On the Mei-Yu front and the associated potential vorticity anomaly

The Mei-Yu (called Baiu in Japanese) front is a particular atmospheric phenomenon in East Asia that occurs during the late spring and early summer period. This thesis presents a combination of theoretical and numerical studies of two important issues regarding the development of Mei-Yu fronts: (1) the initiation of mesoscale convective systems, and (2) the formation of the PV anomaly and its impact on the Mei-Yu frontogenesis.; The organization mechanism of the mesoscale convective systems on the Mei-Yu front has been examined in the context of frontal stability. The results show that the Mei-Yu front itself is unstable due to the presence of a low-level PV anomaly. It is then proposed that the convective systems are organized by the instability of the low-level PV anomaly along the front. The growth rate of the most unstable mode depends on the intensity of cumulus heating, with the e-folding time of the order of less than one day. When the heating intensity is below a critical level, the wavelength is about 8–15 times the cross-front width scale of the PV anomaly, and the instability structure is of the barotropic type but modified by convection. When the heating intensity exceeds the critical level, the disturbance draws its energy almost entirely from heating and the instability structure resembles a system driven purely by cumulus heating. The most unstable wavelength is 1700–2100 km and bears no relationship to the scale of PV anomaly.; The formation of the low-level PV anomaly and its impact on the Mei-Yu frontogenesis have been studied through the numerical simulation of an observed Mei-Yu front. The PV anomaly on the modeled Mei-Yu front is the result of condensational heating. While the PV anomaly on the central sector of the front was caused by stratiform clouds, the PV anomaly on the western sector was mostly caused by cumulus convection. The numerical simulations have verified that the western sector of the Mei-Yu front is intensified mainly by the low-level convergence flow of cumulus convection. The comparison between the numerical simulations, theoretical results and observations is also made in the thesis.