Numerical investigation of development processes of Baiu frontal depressions part I: Case studies

This work investigates the development processes of Baiu frontal depressions (BFDs) using a numerical model. To investigate the effects of upper-level disturbances, latent heating, and baroclinicity on the development of BFDs, case-study numerical simulations are performed. In the present study, two typical cases were selected from BFDs that appeared in June and July, 2000-2007-a BFD that developed in the western part of the Baiu frontal zone (W-BFD) from 26 to 27 June, 2003, and a BFD that had formed in the eastern part of the Baiu frontal zone (E-BFD) from 1 to 3 July, 2003. An available potential energy diagnosis shows that the effect of latent heating is dominant during the W-BFD development, while baroclinicity as well as latent heating is important to the E-BFD development. A sensitivity experiment excluding upper-level potential vorticity (PV) anomalies shows that upperlevel disturbances are important contributors to the development of E-BFDs. The low-level PV and its production associated with latent heating suggest that the W-BFD has a development mechanism driven by latent heating. In the early developmental stage, PV near the W-BFD center is enhanced. This feature is consistent with the nonlinear conditional instability of the second kind mechanism. In the later developmental stage, PV is produced in front of the W-BFD center, in which low-level baroclinicity is large. This process is consistent with a diabatic Rossby vortex. In contrast, the E-BFD develops through a baroclinic instability- like mechanism in the moist atmosphere.