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. © 2017,meteorological society of japan.