After the serious damage due to Nobeoka, Miyazaki prefecture tornado on 17 September 2006 and Saroma, Hokkaido prefecture tornado on 7 November 2006, JMA enhanced their operation related to wind hazards due to tornadoes, downbursts and other damaging gusty winds. For example, they enhanced damage survey of hazardous winds to determine the atmospheric phenomena that caused the damage and the Fujita-scale (F-scale) corresponding to the damage. They also started to issue tornado advisories on prefecture-by-prefecture basis from 26 March 2008 based on radar reflectivity and a mesocyclone observed by the Doppler radars, and meso-scale environmental parameters predicted from numerical models.
While the F-scale originally introduced by Fujita in 1971 was based on damage to typical structures in the United States, it was slightly modified to fit to Japanese structures by himself in 1973. Recently, Enhanced Fujita-scale (EF-scale) was developed in the United States by Texas Tech University in collaboration with a number of scientists and meteorologists, and was implemented by the US Weather Bureau in January 2007. The EF-scale considers 28 Damage Indicators (DIs) which are typical in the United States. Several years later, efforts were made in Canada to slightly modify the EF-scale and was operationally implemented at Environment Canada on 1 April 2013, where the Canadian EF-scale has 31 DIs. Since the DIs in the US and Canada are not directly applicable to Japanese structures, JMA decided to introduce the JEF-scale which is based on DIs typical of Japanese structures. To this end, “The Advisory Committee for Rating Intensity of Tornadoes”, which was chaired by Professor Yukio Tamura, Tokyo Polytech University and consisted of 9 experts in wind engineering and meteorology, was set up in 2013 and developed the JEF-scale through six meetings. The resulting guideline in Japanese has been published on the web site of JMA in December 2015, and its English version is under preparation.
The JEF-scale has 30 DIs. As similar to the EF-scales in the US and Canada, the wind speed in the JEF-scale is estimated from Degree of Damages (DODs) for each DI. The relationships between the wind speeds and DODs of DIs were obtained from the state-of-art knowledge of wind engineering based on large-scale wind tunnel experiments, computer simulations and experiments about damage of structures by tornadoes. In order to determine the relationship between the JEF-scale and the estimated wind speed, the following procedure was taken: Firstly, damage photographs from tornadoes which caused more than 30 damaged structures between 2007and 2013 and three F3 tornadoes (Mobara, Chiba tornado on 11 December 1990, Toyohashi, Aichi prefecture tornado on 24 September 1999, and Saroma, Hokkaido tornado on 7 November 2006) were collected. Secondly, five JMA experts on F-scale rating examined the photographs to determine the corresponding sub-category of F-scale, where each F-scale is divided into three sub-categories (for example, F1 is divided into categories of F1-weak, F1-medium, and F1-strong). Since each sub-category of the F-scale has a corresponding 3-sec mean wind speed, the sub-categories as determined by the experts are converted to 3-sec mean wind speeds, which are then averaged to give a 3-sec mean wind speed U based on the F-scale. Thirdly, five experts in wind engineering examined the photographs and used the DIs and DODs of the JEF-scale to estimate the wind speeds. Finally, a scatter diagram between the average of the wind speeds determined by the experts in wind engineering , V, and that determined by the experts on F-scale rating, U, is plotted, and the correlation between the two kinds of wind speeds are examined by a regression analysis. It is found that a power function fitting gives a better correlation than a linear fitting. The power function that gives the best fitting turned out to be V=2.8 x U0.74. This equation together with the continuity of degree of damage between the same value of F-scale and JEF-scale gives the lower and upper bounds of 3-sec mean wind speeds for JEF as 14xJEF+25 and 14xJEF+38 (m/s), respectively. The JEF-scale of a particular tornado is determined by first finding the maximum wind speed estimated from the DIs and DODs during the damage survey and then consulting the upper and lower bounds of the wind speed for each rank of the JEF-scale.