Bagtasa (2025) Influence of Philippine topography on the pre-landfall intensification of typhoon Rai (2021)

Identification

• Journal: Theoretical and Applied Climatology
• Year: 2025
• Date: 2025-10-17
• Authors: Gerry Bagtasa
• DOI: 10.1007/s00704-025-05843-y

Research Groups

• Institute of Environmental Science & Meteorology, University of the Philippines, Diliman, Quezon City, Philippines

Short Summary

This study investigates the rapid intensification (RI) of Typhoon Rai (2021) prior to its Philippine landfall, revealing that terrain-induced boundary layer wind convergence from the rugged Mindanao topography significantly enhanced its maximum wind speeds by up to 16%.

Objective

• To characterize the rapid intensification (RI) of Typhoon Rai (2021) using observations and numerical simulations to understand the factors leading to its extraordinary RI.
• To investigate the influence of the Philippine landmass on the inner-core dynamics and overall intensification of Typhoon Rai, particularly its sustained intensification up to the point of landfall.

Study Configuration

• Spatial Scale:
  • Numerical simulations: Two-way nested domains with horizontal resolutions of 15 km and 3 km, covering the Western North Pacific and the Philippines.
  • Vertical wind shear (VWS) analysis: Area-averaged around 0–600 km of the tropical cyclone (TC) center.
  • Absolute angular momentum (AAM) analysis: Within a 150 km radius of the TC center.
  • Terrain influence: Began when Typhoon Rai was approximately 990 km from land.
• Temporal Scale:
  • Typhoon Rai event: Rapid intensification from 00:00 UTC on 15 December 2021, to landfall around 06:00 UTC on 16 December 2021.
  • Numerical simulations: Initialized from 00:00 UTC on 12 December 2021, and ended at 00:00 UTC on 19 December 2021.
  • Observational analysis: Focused on the period from 00:00 UTC on 14 December 2021, to 00:00 UTC on 16 December 2021.

Methodology and Data

• Models used:
  • Weather Research and Forecasting (WRF) model (version 4)
  • Single-moment 6-class microphysics scheme (WSM6)
  • Kain-Fritsch cumulus parameterization scheme (outer domain)
  • Yonsei University (YSU) planetary boundary layer (PBL) scheme
• Data sources:
  • Observational:
    • Joint Typhoon Warning Center (JTWC) best-track data (track, intensity)
    • Cooperative Institute for Meteorological Satellite Studies (CIMMS)/University of Wisconsin-Madison real-time Advanced Dvorak Technique (ADT) (intensity)
    • Advanced Microwave Sounder Unit (AMSU) on NOAA polar-orbiting satellites (deep-layer VWS)
    • Naval Research Laboratory (NRL) Next-Generation Weather Satellite Demonstration Project (NexSat) 89-GHz satellite microwave imagery (from GPM GMI, SSMIS, AMSR)
    • Himawari-8 Advanced Himawari Imager (AHI) infrared images
    • Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) Hinatuan radar station (radar reflectivity)
    • Earth Networks Total Lightning Network (lightning data)
    • CIMMS Tropical Cyclone Team TC archive website (atmospheric motion vectors)
  • Reanalysis/Model Input:
    • European Centre for Medium-Range Weather Forecasts (ECMWF) Re-analysis 5th Generation (ERA5) (initial and boundary conditions for WRF)
    • Global Forecast System (GFS) FNL reanalysis data (for sensitivity experiments)
    • Daily Sea Surface Temperature (SST) CDR L4 v2.1 (ESA Climate Change Initiative)

Main Results

• Typhoon Rai underwent extreme rapid intensification (RI) from 36.0 m/s (70 kt) to 77.2 m/s (150 kt) in 24 hours prior to landfall, representing a 99.9th percentile intensity increase.
• The RI occurred in a moderately sheared environment (approximately 7–8 m/s) over warm sea surface temperatures (29.9 °C).
• At the RI onset, a shear-induced wavenumber-one type pattern of asymmetric convection was observed in the downshear-left region, which then advected upshear, wrapped around the eye, and formed deep hot towers, reducing vortex tilt and supporting strong upper-level outflow.
• Numerical simulations demonstrated that the rugged Mindanao terrain enhanced low-level convergence, increasing the inward advection of absolute angular momentum (AAM) in Typhoon Rai's eyewall region.
• This terrain-induced effect increased Typhoon Rai's maximum wind speeds by 6.0% to 16.4% in the six hours preceding landfall compared to a simulation without Philippine landmass.
• The enhanced low-level convergence led to stronger deep convective updrafts and faster warming of the typhoon's inner core, resulting in a higher RI rate (21.1 m/s in the control run vs. 15.6 m/s in the no-terrain run over 24 hours).
• The terrain-altered wind field began influencing the intensification process when Typhoon Rai was approximately 990 km away from the Philippine landmass.

Contributions

• Provides a comprehensive characterization of the extreme rapid intensification of Typhoon Rai (2021) under challenging conditions of moderate vertical wind shear and close proximity to land.
• Quantitatively demonstrates, through numerical simulations, the significant role of Philippine topography in enhancing Typhoon Rai's pre-landfall intensification by modifying boundary layer wind fields and increasing inward absolute angular momentum advection.
• Challenges the conventional understanding that land proximity always leads to tropical cyclone weakening, showing that terrain can, in specific dynamic scenarios, sustain or even further intensify a system.
• Offers a plausible explanation for the observed phenomenon where approximately half of landfalling rapidly intensifying tropical cyclones in the Philippines reach their peak intensity at landfall.
• Emphasizes the critical importance of incorporating terrain-induced atmospheric dynamics into tropical cyclone intensity forecasting, particularly for systems approaching complex coastlines.

Funding

• Philippines’ Department of Science and Technology-Philippine Council for Industry, Energy and Emerging Technology Research and Development (DOST-PCIEERD) under the project entitled ”Analysis of Tropical Cyclone Rapid Intensification in the Philippines: Its Characteristics, Impacts, and Future Projections” (Reference Code: 1211131).

Citation

@article{Bagtasa2025Influence,
  author = {Bagtasa, Gerry},
  title = {Influence of Philippine topography on the pre-landfall intensification of typhoon Rai (2021)},
  journal = {Theoretical and Applied Climatology},
  year = {2025},
  doi = {10.1007/s00704-025-05843-y},
  url = {https://doi.org/10.1007/s00704-025-05843-y}
}