Typhoon Faxai was a very powerful typhoon that made landfall and passed through Japan on September 8-9, 2019. In the previous study that used Total Electron Content (TEC) acquired by the GNSS navigation system, Concentric Traveling Ionospheric Disturbances (CTIDs) were observed within 250 km of Faxai. In this study, to investigate ionospheric disturbances at lower altitudes than cannot be observed using TEC, we examined the ionospheric disturbances associated with Faxai using a High-Frequency Doppler (HFD) sounding system. It was found that disturbances corresponding to the CTIDs were observed by HFD sounding. The horizontal velocity of these disturbances was estimated at about 200 m/s, which is consistent with the horizontal velocities reported by the previous study. The period of the disturbances suggests that the ionospheric disturbances corresponding to the CTIDs is caused by the Internal Gravity Waves (IGWs). Since the gravity wave can propagate from the ground to the upper atmosphere, the CTIDs associated with Faxai distributed at all altitudes with the same velocity in the entire F-region. HFD observation also revealed that the typhoon generated acoustic waves, which caused short period disturbances.

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For more than 50 years, thunder days have been monitored 24 hours a day at 15 airfields in Japan. To analyze the periodic cycles of thunder days and the correlations between airfield pairs on those days, we analyzed long-term data on thunder days from April 1968 to February 2021. The trends of the yearly average thunder days exhibited decreases at nine of the fifteen airfields, increases at four airfields, and plateaus at two airfields. The trends of thunder days in the cold season (November–February) showed decreases at six airfields, increases at four airfields, and plateaus at five airfields. Three of the four airfields that showed increasing trends were located along the Sea of Japan (maximum: 0.17 count/year). In the warm season (March–October), the trends of the thunder days showed decreases at eleven airfields, increases at three airfields (maximum: 0.19 count/year), and a plateau at one airfield. Over the 52-year period, the number of thunder days showed an increasing trend along the Sea of Japan and a decreasing trend along the Pacific Ocean. These trends matched those reported in studies conducted before 1990 that showed increasing and decreasing numbers of thunder days along the Sea of Japan after the 1930s and in the Kanto Plain after the 1950s, respectively. To categorize the data for the 15 airfields, we used a cluster analysis of the correlation coefficients of the thunder day data. The 15 airfields were grouped into four regions in the cold season and four regions in the warm season. Roughly, the largest cluster appeared in the region along the Pacific Ocean in the warm season and in the region along the Sea of Japan in the cold season. In addition, variations in the thunder day data from the 15 airfields were analyzed using Fourier transformation. The predominant periodicities of the thunder days at most airfields along the Pacific Ocean in the warm season ranged from 6 to 7 years. These airfields had single or multiple local minima in the periodicity between 3 and 5 years. In the periodicities of the thunder days in the cold season along the Sea of Japan, one or more peaks appeared in the 2- to 4-year period and in the 4- to 8-year period. The 10- to 11-year cycle was remarkable in Kitagawa’s (1989) analysis for thunder days in the summertime (July and August), while this cycle did not appear or was very weak in our analysis for the warm season (from March to October). Monthly or seasonal variations in thunder days were not analyzed in this paper. This indicates that if analysis periods were changed, other predominant periodicities might appear, such as an 11-year solar cycle.

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