Windward Region Sensitivity and its Effects on Heavy Rainfall Prediction Investigated with Ensemble Systems

Abstract

In this study, we investigated how the prediction of the record-breaking heavy rainfall event that occurred in western Japan in July 2018 was affected by the initial conditions. The most sensitive region was identified and its impact on the verification region was described through ensemble forecasting. Backward trajectory and ensemble sensitivity analyses were conducted to determine the origin of the air mass that reached western Japan, leading to the event. The results consistently indicate that a moist air mass near the Ryukyu Islands, which lies windward of the affected area, was transported by the Western Pacific Subtropical High in the lower troposphere. Observation system experiments were conducted to confirm the importance of windward information, and the resulting statistical verification showed degradation for precipitation forecasts that did not include windward observations. Furthermore, windspeed overestimation in the poor forecast resulted in the precipitation zone being pushed northward, and the weakened convergence led to weaker precipitation than that observed during the actual event.