Importance of Entraining CAPE for Assessing the Tornado Potential in Typhoons and an Exploration of Appropriate Entrainment Rates for Supercells in the Typhoon Environment

Thermodynamic structure of typhoons that spawned tornadoes (tornadic typhoons: TTs) in Japan from 1991 to 2013 have been investigated by a composite analysis using the Japanese 55-year Reanalysis and are compared with those of typhoons that did not spawn tornadoes (non-tornadic typhoons: NTs). In the present study, typhoon-spawned tornadoes are defined as tornadoes that occurred in four large islands of Japan (Kyushu, Shikoku, Honshu, and Hokkaido islands) within 550 km radius from the typhoon center, TTs as typhoons that spawned these tornadoes, and NTs as typhoons which existed at similar locations as TTs but did not spawn any tornadoes throughout their lifetime.

We found that the composited distribution of convective available potential energy (CAPE), which considers the effects of entrainment (entraining CAPE: E-CAPE), is significantly large in the northeast quadrant of TTs where tornadoes frequently occur, and E-CAPE in that quadrant for TTs are larger than those for NTs. On the other hand, ordinary (pseudoadiabatic) CAPE without entrainment is large in the southeastern side and does not account for the distribution of tornado occurrences nor distinguish TTs from NTs. The investigation of the effects of entrainment rate ε reveals that E-CAPE characterized by ε ≈ 20% km^-1 gives the best correspondence between the distribution of tornado locations and that of the composited E-CAPE. Furthermore, E-CAPE value at the location of each tornado often increases to reach a maximum near the time of the tornado occurrences, which does not occur for ordinary CAPE. These results show that E-CAPE can be a good parameter for assessing the potential of typhoon-spawned tornadoes.

The distribution of E-CAPE considerably differs from that of ordinary CAPE because E-CAPE is sensitive to humidity in the mid-troposphere. The northeast quadrant of the typhoon often has relatively moist mid-troposphere in addition to the boundary layer with considerably high equivalent potential temperature, which results in larger E-CAPE in that quadrant. Furthermore, the effects of entrainment lower the equilibrium levels of the lifted parcels, and thus, E-CAPE is effective for detecting the conditionally unstable layer up to about 5 km, which is distinctive of TTs.

Although the entrainment rate ε used in our composite analysis are determined by the consistency between distributions of composited E-CAPE and the tornado locations, its value in the TT environment has not been validated by any observations. The entrainment rate can depend on the vertical wind shear and/or the thermodynamic instability of the environment. To investigate this, we have performed idealized numerical experiment to simulate a mini-supercell in the typhoon environment using the cloud resolving model CM1 ver. 18.3 with a horizontal grid size of 500m. The sounding at Tateno, Ibaraki prefecture, Japan at 2100 JST on 19 September 1990, around which time several tornadoes spawned by mini-supercells in Typhoon 9019 occurred in the Kanto Plain, were smoothed and were used as the basic state of the simulation. A thermal bubble placed near the ground is used to trigger the convection, which evolves into a mini-supercell having a mesocyclone with vertical vorticity larger than 0.01 s^-1. A preliminary examination of the equivalent potential temperature in the storm suggests that an entrainment rate is substantially large. We will report the results of more detailed analysis of the entrainment rate from simulations with a higher resolution at the conference.