Ensemble sensitivity analysis based on convective permitting ensemble simulations is used to understand the processes associated with tropical cyclone (TC) intensification under idealized conditions. Partial correlations between different variables and the future TC intensification rate, with the effect of intensity removed, are used to identify the sensitive factors. It is found that the equivalent potential temperature (θe) in the region from the radius of maximum wind (RMW) to 3 times the RMW below 2 km (hereafter, the sensitive region) has the largest correlation (over 0.7) with 2.5 hour intensity change. It is found that higher θe in the sensitive region is associated with not only a stronger updraft but also an inward shift of vertical motion in the mid-to-upper eyewall. This suggests that higher θe just outside the RMW is favorable to TC not only because of the larger amount of the heating, but also due to the heating location that is closer to the center. Trajectory analysis shows that the parcels in the sensitive region are mainly from the boundary layer inflow and the mid-level inflow. It is found that when the outer rainband is active, the mid-level inflow becomes stronger and is able to bring more low θe air into the boundary layer, and the θe radially inward to the rainband decreases. Verification experiments justify that higher θe around the RMW to 3 times the RMW is favorable to TC intensification, while higher θe away from 5 times the RMW is shown to be unfavorable for TC intensification.

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Current affiliation: Department of Atmospheric Sciences, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan.