Anomaly-based synoptic analysis on the Heavy Rain Event of July 2018 in Japan

The Heavy Rain Event of July 2018 caused huge social impact and economic losses in Japan. In this study, the anomaly-based synoptic analysis is applied to identify the features and structures of the anomalous synoptic systems during the event period. Results show that the heavy rainfall occurred along the trough of anomalous geopotential height (GPH) and the shear line of anomalous winds at the low troposphere. The anomalous synoptic analysis, by removing the temporal climatology from the total variables, can directly reflect the large-scale features of the event, which includes the actual position of the Baiu front, the pathway of anomalous moist air masses associated with anomalous synoptic systems such as the anomalies of Okhotsk cold high and the Northwest Pacific subtropical high. Meanwhile, the opposite signs between 200 and 850 hPa GPH anomalies, which matches observed rainfall records well, could be a good indicator of the potential heavy rain period. The product of the ensemble prediction systems from the European Centre for Medium-Range Weather Forecasts is able to predict such potential anomalous signals of the Heavy Rain Event for 4–5 days in advance.