The Impacts of Tropical Cyclone Anatomy
Mike Stockwell, Operations Manager
Those who work in the shipping and maritime community and closely monitor tropical weather as a part of their daily lives or daily operations should have a basic understanding of the favorable environmental factors that support tropical strengthening.
The ideal conditions for a tropical cyclone to strengthen are:
- Warm sea surface temperatures (above 28C) coupled with deep ocean heat content
- A moisture-rich environment
- Low wind shear at the upper levels of the atmosphere
The unfavorable conditions which favor weakening are:
- Cooling sea surface temperatures which can be accelerated by upwelling (below 28C)
- Stronger wind shear at the levels of the atmosphere
- Dry air entrainment
- Land interaction
However, most are not aware of how the overall size and shape of a system, or even the size and structure of a system’s eye can also affect its development and the hazards they bring.
Tropical Cyclone Size and Shape. A mature, well-developed tropical cyclone is nearly circular in shape, while a weaker, less-develop tropical cyclone will be more “lopsided” and asymmetrical in appearance. The winds of a tropical cyclone are nearly calm near the center of circulation but increase rapidly to a maximum only about 5-30 miles from the center, and then fall off slowly toward the outer extent of the storm. Hurricane (or typhoon) force winds on average extend to about 25 miles from the storm center of a small tropical cyclone, while out to about 150 miles from a large one.
Hurricane Ida at Category 4 strength as it aproaches southern Louisiana on 29 August 2021 (courtesy of NOAA).
Effects of the eyewall. The size and shape of a particular tropical cyclone’s eyewall often changes during its lifespan. In a “typical” cyclone, a single eyewall surrounds a nearly circular eye that is mostly cloud-free. However, eyewalls of strong, long-lived cyclones sometimes contract over time, during which the maximum wind speed typically increases. Then, a new eyewall may begin to form outside of the original contracting eyewall, often from one of the innermost spiral bands. When a tropical cyclone has more than one eyewall at once, it is said to have concentric eyewalls.
After the outer eyewall forms, the inner (original) eyewall may decay, during which the maximum wind speed typically decreases. Eventually, the outer eyewall may become the only one left. The new outer eyewall may then begin to contract, leading to another period of strengthening. This cycle, which may repeat multiple times, is called an eyewall replacement cycle.
Eye Size and Shape. The potential intensity of a tropical cyclone increases when there is a smaller radius of the maximum winds. This is because systems with a smaller eye have a better thermodynamic efficiency, where systems with larger eyes are more prone to dissipation. However, systems with smaller eyes are more susceptible to rapid strengthening and weakening.
The eyes of tropical cyclones are not always circular. Oblong, elliptical eyes are sometimes observed, especially in weaker hurricanes. A strong hurricane may have a polygonal-shaped eyewall, where the eye takes on the appearance of a triangle, square, pentagon, or hexagon. Polygonal eyewalls are often associated with eyewall mesovortices, which are smaller-scale atmospheric swirls that can form within the eye and which can produce extremely strong winds. Eyewall mesovortices may remain nearly stationary relative to the hurricane’s center, or they may rotate around the center within the eye or even pass through the hurricane’s center.
Right side of a tropical cyclone. As a rule of thumb, a tropical cyclone’s right side (relative to the direction it is moving) in the northern Hemisphere is the most dangerous part of the storm because of the effect of the onshore winds coupled with the forward speed of the system. In the southern Hemisphere, the left side of the cyclone is the most dangerous.
The cyclone's hazards will be enhanced on the right side (northern hemisphere) of a cyclone:
- Storm surge levels will reach their maximum levels
- Tornadoes are most common here across land. This is due to a secondary force that is caused by frictional effects from land acting against the onshore winds/forward speed of the system, which can help induce tornadic circulations.
Tropical Cyclones are continually monitored by WRI meteorologists, with the above factors always taken into consideration when providing advice to our clients to manage their assets. As always, our team of meteorologists are also available 24/7 to assist with any weather or route questions.
