Entropy production and coarse graining of the climate fields in a general circulation model

9 We extend the analysis of the thermodynamics of the climate system by investigating the role 10 played by processes taking place at various spatial and temporal scales through a procedure of 11 coarse graining. We show that the coarser is the graining of the climatic fields, the lower is 12 the resulting estimate of the material entropy production. In other terms, all the spatial and 13 temporal scales of variability of the thermodynamic fields provide a positive contribution to the 14 material entropy production. This may be interpreted also as that, at all scales, the temperature 15 fields and the heating fields resulting from the convergence of turbulent fluxes have a negative 16 correlation, while the opposite holds between the temperature fields and the radiative heating 17 fields. Moreover, we obtain that the latter correlations are stronger, which confirms that radiation 18 acts as primary driver for the climatic processes, while the material fluxes dampen the resulting 19 fluctuations through dissipative processes. We also show, using specific coarse-graining procedures, 20 how one can separate the various contributions to the material entropy production coming from 21 the dissipation of kinetic energy, the vertical sensible and latent heat fluxes, and the large scale 22 horizontal fluxes, without resorting to the full three-dimensional time dependent fields. We find that 23 most of the entropy production is associated to irreversible exchanges occurring along the vertical 24 direction, and that neglecting the horizontal and time variability of the fields has a relatively small 25 impact on the estimate of the material entropy production. The approach presented here seems 26 promising for testing climate models, for assessing the impact of changing their parametrizations 27 and their resolution, as well as for investigating the atmosphere of exoplanets, because it allows for 28 evaluating the error in the estimate of their thermodynamical properties due to the lack of high29 resolution data. The findings on the impact of coarse graining on the thermodynamic fields on 30 the estimate of the material entropy production deserve to be explored in a more general context, 31 because they provide a way for understanding the relationship between forced fluctuations and 32 dissipative processes in continuum systems. 33 ∗ Email: [email protected] 2