Potential vorticity diagnostics of a Mei-Yu front case

The present study selects the mei-yu frontal case of 12-13 June 1990 over southeastern China, and performs potential vorticity (PV) diagnostic analysis to discuss the mechanism responsible for the intensification and maintenance of frontal vorticity. The mei-yu front had typical characteristics at the western section, and was shallow with weak baroclinity but strong horizontal shear vorticity. For this particular case, results of piecewise PV inversion indicate that latent heat release associated with both deep convections and stratiform clouds was responsible for the frontal vorticity, and the apparent frontogenesis near 0000 UTC 13 June was driven almost entirely by an outbreak of deep convections along the front, through the conditional instability of the second kind (CISK) mechanism proposed by Cho and Chen. The positive feedback between the mei-yu frontogenesis and cumulus latent heating, in which the front provided low-level convergence and helped organize the convection while latent heating by cumuli generated low-level PV and further enhanced the frontogenesis process, led to rapid growth in shear vorticity along the front. A low-level jet (LLJ) subsequently developed to the immediate south of the front through Coriolis acceleration of isallobaric winds. It is also found that the heating efficiency during this process depended upon the initial low-level PV value when deep convections started, and thus convections south of the front were ineffective in producing a similar response in either PV or wind field. Finally, an upper-level jet streak appeared to provide additional lifting at the outbreak of deep convection in the present case, but its role is secondary compared to the presence of the front itself in providing PV values sufficiently large in the background.