What Is Rapid Intensification? The Secret Behind a Typhoon Becoming a Super Typhoon in 24 Hours
Rapid Intensification (RI) refers to an increase in typhoon wind speeds of more than 15 m/s within 24 hours. It is the most difficult phenomenon to forecast and causes severe disasters. This article analyzes its conditions, case studies, and forecasting challenges.
Definition: A "Violent Upgrade" in 24 Hours
Rapid Intensification (RI) is typically defined as an increase in the maximum sustained wind speed near the typhoon center of ≥15 m/s (approximately 30 knots) within 24 hours. To put it in perspective: a typhoon that holds just a basic ID badge in the morning can transform into a Super Typhoon wearing "level 17 contact lenses" by evening.
Four Conditions Triggering Rapid Intensification
- Very High Sea Surface Temperatures and Deep Warm Water Layers: Sea surface temperatures ≥29°C with a thick warm water layer (high Ocean Heat Content). This ensures the typhoon does not churn up cold water from below, maintaining an uninterrupted energy supply.
- Very Low Vertical Wind Shear: Wind direction and speed are nearly consistent across different atmospheric layers. This allows hot tower convection to stack vertically without being "sheared apart."
- Strong Upper-Level Divergence: The presence of an outflow channel in the upper atmosphere (often aided by interactions with westerly troughs or the edge of the subtropical high). This acts like a chimney, drawing away rising air currents.
- Compact Inner Core Structure: A small, circular eye that contracts (especially after the completion of an eyewall replacement cycle) is most prone to explosive intensification.
Classic Case Studies
- Rammasun (2014): Intensified by approximately 25 m/s within 24 hours after entering the South China Sea. It made landfall in Wenchang, Hainan, as a Super Typhoon, shattering the empirical belief that "no Super Typhoons make landfall in the South China Sea."
- Mangkhut (2018): Underwent continuous RI over the open waters of the Western Pacific, reaching a peak intensity of 65 m/s.
- Noru (2022): Its wind speeds nearly doubled within 24 hours before making landfall in Luzon, Philippines, leaving locals with almost no reaction time.
Why Is RI the "Nightmare" of Forecasters?
While track forecasting has improved significantly over the past 30 years, intensity forecasting has progressed slowly, with RI being the biggest shortfall:
- The outbreak of inner-core convection is a meso- and small-scale process, and global model grid resolutions are often insufficient to capture it.
- Subsurface ocean data is sparse, leading to large errors in estimating ocean heat content.
- When RI occurs 12–24 hours before landfall (near-shore rapid intensification), it leaves extremely little time for upgrading defensive measures.
Implications for the Public
If you see warnings about "rapid intensification near the coast," remain highly alert: Warning signals may be issued with skipped levels (e.g., jumping directly from Blue to Orange). Prepare defenses based on the "upper limit" scenario; do not judge the typhoon's landfall power by its intensity from the previous day.