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Hurricanes: Tempests in a Greenhouse Kerry Emanuel Hurricanes: Tempests in a greenhouse Kerry Emanuel quick Greenhouse gases make Earth’s surface hotter than it would be if the study planet were simply a blackbody radiator. That additional warming is an important driver of hurricanes. Kerry Emanuel is a professor of atmospheric sciences at the Massachusetts Institute of Technology in Cambridge, Massachusetts. The tropics have generally the most benign climates found ated with precipitation absorb most of the entropy produc- on Earth, with gentle breezes and small daily and seasonal tion and spare people from violent winds. But not always. temperature variations. Why, then, do tropical climates breed the most destructive wind storms known? This brief tutorial A Carnot engine explains the paradox and presents an overview of hurricane In the part of the tropics where the sea surface is warm physics. enough and the projection of Earth’s angular velocity vector onto the local vertical axis is large enough, random small- The greenhouse effect scale convective currents sometimes organize into rotating Of the solar energy that streams to Earth, about 30% is re- vortices known as tropical cyclones. In computer models of flected by clouds or the surface, and an additional small per- the tropical atmosphere, such organization can happen spon- centage is directly absorbed by atmospheric water—either taneously, but usually only if a combination of ocean tem- gaseous or condensed in clouds. The radiation that escapes perature and rotation is somewhat higher than those ob- reflection or absorption in the atmosphere is absorbed by the served in nature. In subcritical conditions, some trigger is surface, which transmits energy upward both by radiation necessary to initiate the vortices, and in the terrestrial at- and in vast convective currents whose visible manifestations mosphere tropical cyclones only develop from preexisting are the beautiful cumulus and cumulonimbus clouds that ply disturbances of independent origin. In mathematical parl- the tropical skyscape. The outgoing photons have much ance, tropical cyclones may be said to result from a subcriti- longer wavelengths than the incoming photons, since Earth’s cal bifurcation of the radiative–convective equilibrium state. surface temperature is far lower than the Sun’s. The outgoing About 10% of them develop in the Atlantic Ocean, where the IR radiation is strongly absorbed by clouds and by trace disturbance is often a 100-km-scale “easterly wave” that amounts of certain gaseous components of the atmosphere, forms over sub-Saharan Africa and then moves westward out notably water vapor, carbon dioxide, and methane. Those over the Atlantic. When its maximum wind speed exceeds constituents reradiate both upward and downward. Re- 32 m/s, it, by definition, becomes a hurricane. markably, the surface receives on average more radiation The convective core of a tropical cyclone may be many from the atmosphere and clouds than direct radiation from tens to hundreds of kilometers across, orders of magnitude the Sun. The warming of the surface by back radiation from greater than the few hundred meters’ width of an ordinary the atmosphere is the greenhouse effect. Because of it, Earth’s cumulus cloud. The core’s small surface-to-volume ratio, to- surface temperature is some 35 K higher than its effective gether with the strong stability to horizontal displacement af- blackbody temperature. That difference makes hurricanes forded by the inertial stability of its rotation, greatly reduces possible. mixing between cloudy moist air and clear dry air. In a strong The relatively high surface temperature also means that tropical cyclone, entropy production by the mixing of dry atmospheric radiation exports entropy to space. The reason and moist air is virtually shut down, and dissipation of the is that the atmosphere is heated at approximately the surface wind’s kinetic energy takes over as the primary mechanism temperature, but it cools at the much lower effective emis- for producing entropy. Most of the dissipation occurs in a tur- sion temperature of Earth. In equilibrium, the planet must bulent atmospheric boundary layer within a few hundred generate entropy, and the vast majority of that entropy is pro- meters of the ocean surface. duced in the atmosphere, mainly through the mixing of the The mature hurricane is an almost perfect example of a moist air inside clouds with the dry air outside them and Carnot heat engine whose working fluid may be taken as a through frictional dissipation by falling raindrops and mixture of dry air, water vapor, and suspended condensed snowflakes. Were it not for moisture in the atmosphere, the water, all in thermodynamic equilibrium. The engine is pow- entropy would have to be produced by frictional dissipation ered by the heat flow that is possible because the tropical of the kinetic