The maintenance of the atmospheric circulation requires a continuous production of kinetic energy to balance frictional dissipation. Traditionally, the atmosphere is described as acting as a heat engine that produces kinetic energy by transporting potential energy in the form of heat from warm to cold regions. However, recent studies indicate that moist processes such as diffusion of water vapor, irreversible phase transition and precipitation are responsible for most of the irreversible entropy (a measure of the potential energy unavailable for conversion to motion or kinetic energy) in the atmosphere. It is argued that, as the atmosphere acts as a dehumidifier, its ability to function as a heat engine is reduced: the hydrological cycle severely limits the amount of kinetic energy produced by the atmospheric circulation. This research investigates the connection between the hydrological cycle and the intensity of the circulation. The research component follows three main directions: the development of a new theoretical framework based on the entropy and available potential energy budgets in open systems and transients, the application of this framework in case studies, including shallow convection, hurricanes, and midlatitude storm systems, and the analysis of the global entropy budget, focusing on the impacts of climate change on entropy production and kinetic energy dissipation.
Intellectual merit: The Principal Investigator will (1) develop a new perspective on the maintenance of the general circulation of the atmosphere and its relationship to the hydrological cycle, (2) assess the impacts of future climate change on the number and intensity of extreme events such as thunderstorms and hurricanes.
Broader Impacts: Two new courses (one undergraduate and one graduate) on atmospheric thermodynamics and climate will be introduced. These courses will directly involve students in some of the research activities, including high-end simulations of atmospheric flows. A textbook on irreversible thermodynamics in the atmosphere will be prepared. It is expected that this project will provide a research topic for graduate student.
The research conducted under this Project aimed at answering the question of how thermodynamics can explain the amount of kinetic energy produced by the atmosphere. Broadly speaking, the atmosphere receives energy from the Sun and the Earth's surface at high temperature and pressure, and loses it through the emission of infra-red radiation at a much lower temperature and pressure. Thus, one can think of the atmosphere as acting as a heat engine that continuously generates kinetic energy by transporting energy from a warm source to a colder sink. The atmosphere however differs from the classical prototype for a heat engine - the well-known Carnot cycle - in that a large fraction of the atmospheric energy transport is associated with the hydrological cycle. The energy received from the Sun is used in part to evaporate water at the surface of the Oceans. Water vapor is then transported by the atmospheric circulation, until the air saturates at which point water vapor condenses and a cloud is formed. When water vapor condenses, it releases its latent heat of vaporization. A critical finding here is that this latent heat transport does not act as a classical Carnot cycle. This is demonstrated in Pauluis (2011), which introduces the concept of a steam cycle. This steam cycle be thought of as a Carnot cycle entirely driven by latent heating. Pauluis (2011) shows that the amount generated by such cycle is significantly less than a similar Carnot cycle. Furthermore, the actual work performed by such a steam cycle depends on the relative humidity at the water source and sink. One of the main implications here is that for the same energy transport, an atmosphere an with active hydrological cycle like the Earth, will generate much less kinetic energy than a dry atmosphere such as Mars. The question of how the atmosphere transports the energy has also been investigated. This has lead to the important discovery that the amount of air mass being circulated depens crucially on how one accounts for water vapor. In particular, Pauluis, Czaja and Korty (2008, 2010) have shown that the circulation when computed on moist isentropes is twice as large as what had been reported using different analysis techniques. The enahnced mass transport is tied to the transport of water vapor by the weather systems in the midlatitudes, which thus plays an even more important role in the global atmospheric circulation than previously thought.
