Entropy Production and Coarse Graining of the Climate Fields in a General Circulation Model

TL;DR

This paper investigates how coarse graining of climate fields affects the estimation of entropy production, revealing the dominant vertical dissipation processes and the primary role of radiation in climate thermodynamics.

Contribution

It introduces a coarse-graining approach to analyze the thermodynamics of climate models, highlighting the contributions of different scales and processes to entropy production.

Findings

Most entropy production is linked to vertical irreversible exchanges.

Horizontal and temporal variability have limited impact on entropy estimates.

Radiation acts as the primary driver, with fluxes damping fluctuations.

Abstract

We extend the analysis of the thermodynamics of the climate system by investigating the role played by processes taking place at various spatial and temporal scales through a procedure of coarse graining. The coarser is the graining of the climatic fields, the lower is the resulting estimate of the material entropy production. In other terms, all the spatial and temporal scales of variability of the thermodynamic fields provide a positive contribution to the material entropy production. At all scales, the temperature fields and the heating fields resulting from the convergence of turbulent fluxes have a negative correlation, while the opposite holds between the temperature fields and the radiative heating fields. Moreover, the latter correlations are stronger, which confirms that radiation acts as primary driver for the climatic processes, while the material fluxes dampen the resulting fluctuations through dissipative processes. Using specific coarse-graining procedures, one can separate the various contributions to the material entropy production coming from the dissipation of kinetic energy, the vertical sensible and latent heat fluxes, and the large scale horizontal fluxes. Most of the entropy production is associated to irreversible exchanges occurring along the vertical direction, and neglecting the horizontal and time variability of the fields has a small impact on the estimate of the material entropy production. The approach presented here seems promising for testing climate models, for assessing the impact of changing their parametrizations and their resolution, and for investigating the atmosphere of exoplanets. Our findings deserve to be explored in a more general context, because they provide a way for understanding the relationship between forced fluctuations and dissipative processes in continuum systems.