Available potential energy sources in hurricanes and climate

The atmosphere is often describes as a heat engine that produces kinetic energy by transporting latent and sensible heat from the warm surface to the colder troposphere. The mechanical efficiency can be defined as the ratio of the kinetic energy generated divided by the energy source. Previous studies, such as Pauluis and Held (2002) have argued that the mechanical efficiency of the global atmospheric circulation is relatively weak, on the order of 0.05. In contrast, Emanuel (1986) argues that the mechanical efficiency in hurricanes is high up to 1/3.

In this paper, the mechanical efficiencies of hurricanes and of the atmospheric circulation from the point of view of the sources and sinks of Available Potential Energy (APE). It is argued that, in a moist atmosphere, the generation of APE by surface energy fluxes can achieve a similar efficiency to that of a hurricane. However, under normal conditions, the APE source due to surface heating is balanced by a large destruction of APE associated with the mixing between dry and moist air masses. Scaling arguments indicate that the destruction of APE by diffusion must account for at least 80% of the APE production by surface heating. In contrast, a steady hurricane circulation can only be maintained if all the APE production due to the surface energy flux is converted in kinetic energy. This requires in particular that there be no entrainment of dry air in the planetary boundary layer in the vicinity of the hurricane eyewall.

Reference:

Emanuel, K.A., 1988: The maximum intensity of hurricanes. J. Atmos. Sci., 45, 1143-1155.

Pauluis, O. and I.M. Held, 2002: Entropy budget of an atmosphere in radiative-convective equilibrium. Part I: Maximum work and frictional dissipation. J. Atmos. Sci., 59, 140-149.