Environmental Forcing of Upper-Tropospheric Cold Low on Tropical Cyclone Intensity and Structural Change

The interaction between Typhoon Nepartak (2016) and the upper-tropospheric cold low (UTCL) is simulated to better understand the impact of UTCL on the structural and intensity change of tropical cyclones (TCs). An experiment with -out UTCL is also performed to highlight the quantitative impacts of UTCL. Furthermore, idealized sensitivity experiments are carried out to further investigate the specific TC-UTCL configurations leading to different interactions. It is shown that a TC in-teracting with the UTCL is associated with a more axisymmetric inner-core structure and an earlier rapid intensification. Three plausible mechanisms related to the causality between a UTCL and the intensity change of TC are addressed. First, the lower en-ergy expenditure on outflow expansion leads to higher net heat energy and intensification rate. Second, the external eddy forcing reinforces the secondary circulation and promotes further TC development. Ultimately, the shear-induced downward and radial ventilation of the low-entropy air is unexpectedly reduced despite the presence of UTCL, leading to stronger inner-core convec-tions in the upshear quadrants. In general, the TC-UTCL interaction process of Nepartak is favorable for TC intensification ow-ing to the additional positive effect and the reduced negative effect. In addition, results from sensitivity experiments indicate that the most favorable interaction would occur when the UTCL is located to the north or northwest of the TC ata stable and proper distance of about one Rossby radius of deformation of the UTCL.

Automated summary

The interaction between Typhoon Nepartak and the upper-tropospheric cold low is simulated to understand the impact of the cold low on tropical cyclones. An experiment without the cold low is performed to highlight its quantitative impacts. Sensitivity experiments are carried out to investigate different configurations leading to various interactions. The study shows that the interaction between a tropical cyclone and the cold low is associated with a more axisymmetric inner-core structure and earlier rapid intensification.