Intensity Change of NORU (2017) During Binary Tropical Cyclones Interaction

Abstract

In this study, the intensity change of tropical cyclone (TC) Noru (1705) is investigated by using numerical simulations. TC Noru experienced a weakening stage during the binary interaction with TC Kulap (1706). In the presence of Kulap, the model captures well the observed intensity change. On the contrary, once Kulap is removed, it fails to reproduce the weakening stage of Noru. Possible mechanisms are proposed as follows: During the mutual interaction, the competition of moisture supplies will lead to a dry air layer wrapped around Noru. Meanwhile, due to the circulation of nearby Kulap, the vertical shear is relatively larger in the adjacent of Noru. As proposed by previous studies, the negative impacts by dry air layer will be enhanced through two possible pathways. The first is that the dry air directly impacts the TC inner core convection under vertical wind shears. The second is through the downward entropy flux into boundary layer, by which reducing the boundary layer entropy and thus convection. Dynamically, along with the approach of Kulap, it will induce barotropic instability at the outer region, by which greatly enhances the inner-core asymmetry. The asymmetric component will grow at the expense of the mean flow, and thus TC intensity weakens.