The Formation of a Deep Hub Cloud in the Eye of Tropical Cyclone Trami (2018)

Abstract:

Airborne observation of Tropical Cyclone Trami (2018) was conducted on 25–28 September 2018. The decrease in sea surface temperature (SST) associated with slow movement of the tropical cyclone, and eyewall replacement were observed on 25 September. On 26–27 September, the formation of convective clouds extending into the middle troposphere in the eye (hereafter referred to as deep hub clouds) was identified from photos taken from an aircraft and satellite images. In this study, the formation of the deep hub cloud is investigated, focusing on the contribution of tropical cyclone-ocean interaction. Data from the dropsonde observation, and coupled atmosphere-ocean and non-coupled atmospheric models are used.

The dropsonde data reveal rapid increase in the radius of maximum wind (RMW) and decrease in the maximum wind speed (Vmax). The coupled atmosphere-ocean model successfully reproduces the rapid increase in RMW and the decrease in Vmax, whereas the non-coupled atmospheric model does not. The increase in RMW is consistent with the outward movement of a local maximum of heat flux from the ocean. The formation of the deep hub cloud is well reproduced in the coupled model. Upward motion is observed when the deep hub cloud appears in the eye. In contrast, the deep hub cloud is not reproduced in the non-coupled model. In order to investigate vertical velocity when the deep hub cloud emerges in the eye, secondary circulation is estimated using the Sawyer–Eliassen equation. When convective heating inside the eyewall in the coupled model is given, weak downward motion is generated in the eye, compared to the case where convective heating in the non-coupled model is given.

These results indicate that decrease in convective heating inside the eyewall and the outward movement of the eyewall, which are probably due to the SST decrease associated with the slow-moving tropical cyclone, lead to the weak downward vertical velocity and thus favorable condition for the formation of the deep hub cloud in the eye.