Structure of tornado-spawning tropical cyclones is studied by composite analyses using a reanalysis dataset (JRA-55; Ebita et al., 2011) and idealized numerical experiments.
In the composite analyses, structure of typhoons that spawned tornadoes in Japan (hereafter referred to as tornadic typhoons (TTs)) between 1991 and 2012 is investigated and compared with that of non-tornadic typhoons (NTs), where the composite are made by aligning directions of typhoon movement ( moving-direction composite ) or synoptic-scale vertical wind shear ( shear-direction composite ).
The moving-direction composite for TTs shows that storm-relative environmental helicity (SREH) is large from the right-front to front side of the typhoon center, which is consistent with locations of tornadoes relative to the typhoon center. The shear-direction composite for TTs shows that SREH is large from the downshear-left to downshear side of the typhoon center, which is also consistent with locations of tornadoes. In the case of NTs, distribution patterns of SREH in both composites are similar to those for TTs except that the magnitude of SREH is smaller.
In the idealized numerical experiments, effects of synoptic-scale wind on the structure of tropical cyclones (TCs) and associated tornado environments are studied by placing initial axisymmetric vortices in westerlies with and without vertical shear and examining their evolutions. The experiments without vertical shear (No-shear Exps.) are intended to examine the effects of TCs movement on their structure, while those with vertical shear (Shear Exps.) to examine the effects of synoptic-scale vertical wind shear.
In the No-shear Exps., SREH becomes large from the right-front to front side of the TC center due to southerly flow in the Ekman boundary layer below the geostrophic westerly flow. In the Shear Exps., on the other hand, SREH becomes large from the downshear-left to downshear side of the typhoon center due to direct effects of the sheared westerly and a tilt of the TC vortex to the downshear-left with increasing height. The results of the numerical experiments are used to physically interpret those of the composite analyses.