Crustal deformation, precipitation, and water vapor of the 2019 Typhoons Faxai/Hagibis, and the 2018/2020 heavy rain episodes in SW Japan

Abstract:

Strong typhoons hit the Japanese Islands repeatedly in 2019 autumn. Here we study two of those (Faxai, Category 4, 955 hPa: Hagibis, Category 5, 915 hPa) that made landfalls in central Japan on Sep. 8 and Oct.12, respectively, using three different approaches, i.e. tropospheric water vapor, radar and rain-gauge analyzed precipitation (RRAP), and lithospheric deformation due to stormwater load. The first approach is the recovery of Precipitable Water Vapor (PWV) using the zenith wet delays estimated by the dense GNSS array in Japan GEONET. We reconstructed sea-level PWV of the whole country by using tropospheric delay gradients (Arief & Heki, FES 2020). Then, we compared the total amount of water vapor with RRAP integrated over the country and confirmed these two values are quantitatively similar. Next, we studied vertical crustal movements associated with the water load brought by the typhoon, using the F3/F5 solutions of the GEONET station coordinates. After applying a network filter to remove common mode errors, we confirmed subsidence down to ~2 cm in multiple regions where severe flood occurred, e.g. in Izu-Peninsula, Fukushima, and Nagano for the typhoon Hagibis. Such subsidence was observed to recover with a time constant of 1-2 days reflecting rapid drain of rainwater to ocean due to large topographic slope and proximity to the sea. We could not identify, however, crustal uplift due to the low atmospheric pressure at the center of the typhoon. We estimated the distribution of surface water load from crustal subsidence data using the Green’s function for a point load. However, the estimated total amount of water often exceeded the precipitation. We infer that this is due to non-uniform distribution of GNSS stations (tend to be installed in low-altitude plains) and water concentration near these stations. We also studied the 2018 July and 2020 July heavy rain episodes in SW Japan brought by stationary weather fronts with the same approaches.