Abstract
Extreme rainfall induced by landfalling tropical cyclones (ERLTCs) in China can cause flash floods and other disastrous impacts, so investigating their genesis and mechanism of enhancement has been attracting considerable attention. This study demonstrates that the extreme rainfall of landfalling tropical cyclones (LTCs) possesses two key properties—namely, maintenance of the LTC circulation and a lagging (slowing down or looping) of its movement, and the monsoon surge can provide a positive contribution to these properties. Specifically, diagnostics show that the low-level cyclonic vorticity and upper-level divergence of ERLTCs are significantly stronger than those of NERLTCs (non-extreme-rainfall-producing LTCs). The continuous intensification of the cyclonic rotation in the lower troposphere before the occurrence of extreme rainfall is a significant feature that distinguishes ERLTCs from NERLTCs. Vorticity budget analysis further shows that the relative vorticity advection term contributes the most to the local increase and maintenance of vorticity in the middle and lower troposphere of ERLTCs under the influence of the southwest monsoonal surge, thus demonstrating that the monsoonal surge favors the maintenance of LTC circulation. On the other hand, the activity of the southwest monsoonal surge is mainly manifested in the zonal wind anomaly, and the corresponding strong westerly transport can significantly reduce the zonal component of the steering flow. As a result, the total steering flow can be weakened, which decreases the northwestward translation speed of ERLTCs, and thus the monsoonal surge favors a lagging (slowing down or looping) of LTC movement. These results reveal the mechanism of influence through which the monsoonal surge affects ERLTCs via its direct impacts on the maintenance of their circulation and lagging of their movement—two distinct evolutionary characteristics.
Highlights
Among all the countries in the world, China experiences the largest numbers of landfalling tropical cyclones (TC)
The vertical vorticity equation in the p-coordinate system is used to further diagnose the physical processes that determine the maintenance of Extreme rainfall induced by landfalling tropical cyclones (ERLTCs) (NERLTC) circulation under the influencing of the monsoonal surge, which has been widely used in previous diagnostic studies on TC structure and intensity evolution (Li et al, 2019; Rantanen et al, 2020)
Combined with our previous findings (Zhao et al, 2021), the mechanism through which the monsoonal surge influences ERLTCs is further discussed from a new perspective
Summary
Among all the countries in the world, China experiences the largest numbers of landfalling tropical cyclones (TC). Carried out a systematic study on the torrential rainfall associated with LTCs and revealed that the southwest monsoon transported the water vapor and unstable energy to TC Bilis (2006) via the low-level jet stream, which was highly beneficial to its maintenance and severe torrential rainfall. Lu et al (2012) compared the mechanisms of how the low-latitude monsoonal surge affected the intensification of the torrential rainfall of LTCs Durian (0103) and Bilis (0604). In the first part (Zhao et al, 2021) of this series of studies, we focused on the water vapor budget analysis of the ERLTC under the influence of monsoon surges, and preliminarily exploring impact of the zonal component of monsoonal surges on the slower translational speed of ERLTCs on the rainfall occurrence day. It can be seen that these TCs generated in the Northwest Pacific, landfalling in or affecting Taiwan Province and the mainland China following the northwestward track
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