Reanalysis and Reforecast of Typhoon Vera (1959) Using a Mesoscale Four-Dimensional Variational Assimilation System

Typhoon Vera struck Japan on 26 September 1959, causing the largest meteorological disaster in Japan since World War II, with tragic consequences. Over the past 50 years, numerical weather prediction systems and observation networks have been developed that facilitate more realistic simulation of tropical cyclones (TCs). In this study, the operational mesoscale forecast and analysis systems of the Japan Meteorological Agency (JMA) were applied to the case of Vera with some modifications, and the forecast probability of the natural disasters caused by Vera was investigated. In data assimilation, dropsondes deployed near the TC center and TC bogus data were individually used in addition to conventional observations. In order to reduce the discrepancy between the model states and real observations near the TC core region, due mainly to the insufficiency of the model resolution, we adjusted the observational error of the dropsondes as a function of observation-minus-forecast innovations. The results indicate that both the dropsondes and the bogus data contributed to improving the analysis and a subsequent model forecast with 5-km grid spacing. Initialized using the analysis with dropsondes, the weather distribution (e.g., surface pressure, clouds, and fronts) of the forecast result was similar to the observed weather map. Using this result, a storm surge forecast and a downscaling numerical simulation with horizontal resolution of 1km were also conducted. The intensity and track forecast did not improve compared to the forecast with 5-km grid spacing, but precipitation was better captured due to the higher-resolution orography. The storm surge forecast was comparable to observations, suggesting that the strong wind caused by Vera was well reproduced in the forecast model.