Potential vorticity anomaly and mesoscale convective systems on the Baiu (Mei-Yu) front

It is proposed that the mesoscale convective systems along Baiu (called Mei-Yu in China) fronts are caused by instability of the low-level potential vorticity maximum along the fronts. The growth rate of the most unstable wave depends on the intensity of cumulus heating, with the e-folding time of the order of one day. When the cumulus heating parameter is below a critical value, the wavelength is about 8-15 times the cross-front width scale of the PV anomaly, and the structure of the wave is of the barotropic type but modified by convection. When the heating parameter is above the critical value, the disturbance draws its energy almost entirely from heating and the structure of the wave resembles a system driven purely by cumulus heating. The wavelength of the most unstable wave is about 1700-2100 km and bears little relationship to the width of the background PV anomaly.