{"id":382563,"date":"2025-06-10T16:40:03","date_gmt":"2025-06-10T14:40:03","guid":{"rendered":"https:\/\/climatescience.press\/?p=382563"},"modified":"2025-06-10T16:40:04","modified_gmt":"2025-06-10T14:40:04","slug":"our-atmospheric-heat-engine","status":"publish","type":"post","link":"https:\/\/climatescience.press\/?p=382563","title":{"rendered":"Our Atmospheric Heat\u00a0Engine"},"content":{"rendered":"\n
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From Science Matters<\/a><\/p>\n\n\n\n

By\u00a0Ron Clutz<\/a><\/p>\n\n\n\n

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Climate as heat engine.\u00a0A heat engine produces mechanical energy in the form of work W<\/strong>\u00a0by absorbing an amount of\u00a0heat Qin from a hot reservoir<\/strong>\u00a0(the source) and depositing a smaller amount\u00a0Qout into a cold reservoir<\/strong>\u00a0(the sink).\u00a0(a) An ideal Carnot heat engine<\/strong>\u00a0does the job with the maximum possible efficiency.\u00a0(b) Real heat engines are irreversible<\/strong>, and some work is lost via irreversible entropy production T\u03b4S.\u00a0(c) For the climate system<\/strong>, the ultimate source is the Sun, with outer space acting as the sink. The work is performed internally and produces winds and ocean currents. As a result, Qin = Qout.<\/figcaption><\/figure>\n\n\n\n

A previous post presented Michel Thizon\u2019s description of gravity\u2019s effect on the mass of air functioning as a climate thermostat. Some years ago, Dr. Murry Salby wrote in detail about the troposphere operating as a heat engine and the stratosphere as a refrigerator. This post consists of excerpts from Salby\u2019s textbook entitled\u00a0Physics of the Atmosphere and Climate<\/strong><\/a>. The title is a link to pdf version of the book Salby (2012). Text in italics with my bolds and added images.<\/p>\n\n\n\n

A closed system that performs work through a conversion of heat<\/strong>\u00a0that is absorbed by it is\u00a0a heat engine.<\/strong>\u00a0Conversely, a\u00a0system that rejects heat through a conversion of work<\/strong>\u00a0that is performed on it is a\u00a0refrigerator<\/strong>. In Chap. 6, we will see that\u00a0individual air parcels<\/strong>\u00a0comprising the\u00a0circulation of the troposphere behave as a heat engine.<\/strong>\u00a0 By\u00a0absorbing heat at the Earth\u2019s surface,<\/strong>\u00a0through transfers of radiative, sensible, and latent heat, individual parcels perform network as they\u00a0evolve through a thermodynamic cycle<\/strong>\u00a0(2.13). Ultimately realized as kinetic energy, the heat absorbed maintains the circulation against frictional dissipation. It makes\u00a0the circulation of the troposphere thermally driven.<\/strong><\/em><\/p>\n\n\n\n

In contrast, the circulation of the stratosphere behaves as a radiative refrigerator.<\/strong>  For motion to occur, individual air parcels must have work performed on them.<\/strong> The kinetic energy<\/strong> produced is eventually converted to heat<\/strong> and rejected to space through LW cooling.<\/strong> It makes the circulation of the stratosphere mechanically driven.<\/strong> Gravity waves and planetary waves that propagate upward from the troposphere are dissipated in the stratosphere. Their absorption exerts an influence on the stratosphere analogous to paddle work. By forcing motion that rearranges air, it drives the stratospheric circulation out of radiative equilibrium, which results in net LW cooling to space.<\/strong> Salby (2012) p. 83.<\/em><\/p>\n\n\n\n

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Irreversible processes in the atmosphere.<\/strong>\u00a0Neglecting radiative processes (not shown here), the largest sources of irreversibility in the atmosphere are those\u00a0associated with the hydrologic cycle:<\/strong>\u00a0evaporation, the mixing of moist and dry air, and the melt\u2013freeze cycle (60\u201380% collectively), and the fallout of precipitation (5\u201315%). Those contributions limit the entropy generated by frictional dissipation of the winds (5\u201315%), which ultimately places a\u00a0limit on the work performed by the atmospheric heat engine<\/strong>\u00a0in generating circulations. Percentages are estimated based on global climate simulations12 and idealized high-resolution simulations.8<\/em><\/figcaption><\/figure>\n\n\n\n

