Intercomparison of long-term trends in “Violent typhoon” between multiple tropical cyclone datasets

Tropical cyclones (TCs) are of great scientific and social interest because they cause natural disasters such as strong winds, torrential rains, high ocean waves and storm surges. According to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6), the proportion of Category 3-5 TCs has been increasing with global warming (IPCC 2021). However, for the best track data set, there is a problem of nonuniformity in data quality due to changes in observation and analysis methods. In addition, the definition of maximum wind speed differs among the compiling institutes. These problems are a major obstacle to conducting the climatological characteristics of TC intensity.

The assessment on the proportion of Category 3-5 TCs for the Northwest Pacific (WNP) Ocean is partly based on Mei and Xie (2016), who discussed the long-term trend of maximum wind speed by converting the 10-minute average maximum wind speed in the Regional Specialized Meteorological Center-Tokyo (RSMC-Tokyo) best track data to the 1-minute average maximum wind speed in the U.S. Navy Joint Typhoon Warning Center (JTWC) to overcome the latter problem. However, TC best track reanalysis with data quality more uniformly has been earnestly requested by researchers on TC climatology. The RSMC-Tokyo conducted the best track reanalysis for the period from 1987 to 2016 with the aim of reducing nonuniformity included in the preexisting data set (hereafter the Dvorak reanalysis data; Nishimura et al. 2023). Since this reanalysis data set does not contain maximum wind speeds and central pressures, but the Current Intensity index (i.e., CI number), an additional procedure is needed to convert a CI number to the maximum wind speed.

The purpose of this study is to investigate long-term trends “Violent typhoon” based on the TC Wind Scale of Japan Meteorological Agency (JMA) by making an intercomparison between multiple TC datasets. In this intercomparison, we use RSMC-Tokyo and JTWC best track data and the above-mentioned Dvorak reanalysis data. Also, we introduce a new dataset constructed by applying a conversion equation between the 10-minutes and the 1-minute average maximum wind speed (JMA 1990) before 1987 and the CI number-maximum wind speed conversion table after 1987 when TC intensity estimate starts to apply the Dvorak method.

First, we examine the long-term trends in the number of TCs with two different statistical periods (one after 1977 and the other after 1987) because the method for creating the best track data drastically changed between the periods. The result shows that there was a significant increasing trend for Category 4-5 TCs (corresponding to CI number >= 6.0) on the Saffir-Simpson Hurricane Wind Scale after 1977 only in the JTWC best track data but no significant long-term trend after 1987. The results are consistent with the results of Mei and Xie (2016) and Kawabata et al. (2023). As for the long-term trend in the number of "Violent Typhoon" or above (corresponding to CI number >= 7.0), a significant increasing trend is observed only in the JTWC data after 1977 and a significant increasing trend only in the RSMC-Tokyo data after 1987. This means that the statistical significance of the long-term trend depended on the applying period as well as the data set used. The next step is to examine the average number of TCs per decade. It is found that only for wind speed classification above " Violent Typhoon ", RSMC-Tokyo showed a temporary decrease in the number and JTWC showed a large increase during the two periods from 1977-1986 to 1987-1996.

In our poster session, we will present the results of further investigation into the causes of the discrepancies between the datasets. In addition, we will discuss issues in conducting research of long-term trends of TCs related to the data quality and the analyzing method because of changes in the techniques on the best track data analysis.

References:

• IPCC, 2021, Climate Change 2021, doi:10.1017/9781009157896
• Japan Meteorological Agency, 1990, Guidelines for Forecast Operations – Typhoon forecast, 150pp. (in Japanese)
• Kawabata et al., 2023, JMSJ, doi:10.2151/jmsj.2023-025
• Mei and Xie, 2016, NG, 9(10), 753-757.
• Nishimura et al., 2023, Typhoon Center Technical Review (in press)