Hurricanes: Their Nature and Impacts on Society - Climate Science ...

TROPICAL CYCLONES ON PLANET EARTH
of the storm. A disruption in the outflow from the hurricane, due to the upperlevel
divergence being replaced by upper-level convergence, as might occur due
to large-scale atmospheric circulation changes, can also weaken <strong>and</strong> even
dissipate a storm.
<strong>Hurricanes</strong> also weaken rapidly when they traverse over cool water or inl<strong>and</strong>
after they make l<strong>and</strong>fall (Kaplan <strong>and</strong> DeMaria 1995). There are two major
reasons for the decay of hurricanes over a cool ocean or after l<strong>and</strong>fall. First, a
hurricane requires that the warmest temperatures which are associated with the
storm be in its center in order for the heat engine to work. However, as air spirals
into a hurricane, it exp<strong>and</strong>s as a result of the lower pressures closer to the eye.
Unless heat is added, this expansion results in cooling. (The same process occurs
when air is let out of a tire. The exp<strong>and</strong>ing air at the nozzle from the pressurized
tire is substantially colder than the surrounding air.) The cooling works against
maintaining the heat engine.. This cooler air is not a favorable environment for
continued thunderstorm development. Over l<strong>and</strong> <strong>and</strong> cool ocean surface waters,
there is no heat source to counteract this cooling. The direct result is that
thunderstorms weaken within the eye wall. The low central pressure of the
tropical cyclone correspondingly rises as the coupling between the lower <strong>and</strong>
upper troposphere is reduced <strong>and</strong>, as a result, the divergent winds in the upper
levels of the tropical cyclone are diminished in strength. Thus, the eye wall tends
to be destroyed as the hurricane weakens to tropical storm strength. The second
reason that hurricanes decay over l<strong>and</strong> <strong>and</strong> cold oceans is that they lose the
unlimited source of water vapor from the warm ocean water which is essential
for maintaining the strong thunderstorms in the eye wall region...
Wind speeds also decrease near the ground after l<strong>and</strong>fall. While not directly
related to a hurricane's decay, this decrease in wind speed near the ground is
due to the aerodynamic roughness of the l<strong>and</strong>. Trees, buildings, <strong>and</strong> even
grassl<strong>and</strong>s tend to be rough surfaces compared to the ocean, with the result
that wind is decelerated. One major result of this difference in roughness is that,
even if the wind well above the surface remained constant at l<strong>and</strong>fall, the
greater retardation of the flow by the rougher surface over l<strong>and</strong> would result in
slower wind speeds at the surface.
The cooler air over l<strong>and</strong> magnifies this reduction in wind speed near the
surface even further. Therefore, while the winds well above the ground may
even accelerate, as those levels tend to become decoupled from frictional
retardation by the surface, the winds near the surface can become quite weak.
While this decoupling of near surface flow from the winds aloft does not
.For example, air which originates at a pressure of 1000 mb <strong>and</strong> 80°F (27°C) in the region
surrounding the storm would cool to 64°F (18°C) at a pressure near the center of a storm of 900
mb, unless heat were added.
..A surface of water evaporates at a rate which is directly related to its surface temperature.
For the same amount of moisture just above the surface, <strong>and</strong> a surface pressure of 1000 mb, an
ocean area with a temperature of 80°F (27°C), for example, will evaporate at a rate about 64%
greater than when the surface is at 64°F (18°C).
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