energy of air molecules. The resulting air mo- ocean and atmosphere are not in thermal equilibrium. This tion would be too violent to permit air travel. disequilibrium arises because, thanks to the greenhouse ef- Water in the atmosphere thus has a paradoxical effect on fect, the ocean must lose heat by direct, nonradiative transfer climate. It is far and away the most important greenhouse to the atmosphere to balance the absorption of solar radia- molecule in the atmosphere and is responsible for a surface tion and back radiation from the atmosphere and clouds. The temperature increase that requires the production of entropy. heat transfer is accomplished mostly by evaporation of water, On the other hand, mixing and irreversible processes associ- which has a large heat of vaporization. To maintain substan- 2 August 2006 Physics Today © 2006 American Institute of Physics, S-0031-9228-0608-XXX-X The hurricane as a Carnot heat engine. This two-dimensional plot of the thermody- namic cycle shows a vertical cross section of the hurricane, whose storm center lies along the left edge. Colors depict the entropy dis- tribution; cooler colors indicate lower entropy. The process mainly responsible for driving the storm is the evapo- ration of seawater, which transfers energy from sea to air. As a result of that transfer, air spirals inward from A to B and acquires entropy at a constant temperature. It then undergoes an adiabat- ic expansion from B to C as it ascends within tial evaporation rates, the air a short distance above the sea the storm’s eyewall. Far from the storm center, symbolically surface must be much drier than would be the case were it in between C and D, it exports IR radiation to space and so equilibrium with the sea. loses the entropy it acquired from the sea. The depicted com- The figure illustrates the four legs of a hurricane Carnot pression is very nearly isothermal. Between D and A the air cycle. From A to B, air undergoes nearly isothermal expan- undergoes an adiabatic compression. Voilà, the four legs of a sion as it flows toward the lower pressure of the storm cen- Carnot cycle. ter while in contact with the surface of the ocean, a giant heat reservoir. As air spirals in near the surface, conservation of angular momentum causes the air to rotate faster about the Here Ts is the ocean temperature and To is the temperature storm’s axis. Evaporation of seawater transfers energy from of the outflow. Those temperatures and E may be easily esti- the sea to the air and increases the air’s entropy. mated from observations of the tropics, and v as given by the Once the air reaches the point where the surface wind is above equation is found to provide a good quantitative upper strongest—typically 5–100 km from the center of the hurri- bound on hurricane wind speeds. Several factors, however, cane—it turns abruptly (point B in the figure) and flows up- prevent most storms from achieving their maximum sus- ward within the sloping ring of cumulonimbus cloud known tainable wind speed, or “potential intensity.” Those include as the eyewall. The ascent is nearly adiabatic. In real storms cooling of the sea surface by turbulent mixing that brings the air flows out at the top of its trajectory (point C in the fig- cold ocean water up to the surface and entropy consumption ure) and is incorporated into other weather systems; in ide- by dry air finding its way into the hurricane’s core. alized models one can close the cycle by allowing the heat ac- The thermodynamic cycle of a hurricane represents only a quired from the sea surface to be isothermally radiated to glimpse of the fascinating physics of hurricanes; more com- space as IR radiation from the storm outflow. Finally, the plete expositions are available in the resources given below. cycle is completed as air undergoes adiabatic compression The transition of the tropical atmosphere from one with or- from D to A. dinary convective clouds and mixing-dominated entropy The rate of heat transfer from the ocean to the atmos- production to a system with powerful vortices and dissipa- phere varies as vE, where v is the surface wind speed and E tion-driven entropy production remains a mysterious and in- quantifies the thermodynamic disequilibrium between the adequately studied phenomenon. This may be of more than ocean and atmosphere. But there is another source of heat; academic interest, as increasing concentrations of green- house gases increase the thermodynamic disequilibrium of the dissipation of the kinetic energy of the wind by surface the tropical ocean–atmosphere system and thereby increase friction. That can be shown to vary as v3. According to Carnot, the intensity of hurricanes. the power generation by the hurricane heat engine is given by the rate of heat input multiplied by the thermodynamic efficiency: Additional resources If the storm is in a steady condition, then the power gen- K.
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