The Life Cycle of a Typhoon
Author: 阿白特尔
Everything has a life cycle, and typhoons are no exception. In this article, we will follow a typhoon through its birth, growth, maturity, and eventual demise.
The birth of a typhoon
A typhoon is a tropical cyclone that forms in the northwestern Pacific. Tropical cyclones develop over many of the world’s oceans. Those in the eastern Pacific and North Atlantic are called hurricanes; those in the Indian Ocean and South Pacific are called cyclones; and those in the northwestern Pacific are called typhoons. Despite the different names, they form and operate through the same mechanisms and belong to the same type of weather system. How, then, is a tropical cyclone born?
Across the vast tropical ocean, intense sunlight has warmed a deep layer of seawater. Water evaporates rapidly from the warm surface, producing broad expanses of moist, vapor-rich air. A disturbance then develops in the atmosphere. It may come from a monsoon, a tropical atmospheric wave, or an upper-level cold-core vortex. Whatever the source, warm, humid air begins moving toward a common center. The disturbance gathers large quantities of this air at the center, where it rises. At the lower temperatures aloft, water vapor condenses into droplets. Condensation releases heat, warming the surrounding air, making it less dense and still more likely to rise. As the updraft strengthens, it draws in increasing amounts of air from every direction, continually supplying more moisture. More vapor condenses and releases more heat, which strengthens the updraft again. A positive-feedback loop has formed: condensation strengthens the updraft; the updraft brings in more vapor; and the condensation of that vapor produces an even stronger updraft. As the cycle continues, the updraft intensifies and more air converges from the surroundings. The Coriolis force makes the converging air rotate around the center. When the maximum sustained wind in this circulation reaches Beaufort force 8, a tropical storm is born.
This account shows that the key to a typhoon’s birth is the positive-feedback loop between vapor condensation and rising air, sometimes called the typhoon’s “engine.” Typhoons consequently cannot form just anywhere. Land and colder seas do not supply enough water vapor to start the engine, so typhoons generally cannot form there. In places where low-level and upper-level winds differ greatly, the winds tilt the storm’s lower and upper structure out of alignment. Its engine becomes distorted: condensation aloft can no longer strengthen the inflow and ascent below effectively, making development difficult. At extremely low latitudes, meanwhile, the Coriolis force is too weak to set the converging air into rotation. The system remains an ordinary cluster of convective clouds rather than developing into a larger storm.
The growth of a typhoon
A strengthening typhoon passes through several stages. In ascending order of wind speed, China classifies them as tropical storm, severe tropical storm, typhoon, severe typhoon, and super typhoon. The United States uses a similar scale for hurricanes: tropical storm, followed by Categories 1 through 5. Although the categories differ, they serve the same purpose. Reporting a storm’s category tells the public how intense it is and allows people to prepare both mentally and materially.
Does every typhoon pass through all these categories and reach the highest one? No. Many remain tropical storms throughout their lives. Some slowly climb to typhoon strength; others intensify steadily into super typhoons; and a few leap through several categories to become super typhoons within a single day. Five main factors constrain or promote intensification.
1. Ocean heat content and sea-surface temperature
The higher the sea-surface temperature and the greater the ocean heat content, the more energy a typhoon can obtain and the easier it is for the storm to strengthen. If a typhoon lingers in one place long enough to exhaust the available heat in the water below, it will weaken rapidly.
2. Upper-level outflow
A typhoon lifts low-level air to high altitudes, and that air must go somewhere once it arrives. It flows outward in every direction. When the surrounding environment allows efficient outflow, air can leave smoothly, making room for more warm, humid air to rise and helping the typhoon strengthen.
3. Vertical wind shear
This term describes the mismatch between upper-level and lower-level winds mentioned above. Excessive vertical wind shear separates a typhoon’s upper and lower structure. A storm effectively torn in two is unlikely to become very strong.
