A Case Study on a Quasi-Stationary Meiyu Front Bringing About Continuous Rainstorms with Piecewise Potential Vorticity Inversion

A 4-day persistent rainstorm resulting in serious flooding disasters occurred in the north of Fujian Province under the influences of a quasi-stationary Meiyu front during 5-8 June 2006. With 1°× 1°latitude and longitude NCEP reanalysis data and the ground surface rainfall, using the potential vorticity (PV)analysis and PV inversion method, the evolution of main synoptic systems, and the corresponding PV and PV perturbation (or PV anomalies) and their relationship with heavy rainfall along the Meiyu front are analyzed in order to investigate the physical mechanism of the formation, development, and maintenance of the Meiyu front. Furthermore, the PV perturbations related to different physics are separated to investigate their di erent roles in the formation and development of the Meiyu front. The results show: the formation and persistence of the Meiyu front in a quasi-WE orientation are mainly due to the maintenance of the high-pressure systems in its south/north sides (the West Pacific subtropical high/ the high pressure band extending from the Korean Peninsula to east of North China). The Meiyu front is closely associated with the PV in the lower troposphere. The location of the positive PV perturbation on the Meiyu front matches well with the main heavy rainfall area along the Meiyu front. The PV inversion reveals that the balanced winds satisfying the nonlinear balanced assumption represent to a large extent the real atmospheric flow and its evolution basically reflects the variation of stream flow associated with the Meiyu front. The unbalanced flow forms the convergence band of the Meiyu front and it mainly comes from the high-pressure system in the north side of the Meiyu front. The positive PV perturbation related to latent heat release in the middle-lower troposphere is one of the main factors influencing the formation and development of the Meiyu front. The positive vorticity band from the total balanced winds is in accordance with the Meiyu front band and the magnitude of the positive vorticity from the balanced wind is very close to that from real winds. The PV perturbation in the boundary layer is to a certain degree favorable for the formation and development of Meiyu front. In general, the lower boundary potential temperature perturbation is not bene cial to the formation and development, which is attributed to the relatively low surface temperature due to surface evaporation and solar short-wave radiation reduction shaded by clouds on the Meiyu front band, however,it has some diurnal variation. The effect of PV perturbation in the upper troposphere on the formation and development of the Meiuyu front is relatively weaker than others' and not beneficial to the formation and development of the Meiyu front, but it is enhanced in the period of Meiyu front's fast southward movement when the deep North China trough develops and moves southeastward. Rest PV perturbation unrelated to latent heat release in the middle-lower troposphere plays a certain role in the Meiyu front's fast southward movement. Lastly, it should be pointed out that the different PV perturbations maybe play a different role in different stages of the Meiyu front development.