Tropical Cyclone Fullness as a Bound on Tropical Cyclone Intensification
Abstract
Wind structure is a significant factor that modulates the internal dynamical features of tropical cyclones (TC). The concept of TC fullness, defined as a ratio of the extent of the outer-core wind skirt (i.e., DR=R34-RMW) to the outer-core size (TCF=DR/R34=1-RMW/R34), describes the overall characteristics of wind structure in both inner- and outer-core region. About 64% of the major TCs (category 3-5) could achieve high fullness (TCF≥0.8) during their weak stage (TS, category 1-2) before intensifying rapidly to major status. Therefore, it is reasonable to anticipate a critical role that fullness plays in controlling the intensification of TCs.
Based on global intensifying TCs (intensification rate, IR, is greater than or equal to 5kt in 12h) from IBTrACs dataset during 2001 to 2019, the combined effect of intensity and fullness on TC intensification is discussed. Positive vorticity is required for an inertial stable vortex, which set a lower bound on TCF, TCFmin=1-V34/Vmax, where Vmax is the wind speed at RMW and V34 equals to 34 kt/s. TCFmin increases as the increase of intensity, leading to a strong positive relationship between intensity and TCF for intensifying TCs (the correlation efficiency is about 0.72). The deviations of TC fullness from the lower bound TCFmin at the corresponding intensity is measured by ΔTCF (ΔTCF=TCF-TCFmin). ΔTCF is related to the vorticity in the outer-core region that contributes to the acceleration of cyclonic wind in a TC. Our results show that, as the increase of IR, TCs are more likely to have moderate ΔTCF (0.1≤ΔTCF≤0.3), which indicates a favorable match between intensity and fullness during TC intensification. About 51% of slowly intensifying TCs (12-hr IR≤15kt) possess moderate ΔTCF, while the percentage increases to 62% for rapid intensification TCs (RI, 12-hr 15kt≤IR<30kt) and further increases to 69% for extreme intensification TCs (12-hr IR≥30kt, account for about 6% of the samples). On the other hand, TCs with moderate ΔTCF are more capable of achieving higher intensification rate than their counterparts. The exceeding probability of reaching extreme IR for TCs with moderate ΔTCF nearly double those with too low or too high ΔTCF. Since TCFmin increases rapidly with the increase of intensity in the region of Vmax≤100kt where most of RI TCs occur, a moderate ΔTCFwill lead to moderate to high fullness. For TCs with favorable configuration of intensity and fullness (0.1≤ΔTCF≤0.3), grater IR is more likely to be associated with higher fullness. Compared to TCs with TCF<0.6, those with TCF≥0.6 are 20% more likely to reach RI, and are 4 times the probability of obtaining extreme IR. This study finds out the restriction on wind structure for the maintenance and intensification of TCs, and emphasizes the significance of the cooperative effect of intensity and fullness during intensification. The findings here suggest that fullness could be considered as an indicator in forecast models to help achieving the challenging task in the prediction of intensification.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMA136...03G
- Keywords:
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- 3315 Data assimilation;
- ATMOSPHERIC PROCESSES;
- 3360 Remote sensing;
- ATMOSPHERIC PROCESSES;
- 3372 Tropical cyclones;
- ATMOSPHERIC PROCESSES;
- 4313 Extreme events;
- NATURAL HAZARDS