A diagnostic case study of Mei-yu frontogenesis and development of wavelike frontal disturbances in the subtropical environment

During 6-7 June 2003, a mei-yu front over southern China, with active mesoscale convective systems (MCSs) along it and to its south, intensified rapidly in 24 h to possess a strong cyclonic circulation with relative vorticity centers reaching 1.7 × 10^-4 s^-1 and a low-level jet (LLJ) of 22.5 m s^-1 at 850 hPa. Moreover, both the mass and wind fields and convection developed a wavelike structure at meso-α scale along the front. Using mainly gridded analyses from the European Centre for Medium-Range Weather Forecasts (ECMWT), the present study documented the evolution of this case and diagnosed the mechanisms responsible for its rapid intensification and the development of mesoscale disturbances using methods including the piecewise potential vorticity (PV) inversion. Results indicate that this mei-yu front was characterized by strong horizontal shear and moderate temperature gradient, and its development was not baroclinic in nature. The initiation of a wavelike structure along the frontal shear zone was consistent with barotropic: instability. The growth of mesoscale frontal disturbances (cyclogenesis) was a result of the nonlinear mechanism similar to the conditional instability of the second kind (CISK) in which the frontal PV centers and cumulus convection reinforce each other through a positive feedback process. As the MCSs were both intense and persistent, the latent heating was highly efficient and caused significant enhancement of the mei-yu frontal system, even at latitudes below 25°N. The vorticity budget analysis indicates that the front was maintained mainly by a stretching effect, while its gradual eastward extension and slow southward migration were linked to horizontal advection and a tilting effect, respectively. Induced by the MCSs, the transverse circulation south of the front was exceptionally strong, and the LIJ developed within its lower branch of northward-directed ageostrophic flow through Coriolis torque.