Application of the second law to the atmosphere: impacts of the third-law definition for the moist-air entropy

TL;DR

This paper explores how applying the third law of thermodynamics to define moist-air entropy affects entropy flux calculations in the atmosphere, potentially altering our understanding of atmospheric thermodynamics and equilibrium.

Contribution

It introduces a new approach to defining moist-air entropy based on the third law, contrasting with traditional methods, and examines its implications for atmospheric entropy budgets.

Findings

Significant impact of moisture on entropy flux calculations.

Potential modification of atmospheric entropy budgets.

Implications for atmospheric equilibrium and radiative processes.

Abstract

Calculations of entropy fluxes and production rate have been evaluated with some success to study atmospheric processes. However, recurring questions arise as to how best to take into account entropy flux due to radiation, for example. This article raises another kind of question: how to define the entropy of the atmosphere itself, which is composed of variable proportions of dry air (nitrogen, oxygen, argon, etc.) and water (vapour, liquid, ice). The specific values of the entropy for such a variable composition system depend on the reference values of its components. Most of the current definitions are based on entropies set at zero for dry air and liquid water at zero degrees Celsius. Differently, the third law of thermodynamics assumes that the entropy of all species cancels out for the more stable solid state at the zero of absolute temperatures. In this paper, we analyze the possible consequences of this absolute definition of entropy of moist air on the calculation of entropy fluxes. The impacts of moisture are significant and these new calculation methods seem to be able to modify the budgets of atmospheric entropy, with possible impacts on the nature of the equilibrium of the atmosphere resulting from entropic imbalances induced by radiations.