The Diurnal Variation of Deep Convective Cells in Tropical Cyclones Undergoing Rapid Intensification

The deep convective cells within the radius of maximum wind (RMW) could be an important precursor for undergoing the rapid intensification (RI) process in tropical cyclones (TCs). To address this interesting feature, the present study has selected four RI cases (Nepartak 1^st, Meranti 14^th, Chaba 18^th, Nockten 26^th) in 2016 in the western North Pacific.

Based on the Himawari-8 satellite imagery (11.2 µm), the anvil core and area are defined as the brightness temperature (Tbb) is lower than 195 K and 205 K, respectively. Except for the case of Nockten, the number of anvil core and area within 200 km radius from TC center tends to be maximum during nighttime, while its number tends to be minimum during daytime. When there is an explosive increase of anvil core and area, the RI event takes place in all cases. Most large intensification generally occurs during nighttime.

Based on water vapor channel (6.2 ~ 7.3 µm) of Himawari-8 satellite imagery, we have investigated how the eye structure could be formed in the mid and upper troposphere. Before TCs have the distinct eye structure, the anvil core and area shows the similarity to hook-echo shape. Few hours later, the anvil core and area rapidly surround the relatively dry area, and after that, the relatively dry area is positioned to the TC center and stuck in anvil area. It thus leads to the distinct eye structure during RI period. We think that the increasing lower Tbb area like anvil core and area and its duration time in the upper troposphere especially in nighttime could be a significant precursor to RI.