Dynamical and Thermodynamical Roles of the Upper-Tropospheric Trough on the Development of the Heavy Rainfall Event over Western Japan in July 2018

This study investigates the dynamical and thermodynamical roles of an upper-tropospheric trough on the heavy rainfall event over western Japan in July 2018. Quasi-geostrophic potential vorticity (QGPV) piecewise inversion was used to extract the anomalies of geopotential and temperature corresponding to the trough. Additionally, quasi-geostrophic omega equation was solved to diagnose dynamically induced vertical motion fields. The results showed that the dynamical ascent enhanced by the trough was prominent in the upper-to-middle troposphere but limited in the lower troposphere. The lower-level dynamical ascent, which played an important role in the vertical transport of moisture preceding the rainfall, was found to correspond to a depression that developed along the Baiu front as the trough approached. The examination of thermodynamical effects indicated that a negative temperature anomaly beneath the trough led to an increase in convective available potential energy (CAPE), primarily in the upstream region of the rainfall region. Other atmospheric stability indices also indicated enhanced instability due to the presence of the trough, suggesting that the trough contributed to the intensification of the rainfall event from a thermodynamic point of view.