Upper-tropospheric potential vorticity anomaly before the rapid intensification of Typhoon Mujigae (2015) and its response to reduced SST

As the strongest typhoon in 2015, the rapid intensification (RI) process of Typhoon Mujigae is simulated at the cloud-resolving scale by utilizing the Weather Research and Forecasting model, coupled with observed sea surface temperature (SST) and reduced SST as the control and sensitivity experiments, respectively. The spatiotemporal distribution characteristics of upper-tropospheric potential vorticity (PV) and its response to SST changes are analyzed simultaneously. The results show a significant upper-tropospheric PV anomaly 6 h before RI, which indicates the approaching RI of Typhoon Mujigae. Deep convection overlies the strong signals of upper-tropospheric PV anomalies. More insight into the PV budget analysis verifies that the diabatic heating effect and vertical advection terms associated with deep convection dominate the upper-tropospheric PV anomaly, inferring that deep convection plays a crucial role in the upper-tropospheric PV anomaly. As SST decreases, diabatic heating and vertical advection effects are weakened due to reduced deep convection; therefore, the PV anomaly in the model run is not stronger than that in the control run, and the RI process of Mujigae disappears. Therefore, from the viewpoint of upper and lower-level interactions, how SST impacts upper-tropospheric PV anomalies and their interactions with RI are clarified. Thus, in addition to SST, the upper-tropospheric PV anomaly might be a significant factor in differentiating between RI and non-RI tropical cyclones.