All tropical cyclones rotate around an area of lowatmospheric pressure near the Earth's surface. The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth's surface at sea level.[1] Tropical cyclones are characterized and driven by the release of large amounts of latent heat of condensation as moist air is carried upwards and its water vapor condenses. This heat is distributed vertically, around the center of the storm. Thus, at any given altitude (except close to the surface where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.[2]Rainbands are bands of showers and thunderstorms that spiral cyclonically toward the storm center. High wind gusts and heavy downpours often occur in individual rainbands, with relatively calm weather between bands. Tornadoes often form in the rainbands of landfalling tropical cyclones.[3]Annular hurricanes are distinctive for their lack of rainbands.[4] While all surface low pressure areas require divergence aloft to continue deepening, the divergence over tropical cyclones is in all directions away from the center. The upper levels of a tropical cyclone feature winds headed away from the center of the storm with an anticyclonic rotation, due to the Coriolis force. Winds at the surface are strongly cyclonic, weaken with height, and eventually reverse themselves. Tropical cyclones owe this unique characteristic to requiring a relative lack of vertical wind shear to maintain the warm core at the center of the storm.[5][6]
A strong tropical cyclone will harbor an area of sinking air at the center of circulation, developing into an eye. Weather in the eye is normally calm and free of clouds, however, the sea may be extremely violent.[3] The eye is normally circular in shape, and may range in size from 3 to 370 km (2–230 miles) in diameter.[7][8] Intense, mature hurricanes can sometimes exhibit an inward curving of the eyewall top that resembles a football stadium; this phenomenon is thus sometimes referred to as the stadium effect.[9]
There are other features that either surround the eye, or cover it. The central dense overcast is the shield of cirrus clouds produced by the eyewall thunderstorms;[10] in weaker tropical cyclones, the CDO may cover the eye completely.[11] The eyewall is a band around the eye, in which the greatest wind speeds are found, and where clouds reach the highest and precipitation is the heaviest. The heaviest wind damage occurs where a hurricane's eyewall passes over land.[3] Associated with eyewalls are eyewall replacement cycles, which occur naturally in intense tropical cyclones. When cyclones reach peak intensity they usually - but not always - have an eyewall and radius of maximum winds that contract to a very small size, around 5 to 15 miles (10–25 km). At this point, some of the outer rainbands may organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum. During this phase, the tropical cyclone weakens (i.e. the maximum winds die off a bit and the central pressure goes up), but eventually the outer eyewall replaces the inner one completely. The storm can be of the same intensity as it was previously or, in some cases, it can be even stronger after the eyewall replacement cycle. Even if the cyclone is weaker at the end of the cycle, the fact that it has just undergone one and will not undergo another one soon will allow it to strengthen further, if other conditions allow it to do so.[12]
Mechanics
Structurally, a tropical cyclone is a large, rotating system of clouds, wind, and thunderstorms. Its primary energy source is the release of the heat of condensation from water vapor condensing at high altitudes, the heat being ultimately derived from the sun. Therefore, a tropical cyclone can be visualized as a giant vertical heat engine supported by mechanics driven by physical forces such as the rotation and gravity of the Earth.[13] In another way, tropical cyclones could be viewed as a special type of Mesoscale Convective Complex, which continues to develop over a vast source of relative warmth and moisture. Condensation leads to higher wind speeds, as a tiny fraction of the released energy is converted into mechanical energy;[14] the faster winds and lower pressure associated with them in turn cause increased surface evaporation and thus even more condensation. Much of the released energy drives updrafts that increase the height of the storm clouds, speeding up condensation.[15] This gives rise to factors that provide the system with enough energy to be self-sufficient and cause a positive feedback loop, where it can draw more energy as long as the source of heat, warm water, remains. Factors such as a continued lack of equilibrium in air mass distribution would also give supporting energy to the cyclone. The rotation of the Earth causes the system to spin, an effect known as the Coriolis effect, giving it a cyclonic characteristic and affecting the trajectory of the storm.
The factors to form a tropical cyclone include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist and allow it to create a feedback loop by maximizing the energy intake possible – for example, such as high winds to increase the rate of evaporation – they can combine to produce the violent winds, incredible waves, torrential rains, and floods associated with this phenomenon.
Deep convection as a driving force is what primarily distinguishes tropical cyclones from other meteorological phenomena.[16] Because this is strongest in a tropical climate, this defines the initial domain of the tropical cyclone. By contrast, mid-latitude cyclones draw their energy mostly from pre-existing horizontal temperature gradients in the atmosphere.[16] To continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the needed atmospheric moisture. The evaporation of this moisture is accelerated by the high winds and reduced atmospheric pressure in the storm, resulting in a positive feedback loop. As a result, when a tropical cyclone passes over land, its strength diminishes rapidly.[17]
The passage of a tropical cyclone over the ocean can cause the upper ocean to cool substantially, which can influence subsequent cyclone development. Cooling is primarily caused by upwelling of cold water from below due to the wind stresses the tropical cyclone itself induces upon the upper layers of the ocean. Additional cooling may come from cold water from falling raindrops. Cloud cover may also play a role in cooling the ocean by shielding the ocean surface from direct sunlight before and slightly after the storm passage. All these effects can combine to produce a dramatic drop in sea surface temperature over a large area in just a few days.[18]
Scientists at the National Center for Atmospheric Research estimate that a tropical cyclone releases heat energy at the rate of 50 to 200 trillionjoules per day.[15] For comparison, this rate of energy release is equivalent to exploding a 10-megaton nuclear bomb every 20 minutes[19] or 200 times the world-wide electrical generating capacity per day.[15]
While the most obvious motion of clouds is toward the center, tropical cyclones also develop an upper-level (high-altitude) outward flow of clouds. These originate from air that has released its moisture and is expelled at high altitude through the "chimney" of the storm engine.[13] This outflow produces high, thin cirrus clouds that spiral away from the center. The high cirrus clouds may be the first signs of an approaching tropical cyclone.[20]
