Archived: Structure and Motion

The structure of tropical cyclones is determined by the latent-heat release mechanism that forms them. The graphic here does a better job than words in describing what these storms look like up close.

Features of a tropical cyclone

Eye – The eye is the centre of the storm and is where the lowest pressure is found. The eye is roughly circular and is an area of warm subsiding air that is generally free of clouds and weather, although the sea surface will generally be an area of chaotic seas coming in from all directions toward the centre. The eye can become obscured and filled with cloud if the storm is embedded within a strong atmospheric flow. Higher up in the storm the eye can be 10°C warmer than the surrounding air. A hurricane eye averages 30-60 km in diameter but can be less than 10 km or as much as 200 km across.

Eyewall – The eyewall is the ring of deep convection surrounding the eye. This is where the highest surface winds are found. There are strong updrafts and downdrafts in the eyewall, but  overall the updrafts are stronger and more frequent.

Spiral rain bands – The convection in tropical cyclones organizes itself into long, narrow rain bands which spiral in towards the centre of the storm. The rain bands are oriented in the same direction as the horizontal wind.

Surface winds – In the Northern Hemisphere, the surface winds of a tropical cyclone rotate in a counter-clockwise direction. As shown in the diagram, the winds are zero at the centre of the eye, increase dramatically in the eyewall, then decrease gradually farther away from the storm. Any location over which the eye of a hurricane passes experiences the strongest winds twice, on each side of the eyewall. The first exposure to eyewall winds comes through a gradual building of the winds until the peak winds are experienced. Winds then drop to calm in the eye. When the eyewall at the other side of the storm arrives, the winds increase almost instantaneously to their maximum value, and from the opposite direction.

Winds in the storm – Wind speeds increase the higher you move from the earth’s surface. In the eyewall, winds reach a maximum strength about 450-600 m (~1500-2000 feet) above the surface. Depending on the underlying stability of the air near the surface, the increase in wind between the surface and this maximum can be so sharp that the wind at the top of a high-rise office building could be one Saffir-Simpson category higher than the wind near the surface. Above the maximum wind, the winds then decrease with height.

This profile of wind speed vs. height in a hurricane is an average of values obtained from dropsonde data (dropsondes are instruments that are ejected into the storm during air reconnaissance). The vertical scale is measured in feet. The horizontal scale is measured as a percentage of wind speeds measured by air reconnaissance that typically flies near the 10 000-feet level in the hurricane. The data have been plotted this way because of a long-standing practice of estimating surface winds based on flight-level winds (which was necessary before dropsondes were introduced in 1998).

Winds above the storm – If a hurricane is well-developed, the winds at the top rotate clockwise--the opposite direction of the surface winds. This is because a hurricane is supported by an area of high pressure aloft that helps to vent the rising air and serve as a “chimney” to evacuate the air before it piles up and collapses on itself. Evidence of this upper-level high is seen by the clockwise rotation on animated satellite imagery and tells forecasters that the cyclone is well-developed.

Size, shape and beauty of a tropical cyclone

Tropical cyclones are generally measured by the reach of their gale-force winds. These storms can be very small, measuring less than 100 km across, or they can be monsters spanning over 1000 km. Category 4 hurricanes Andrew (1992) and Floyd (1999), shown below, clearly demonstrate that size and intensity are not related.

This 1998 infrared satellite image shows four different hurricanes that existed at the same time in the Atlantic and Gulf of Mexico (Georges, Jeanne, Karl, and Ivan). The size and shape of each one make it clear that they don’t come in a uniform shape or size.

The danger posed by hurricanes does not steal from their beauty--beauty that stems from both the simple and the complex. This satellite image of Category 5 Hurricane Isabel (2003) shows the eye to be filled with a series of smaller vortices that revolved around the larger centre of the storm. This particular image proved to be an important verification of a structure that had been theorized in a paper by Kossin and Schubert in 2001.

High-resolution satellite images of the top of a hurricane, such as this image of Category 5 Hurricane Wilma (2005), show the sloping nature of the top of the vortex. This phenomenon, known as the “stadium effect,” is like the sloping walls of Rome’s Colliseum. A U.S. research aircraft flew into Hurricane Katrina in 2005, and one of the crew snapped a shot near the top of the storm, capturing a close-up view of the “stadium-effect.”

Tropical cyclone motion

We live at the bottom of an ocean of air above us, and that ocean is filled with many currents that move the weather around the globe. Tropical cyclones simply move with the currents in which they are embedded. Because tropical cyclones extends 25 000-35 000 feet through the atmosphere, they generally move in the direction and with the average speed of the deep air current layer. If these wind currents are very weak then other forces start to dominate the motion, however, that is never the case by the time tropical cyclones reach Canadian waters because our air currents are never that weak.

As tropical cyclones move into mid-latitudes and become “involved with” the atmosphere, they slow down with respect to the wind currents and eventually move at only half the speed of the deep layer average wind. However, with respect to the ground, the storm systems actually speed up because the wind currents aloft can be 3-6 times faster than their tropical counterparts. See extratropical transition.