the horizontal structures of the warm currents between NP and NA.