Thermodynamical impacts on the boundary layer imbalance during secondary eyewall formation

It is well realized that the planetary boundary layer (PBL) imbalance is essential for the secondary eyewall formation (SEF) in tropical cyclones (TCs). The purpose of this study is to investigate the thermodynamic processes in inducing the PBL imbalance. While the control run in an idealized simulation is capable of generating the SEF, simulations with varying fall speeds of raindrops (FSR) in the microphysics resulted in different outcomes for the formation of SEF. The SEF occurs only within a specific range of FSR relative to its default formulation in the model. The gradient wind equation in the PBL is used to diagnose the imbalance leading to SEF. In the case with a formation of SEF, a substantial downdraft induced by TC outer rainbands is observed, leading to positive agradient force (AF). Such downdraft favors the formation of a moat region. Furthermore, the adiabatic warming induced by the downdrafts also modifies the temperature field and the radial gradient of pressure field, contributing to the PBL imbalance. While previous studies have emphasized the importance of the broadening of tangential wind as a trigger for positive AF, our results highlight the importance of the local pressure gradient imbalance within the PBL.