In this study, statistical methods and sensitivity experiments of WRF model were used to explore the enhancement mechanism of weak tropical cyclones (TCs) crossing Luzon. Statistical results show that the number of tropical depression (TD) before landing was the largest (46%) among the 212 TCs crossing Luzon from 1951 to 2017. Further, about 79% TDs would be more intensified after they crossed the Luzon island, and the proportion is higher in September and October which is 87% and 88%, respectively. Through harmonic analysis of the multi-year monthly wind field of enhanced years, it is found that the large-scale environmental flow field in September and October is beneficial to the enhancement of TCs crossing Luzon. High-level out-flow of TCs is connected with westerly jet in the north and easterly jet in the south after crossing Luzon, which strengthens the TCs’ upper divergence. At the low level, cyclonic circulation would form on the west side of Luzon Island due to the monsoon transition in autumn, which favors the low-level cyclonic vortices of TC after it crossed Luzon from east to west. The above-mentioned environmental flow field typically existed in October 2017, which was conducive to the rapid intensification of Khanun (1720) after it crossed Luzon Island. The flow field and topographic sensitivity experiments are designed to explore the enhancement mechanism of weak TCs over Luzon Island. The sensitivity experiments verified the results of harmonic analysis, and it further reveals that the westerly jet plays a greater role in the enhancement of TCs crossing the island. The topographic sensitivity experiments show that the process of enhancing TCs by the topography of Luzon Island occurs in three stages: before TC landing, after landing and leaving islands. Overall, favorable large-scale environmental flow field and Luzon topography are both possible reasons for the enhancement of weak TCs crossing Luzon, in which the former plays a leading role in this phenomenon.
 

tropical cyclone,luzon island,enhancement