The Process of Hurricane Landfall
As the immensely powerful Hurricane Matthew (Category 4 as of this writing) tears through the Bahamas and bears down on Florida, now seems as good a time as any to discuss the behavior of hurricanes as they cross over coastlines onto land. Since the greatest impacts at landfall occur in and near the eyewall, let’s begin our discussion there.
The sometimes-cloudless center of rotation of a hurricane (most commonly referred to as the “eye”) has on its periphery the “eyewall,” a structure containing the most intense rains and winds in the entire storm (see the example of Jeanne from 2004, making landfall not far from where Matthew is forecast to hit). However, even a well-developed, highly symmetric eyewall is not an equal-opportunity destructive force, as the diagram below illustrates. In the Northern Hemisphere, a hurricane’s right-front quadrant (the northeastern quarter of the storm) contains the most forceful winds because the motion of the storm itself contributes to these wind speeds. As a direct result of these more intense winds, hurricanes are usually able to accumulate the greatest amount of water on their NE sides as well, leading to maximum storm surges occurring slightly to the east of the center of landfall.
While the aggregate effects of high winds, heavy rains, and storm surge can obviously result in terrible damage, the winds themselves are often not as severe as advertised. This isn’t a product of media hype – rather, it’s a result of friction. The wind speed attached to a hurricane represents the maximum sustained wind anywhere in the storm while it’s over water. Therefore, a “140mph (225kph) hurricane” may only be producing 140mph winds over a small area, and these winds begin decelerating when they start interacting with objects that slow their momentum (land, buildings, trees, etc.). Unfortunately, much of this kinetic energy from the slowing wind is transferred to objects on the ground, resulting in snapped trees, airborne debris, and damaged buildings.
As a storm’s center spends more time near or over land, the storm continues to weaken. The effects of friction continue to accumulate, slowing the storm’s wind and rotation. Moreover, the loss of warm ocean waters beneath the hurricane’s thunderstorms means that its chief source of energy is lost, and the intensity of its rains and winds further decrease. Even large and powerful major hurricanes survive for just a handful of hours to perhaps a day or so over land. With a storm like Matthew, we can only hope that its projected track grinding along the coast will lead to a quick demise.
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