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<p>Typhoon (TY) Rai (locally named “Odette”) was a category 5 TY that made landfall in the southeastern Philippines on 16 December 2021, with maximum 1-minute (10-minute) sustained winds of 150 kt (105kt). TY Rai underwent rapid intensification (RI) in the 24&#xa0;h leading up to landfall, intensifying from 70 to 150 kt starting from 0Z on December 15, 2021. This resulted in a large error in the landfall intensity forecast from 85 kt the night before to 140 kt on the morning of landfall day, which led to miscalculated disaster mitigation responses and humanitarian efforts, making TY Rai the second costliest TY in Philippine history. This study examines the processes that contributed to the RI of TY Rai. Analysis reveals that TY Rai rapidly intensified in a moderately sheared (~ 7–8&#xa0;m/s) environment over warm sea surface temperature (29.9&#xa0;°C). At the RI onset, a shear-induced wavenumber-one type pattern of asymmetric convection in the downshear-left region around the TY center was observed. The convective region was advected upshear and wrapped around the TY eye, forming deep hot towers, which helped reduce vortex tilt and support strong upper-level outflow. This was enhanced by increased low-level convergence from the modified wind field induced by the rugged Mindanao terrain as TY Rai approached land. This terrain-induced boundary layer wind convergence increased the inward advection of absolute angular momentum during the RI period, which increased its maximum wind speeds up to 16%, and may explain why half of landfalling RI tropical cyclones strengthen up to the point of landfall.</p>

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Influence of Philippine topography on the pre-landfall intensification of typhoon Rai (2021)

  • 期刊论文

  • 发表日期：

    2025年10月17日

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  • Gerry Bagtasa

摘要

Typhoon (TY) Rai (locally named “Odette”) was a category 5 TY that made landfall in the southeastern Philippines on 16 December 2021, with maximum 1-minute (10-minute) sustained winds of 150 kt (105kt). TY Rai underwent rapid intensification (RI) in the 24 h leading up to landfall, intensifying from 70 to 150 kt starting from 0Z on December 15, 2021. This resulted in a large error in the landfall intensity forecast from 85 kt the night before to 140 kt on the morning of landfall day, which led to miscalculated disaster mitigation responses and humanitarian efforts, making TY Rai the second costliest TY in Philippine history. This study examines the processes that contributed to the RI of TY Rai. Analysis reveals that TY Rai rapidly intensified in a moderately sheared (~ 7–8 m/s) environment over warm sea surface temperature (29.9 °C). At the RI onset, a shear-induced wavenumber-one type pattern of asymmetric convection in the downshear-left region around the TY center was observed. The convective region was advected upshear and wrapped around the TY eye, forming deep hot towers, which helped reduce vortex tilt and support strong upper-level outflow. This was enhanced by increased low-level convergence from the modified wind field induced by the rugged Mindanao terrain as TY Rai approached land. This terrain-induced boundary layer wind convergence increased the inward advection of absolute angular momentum during the RI period, which increased its maximum wind speeds up to 16%, and may explain why half of landfalling RI tropical cyclones strengthen up to the point of landfall.