24ARAM Statistical Analysis of Aviation Turbulence in the Middle–Upper Troposphere over Japan

This study examined the statistics of aviation turbulence that occurred in Japan between 2006 and 2018 by analyzing the Pilot Weather Report (PIREP). In total, 81,639 turbulence events, with moderate or greater intensity, were reported over this period. The monthly number of turbulence has an annual periodical variation as observed in different regions by previous studies. The number of turbulence cases is high from March to June and low in July and August. Higher number of turbulence cases are experienced along the major flight routes in Japan, especially around Tokyo, for the active period between 9:00 and 20:00 local time. The number of cases of turbulence peaks when the flight reaches an altitude of 33000 ft (FL330), while it reduces when the flight altitude is less above FL380 and below FL280. The statistical features are not largely different among among the four seasons; however, there are some exceptions. For instance, the number of turbulence increases in high (low) altitudes in summer (winter). Considering the number of flights, it is evident that the frequency of turbulence is higher in altitudes between FL200 and FL350, whereas the flight number is high below FL200 and above FL350. The frequency of turbulence increase around the eastern side of the main island of Japan where the lee wind of the jet stream overflows a mountainous region.