A diagnostic study on heavy rainfall induced by landfalling Typhoon Utor (2013) in South China: 2. Postlandfall rainfall

Abstract In this part, mechanisms responsible for the maintenance of Typhoon Utor (2013) after landfall and its associated heavy rainfall in South China were investigated with methods including piecewise potential vorticity (PV) inversion. The focus was on the monsoonal influence and the interaction between Utor and the mesoscale convective systems (MCSs) embedded in the southwesterly monsoon flow. The results show that after landfall Utor underwent coalescence with the cyclonic monsoon gyres, related to the quasi‐biweekly oscillation (QBWO) and the Madden‐Julian oscillation (MJO). The QBWO cyclonic gyre, which better developed and coalesced with the tropical cyclone (TC) vortex, enhanced the large‐scale southwesterly monsoon flow southeast of the TC, providing favorable condition for initiation of convection and organization of MCSs in the eastern outer rainband of the TC. Latent heat release in the MCSs provided a positive feedback to enhance the TC circulation and the southwesterly monsoon flow, slowing down the decay of Utor and sustaining heavy rainfall. Piecewise PV inversion confirmed that the nonlinear balanced flow inverted from the PV anomaly associated with latent heating in MCSs led to an increase of the low‐level southwesterly flow south of the TC by over 4 m s−1, which contributed notably to the formation of the low‐level jet and moisture convergence in the eastern outer rainband. It is suggested that the positive feedback between the outer rainband MCSs and the southwesterly monsoon flow is a major mechanism responsible for the maintenance of Typhoon Utor after its landfall and the associated postlandfall heavy rainfall over South China.