Structure and Environment of Tornado-spawning Tropical Cyclones: Composite Analyses and Idealized Numerical Experiments

Structure and environment of tornado-spawning tropical cyclones are studied by means of composite analyses using reanalysis dataset (JRA-55) and idealized numerical experiments. In the composite analyses, the structure of 35 typhoons that spawned a total of 52 tornadoes in Japan between 1991 and 2012, hereafter referred to as tornadic typhoons (TTs), is investigated and compared with that of 199 non-tornadic typhoons (NTs), where NTs having similar strength and locations as TTs are objectively chosen out of typhoons that did not spawn any tornadoes. Two types of composite analyses are made: one is “moving-direction composite” in which a mean structure of typhoons is obtained by aligning their moving directions, and the other “shear-direction composite” in which the structure is obtained by aligning the direction of synoptic-scale vertical wind shear vector, where the shear vector is defined by the difference of the area-mean wind vector at 200 hPa over a circle of 500 km radius around the typhoon center from that at 850 hPa. The moving-direction composite for TTs shows that storm relative environmental helicity (SREH)^[1][2] is large from the right-front to the front side of the typhoon center, which is consistent with a distribution of observed tornadoes relative to the typhoon center (Fig. 1(a)). The shear-direction composite for TTs shows that SREH is large from the downshear-left to the downshear side of the typhoon center, which is also consistent with the distribution of observed tornadoes (Fig. 1(b)). In both composites, distribution patterns of SREH for NTs are similar to those for TTs, but the magnitude of SREH for NTs is much smaller than that for TTs. When TTs and NTs are further classified by their symmetry about their moving directions using the parameter “B” of Cyclone Phase Space (CPS)^[3], the following findings are obtained: TTs contain more asymmetric typhoons than NTs do; the number of tornadoes spawned by an asymmetric TT is larger than that by a symmetric TT; in both moving-direction and shear-direction composites for TTs, SREH calculated from composited wind fields for asymmetric typhoons are larger than those for symmetric typhoons. In the idealized numerical experiments, effects of synoptic-scale wind on the structure of tropical cyclone (TC) and associated SREH distribution are studied by the Weather Research and Forecasting Model ver. 3.4.1. The calculation domain is 3000 km x 3000 km x 25 km on an f-plane at 20°N. The horizontal grid size is 5 km, and 40 unequally-spaced grids are distributed in the vertical. The boundary condition is cyclic in the zonal direction, and symmetric in the meridional direction. The initial thermodynamic state is given by the mean hurricane sounding^[4]. The sea surface temperature is fixed at 28°C at the meridional center of the domain, where its meridional gradient is taken to be equal to that of the initial air temperature at the lowest grid level. The experiments are started by placing an axisymmetric vortex in a horizontally homogeneous westerly either with or without vertical shear, and the structure of its developed stage is examined. The experiments without vertical shear (hereafter referred to as “No-shear Exps.”) are designed to examine the effects of TC movement on its 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 the front side of the TC center because a low-level southerly flow in the Ekman boundary layer associated with the synoptic geostrophic westerly enhances local vertical wind shear in the right-front side of the TC vortex. In the Shear Exps., on the other hand, SREH becomes large from the downshear-left to the downshear side of the TC center. This is caused by both direct effects of the sheared westerly and the structure of the TC vortex that tilts downshear-left with increasing height. The asymmetries of the simulated TC vortices are consistent with the results of the composite analyses. Both of the composite analyses and numerical experiments show that tornado environment in a TC is closely related to an asymmetric structure caused by TC movement and synoptic-scale vertical wind shear, and the asymmetric structure is more significant in TTs than in NTs if the TCs have similar strength. REFERENCES [1]Davies-Jones, R. P., D. Burgess, and M. Foster, 1990: Test of helicity as a tornado forecast parameter. Preprints, 16th Conf. on Severe Local Storms, Kananaskis Park, AB, Canada, Amer. Meteor. Soc., 588-592. [2]Bunkers, M. J., B. A. Klimowski, J. W. Zeitler, R. L. Thompson, and M. L. Weisman, 2000: Predicting supercell motion using a new hodograph technique. Wea. Forecasting, 15, 61-79. [3]Hart, R. E., 2003: A cyclone phase space derived from thermal wind and thermal asymmetry. Mon. Wea. Rev., 131, 585-616. [4]Jordan, C. L., 1958: Mean soundings for the West Indies area. J. Atmos. Sci., 15, 91-97.