4. Moisture
Air flows into a typhoon at low levels from all directions. If the surrounding air is very dry, the storm may lack moisture. Without enough water vapor to condense and release the heat that powers its engine, rapid intensification is difficult.
5. Consolidation rate
A developing typhoon sometimes forms more than one center. If the incipient storm has multiple centers, rapid consolidation is necessary before it can reach a high intensity. Otherwise, the competing centers fight among themselves, and this internal conflict leaves the storm as a whole weak. When most of the five conditions are favorable, a storm may develop into a super typhoon and reach true maturity.
The maturity of a typhoon
As a typhoon strengthens, the inertial centrifugal force associated with its spiraling motion gradually displaces convection away from the center. Winds and rain at the center itself become weaker, and blue sky and white clouds may even appear. This region of suppressed convection that develops at the center of a powerful typhoon is the eye. Seen from space, it resembles a calm eye surrounded by clusters of clouds filled with violent wind and rain. The formation of an eye marks a typhoon’s true maturity.
The eye contains the lowest air pressure in the typhoon. Moving outward from it, the first feature is a rapidly rotating wall of cloud known as the eyewall, where the typhoon’s strongest winds occur. Beyond the eyewall lies a roughly circular region of vigorous convection covered by the cloud tops of eyewall cumulonimbus clouds. This is the central dense overcast (CDO). Farther out, spiral rainbands extend from the storm, with wind and rain generally weakening with distance. At the typhoon’s outermost edge, descending air around the storm often produces clear skies and oppressive heat. In Chinese meteorological slang, this typhoon-induced heat is vividly described as “feeding on subsidence.”
How can we determine a typhoon’s intensity? Under the Dvorak technique for analyzing tropical cyclones, two main factors are considered: the temperature of the CDO cloud tops and the temperature of the eye. If a sufficiently broad ring of cloud tops falls into an extremely cold color range, generally below −60°C, while the eye is comparatively warm, generally above 0°C, the typhoon can be considered exceptionally intense.
After reaching maturity, a typhoon often undergoes a process called an eyewall replacement cycle. A mature typhoon acts like an enormous pump, drawing moisture inward from every direction, lifting it through the eyewall, and then expelling it aloft. Some moisture, however, rises early and triggers strong convection before it reaches the eyewall. If that convection forms a ring around the existing eyewall, the typhoon develops concentric eyewalls—a large eye surrounding a smaller one. The energy then becomes more dispersed and the storm weakens to some degree. The inner eye gradually disappears after the outer eye cuts off its moisture supply. Once the outer eye becomes the sole eyewall, the typhoon can strengthen again. Not every typhoon completes an eyewall replacement successfully; a failed cycle can prevent the storm from reaching very high intensity.
The demise of a typhoon
Everything that begins must end. How does a typhoon die?
There is no single answer. Some make landfall and gradually dissipate as terrain creates friction and cuts off their moisture. Some enter the mid-latitudes and, under the influence of fronts, transition into extratropical cyclones. Some fall into a trap of vertical wind shear, are split across the middle, and rapidly collapse. Others move over cold water, where their engines sputter out and stall.
An ordinary typhoon has a life cycle of roughly ten days from formation to dissipation. During those ten days it gathers energy, displays its power, drives fierce winds and enormous waves, and finally disappears. Some typhoons live much longer. Typhoon Noru in 2017, for example, was pulled about by other storms and wandered for 28 days before making landfall in Japan and dissipating. Its life followed a winding, difficult course with many reversals. Interested readers can watch animations of its clouds and appreciate the force of life embodied by a typhoon.
Some readers may now ask: a typhoon is a disaster, so why describe it with such affection? Calling it only a disaster is one-sided. Typhoons bring both benefits and harm. They cause destructive winds, unnecessary torrential rain, and flooding where they make landfall, but they also help balance Earth’s climate and transport heat and water vapor. A violent phenomenon sometimes occurs because it prevents something still more violent. A typhoon may look ferocious, yet it helps preserve a deeper balance.
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