Statistical analysis of Aviation Turbulence in 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 Report (PIREP). In total, 81,639 turbulence events, with moderate or greater intensity, were reported over this period. The monthly number of turbulence cases 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 above FL380 and below FL280. The statistical features are not largely different among the four seasons; however, there are some exceptions. For instance, the number of turbulence cases is large in high altitudes in summer and small in low altitudes in winter. Considering the number of flights, it is evident that the frequency of turbulence is higher in altitudes between FL200 and FL350, although the number of flights is low in this altitude region. The number of convectively induced turbulence is relatively large during the daytime in summer compared with the other seasons. Large number of mountain wave turbulence is observed around the mountainous region in fall and winter when the jet stream flows over Japan.