Changes of thermodynamic state that accompany vertical motion follow from the distribution of atmospheric mass, which is determined ultimately by gravity.<\/strong> In the absence of motion, Newton\u2019s second law applied to the vertical reduces to a statement of hydrostatic equilibrium (1.16). Gravity is then balanced by the vertical pressure gradient force. This simple form of mechanical equilibrium<\/strong> is accurate even in the presence of motion because the acceleration of gravity is, almost invariably, much greater than vertical acceleration of individual air parcels<\/strong>. Only inside deep convective towers and other small-scale phenomena is vertical acceleration large enough to invalidate hydrostatic equilibrium.<\/em><\/p>\n\n\n\n

Because it is such a strong body force, gravity must be treated with some care. Complications arise from the fact that the gravitational acceleration experienced by an air parcel does not act purely in the vertical. It also varies with location. According to the preceding discussion, gravity is large enough to overwhelm other contributions in the balance of vertical forces.<\/strong> The same holds for the balance of horizontal forces. Horizontal components of gravity that are introduced by the Earth\u2019s rotation and other sources must be balanced by additional horizontal forces.<\/strong> Unrelated to air motion, those additional forces unnecessarily complicate the description of atmospheric motion.  Salby (2012) p. 150.<\/em><\/p>\n\n\n\n

The temperature of a dry air parcel decreases with its altitude at the dry adiabatic lapse rate.<\/strong> To a good approximation, the same holds for a moist air parcel<\/strong> under unsaturated<\/strong> conditions \u2013 because the trace abundance of water vapor modifies thermal properties of air only slightly. Under saturated<\/strong> conditions, the adiabatic description of air breaks down<\/strong> due to the release of latent heat that accompanies the transformation of water from one phase to another. Latent heat<\/strong> exchanged with the gas phase then offsets adiabatic cooling and warming, which accompany ascending and descending motion. Salby (2012) p. 162<\/em><\/p>\n\n\n\n

Net heat absorption and work performed by individual air parcels make the general circulation of the troposphere<\/strong> behave as a heat engine, one that is driven thermally by heat transfer at its lower and upper boundaries.<\/strong> Work performed by individual parcels is associated with a redistribution of mass:<\/strong> Air that is effectively warmer and lighter at the lower boundary is exchanged with air that is effectively cooler and heavier at the upper boundary. This redistribution of mass represents a conversion of potential energy into kinetic energy.<\/strong> The conversion of energy maintains the general circulation against frictional dissipation<\/strong>. Salby (2012) p. 163<\/em><\/p>\n\n\n\n

The idealized behavior just described relies on heat transfer being confined to the lower and upper boundaries of the layer, where an air parcel resides long enough for diabatic influences<\/strong> to become important. Between the boundaries, the time scale of motion is short. For motion that operates on longer time scales, typical of the stratosphere, the evolution of an individual air parcel is not adiabatic.<\/strong><\/em><\/p>\n\n\n\n

Radiative transfer<\/strong> is the primary diabatic influence outside the boundary layer and cloud. It is characterized by cooling rates of order 1 K day\u22121 in the troposphere<\/strong> (see Fig. 8.24). Cooling rates as large as 10 K day\u22121 occur in the stratosphere and near cloud<\/strong> (Fig. 9.36). (2012) p. 164<\/em><\/p>\n\n\n\n

Unlike the troposphere, buoyancy in the stratosphere opposes vertical motion<\/strong> because, invariably, warm (high-\u03b8) air overlies cool (low-\u03b8) air.<\/strong> To exchange effectively-heavier air at lower levels with effectively-lighter air at upper levels, work<\/strong> must be performed against<\/strong> the opposition of buoyancy<\/strong>. The rearrangement of mass represents a conversion of kinetic energy<\/strong> (that of the waves driving the motion) into potential energy.<\/strong> Manifest in temperature, the potential energy is dissipated thermally through LW emission to space<\/strong>. (2012) p. 168<\/em><\/p>\n\n\n\n

See Also<\/strong><\/p>\n\n\n\n

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Fearless Physics from Dr. Salby<\/a><\/blockquote>