A subtropical quasi-stationary front, called the Baiu (Meiyu) front in Japan (China), extends from south China to the northwestern Pacific through the Japan Islands in the rainy season, which is the period from June to July. In the frontal zone, a number of Baiu frontal depressions (hereafter, BFDs) form and develop. Precipitation associated with BFDs is the primary water resource in East Asia, while heavy rainfall produced by highly-developed BFDs often causes flash flooding and landslips. Thus, it is important to elucidate the development processes of BFDs.
Tochimoto and Kawano (2012) classified BFDs into two groups; W-BFDs (E-BFDs) peak in the region west (east) of 140E, and examined the development processes of each group of BFD using a reanalysis data set. They concluded that the vertical coupling between upper- and lower-level disturbances and latent heating contribute to the development of both types of BFDs. The contribution of latent heating is larger in the W-BFD development. It is suggested that low-level baroclinicity is more important to the E-BFD development.
In this study, to further investigate the effects of upper-level disturbances, latent heating and baroclinicity on the development of BFDs, numerical simulations are performed using the initial and boundary conditions calculated from reanalysis data. An available potential energy diagnosis shows that the effect of latent heating on the W-BFD development is dominant, while baroclinicity as well as latent heating is important to the E-BFD development. A comparison between the control runs and the simulations without upper-level potential vorticity (PV) anomalies shows that upper-level disturbances are greater contributors to the development of E-BFDs. The effects of latent heating and baroclinicity on the development of BFDs are reconfirmed by idealized numerical simulations with zonally homogeneous environmental fields.