Environmental parameters for forecasting tornado outbreak on the outer rain band of typhoon

A typhoon is one of major meteorological disturbance causing tornado outbreak in Japan. More than 70 % of the tornadoes accompanied by typhoons occur at forward and right hand side of traveling direction in the outer rain bands as shown in Fig. 1. Therefore, the environmental parameter that becomes large in the outer rain band is expected to be a good index for predicting tornado outbreak. The objective of the present study is to decide the best environmental parameter for the prediction of tornado outbreak accompanied by typhoon. We simulated the typical cases that tornadoes occurred in the outer rain bands of the typhoons from 1991 to 2010 and compared various environmental parameters, i.e., SReH, CAPE, EHI and so on. We also introduced new parameter, Energy Vorticity Index (EVI). EVI is a product of CAPE and vertical vorticity near surface. The distribution of EHI when the Nobeoka City was attacked by the F2 tornado occurred in the outer rain band of Typhoon 200613 is shown in Fig. 2. Because the state shown in this figure is one hour after when the tornado attacked the Nobeoka City, the outer rain band moved 40 km north to the Usuki City where the damage was also caused by the violent gust. EHI is slightly large around the Usuki City, but large EHI region spreads in broad area. SReH at the same time is high around the center of the typhoon as shown in Fig. 3. Therefore, these parameters are found not to be good indices for predicting tornado outbreak. On the other hand, the region where EVI is large clearly shows the outer rain band as shown in Fig. 4. We expect that this environmental parameter becomes a good index for tornado prediction.