Tropical Depression Aere (2022) and Flooding in Western Japan: The Role of Atmospheric River

On the morning of July 5, 2022, Tropical Depression (TD) Aere made landfall at Nagasaki, Japan inducing heavy rainfall. Despite its initial classification as a weak tropical storm with a central sea level pressure of 1,000hPa and an asymmetric structure, TD Aere’s aftermath revealed flash floods across western Japan. Our investigation posits that the continuous moisture transport from the atmospheric river played a pivotal role in the flooding, challenging conventional notions about weak tropical storms. TD Aere’s elongated structure along the periphery of the western North Pacific High (WNPH) resembled an atmospheric river, with plume-like moisture flux originating from the southwest Warm Pool region. The unusual persistence of convection even after Aere’s landfall could be attributed to sustained latent heat support. Furthermore, we found that Aere did not interact with other extratropical systems, and the absence of synoptic-scale forcing suggests that moisture transport is the primary factor enabling Aere’s unforeseen consequences of flooding over the western Japan. Comparing the in-situ field observations (radiosondes and dropsondes) from the PRECIP/AHOPE/TPARC-II field campaign and the ERA5 reanalysis, ERA5 reanalysis had a significant deviations from in-situ observations for low-level winds and humidity for the upstream area of Aere. ERA5's resolution might be too coarse to capture subtle changes in moisture levels that are crucial for precise forecasting. Specifically, narrow moisture channels may be smoothed over, leading to underestimation of moisture transport. Given that reanalysis data tends to inaccurately predict low-level moisture content and winds, it is essential to correct future models to provide representative atmospheric profiles. This improvement could lead to better predictions of heavy rainfall events, associated with multi-scale weather events such as interactions between atmospheric river and tropical storm.