Francesco Ragone — Page 2

arXiv2011

New Results on the Thermodynamical Properties of the Climate System

In this paper we exploit two equivalent formulations of the average rate of material entropy production in a planetary system to propose an approximate splitting between contributions due to vertical and eminently horizontal processes. Our approach is based only upon 2D radiative fields at the surface and at the top of atmosphere of a general planetary body. Using 2D fields at the top of atmosphere alone, we derive lower bounds to the rate of material entropy production and to the intensity of the Lorenz energy cycle. By introducing a measure of the efficiency of the planetary system with respect to horizontal thermodynamical processes, we provide insight on a previous intuition on the possibility of defining a baroclinic heat engine extracting work from the meridional heat flux. The approximate formula of the material entropy production is verified and used for studying the global thermodynamic properties of climate models (CMs) included in the PCMDI/CMIP3 dataset in pre-industrial climate conditions. It is found that about 90% of the material entropy production is due to vertical processes such as convection, whereas the large scale meridional heat transport contributes only about 10%. The total material entropy production is typically 55 mWK-1m-2, with discrepancies of the order of 5% and CMs' baroclinic efficiencies are clustered around 0.055. When looking at the variability and co-variability of the considered thermodynamical quantities, the agreement among CMs is worse, suggesting that the description of feedbacks is more uncertain.

physics.ao-ph↗

arXiv2010

Energetics of PCMDI/CMIP3 Climate Models: Energy Budget and Meridional Enthalpy Transport

We analyze the PCMDI/CMIP3 simulations performed by climate models (CMs) using pre-industrial and SRESA1B scenarios. Relatively large biases are present for most CMs when global energy budgets and when the atmospheric, oceanic, and land budgets are considered. Apparently, the biases do not result from transient effects, but depend on the imperfect closure of the energy cycle in the fluid components and on inconsistencies over land. Therefore, the planetary emission temperature is underestimated. This may explain the CMs' cold bias. In the pre-industrial scenario, CMs agree on the location in the mid-latitudes of the peaks of the meridional atmospheric enthalpy transport, while large discrepancies exist on the intensity. Disagreements on the location and intensity of the oceanic transport peaks are serious. With increased $CO_2$ concentration, a small poleward shift of the peak and an increase in the intensity of the atmospheric transport of up to 10% are detected in both hemispheres. Instead, most CMs feature a decrease in the oceanic transport intensity in the northern hemisphere and an equatorward shift of the peak in both hemispheres. The Bjerkens compensation mechanism is active both on climatological and interannual time scales. The peak of the total meridional transport is typically around $35^\circ$ in both hemispheres and scenarios, whereas disagreements on the intensity are relevant. With increased $CO_2$ concentration, the total transport increases by up to 10%, thus contributing to polar amplification. Advances in the representation of physical processes are definitely needed for providing a self-consistent representation of climate as a non-equilibrium thermodynamical system.

physics.ao-ph↗