This paper describes the main characteristics of CNRM-CM6-1, the fully coupled atmosphere-ocean general circulation model of sixth generation jointly developed by Centre National de Recherches Meteorologiques (CNRM) and Cerfacs for the sixth phase of the Coupled Model Intercomparison Project 6 (CMIP6). The paper provides a description of each component of CNRM-CM6-1, including the coupling method and the new online output software. We emphasize where model's components have been updated with respect to the former model version, CNRM-CM5.1. In particular, we highlight major improvements in the representation of atmospheric and land processes. A particular attention has also been devoted to mass and energy conservation in the simulated climate system to limit long-term drifts. The climate simulated by CNRM-CM6-1 is then evaluated using CMIP6 historical and Diagnostic, Evaluation and Characterization of Klima (DECK) experiments in comparison with CMIP5 CNRM-CM5.1 equivalent experiments. Overall, the mean surface biases are of similar magnitude but with different spatial patterns. Deep ocean biases are generally reduced, whereas sea ice is too thin in the Arctic. Although the simulated climate variability remains roughly consistent with CNRM-CM5.1, its sensitivity to rising CO 2 has increased: the equilibrium climate sensitivity is 4.9 K, which is now close to the upper bound of the range estimated from CMIP5 models.

Evaluation of CMIP6 DECK Experiments With CNRM-CM6-1

If model parameterizations of unresolved physics, such as the variety of upper ocean mixing processes, are to hold over the large range of time and space scales of importance to climate, they must be strongly physically based. Observations, theories, and models of oceanic vertical mixing are surveyed. Two distinct regimes are identified: ocean mixing in the boundary layer near the surface under a variety of surface forcing conditions (stabilizing, destabilizing, and wind driven), and mixing in the ocean interior due to internal waves, shear instability, and double diffusion (arising from the different molecular diffusion rates of heat and salt). Mixing schemes commonly applied to the upper ocean are shown not to contain some potentially important boundary layer physics. Therefore a new parameterization of oceanic boundary layer mixing is developed to accommodate some of this physics. It includes a scheme for determining the boundary layer depth h, where the turbulent contribution to the vertical shear of a bulk Richardson number is parameterized. Expressions for diffusivity and nonlocal transport throughout the boundary layer are given. The diffusivity is formulated to agree with similarity theory of turbulence in the surface layer and is subject to the conditions that both it and its vertical gradient match the interior values at h. This nonlocal “K profile parameterization” (KPP) is then verified and compared to alternatives, including its atmospheric counterparts. Its most important feature is shown to be the capability of the boundary layer to penetrate well into a stable thermocline in both convective and wind-driven situations. The diffusivities of the aforementioned three interior mixing processes are modeled as constants, functions of a gradient Richardson number (a measure of the relative importance of stratification to destabilizing shear), and functions of the double-diffusion density ratio, Rρ. Oceanic simulations of convective penetration, wind deepening, and diurnal cycling are used to determine appropriate values for various model parameters as weak functions of vertical resolution. Annual cycle simulations at ocean weather station Papa for 1961 and 1969–1974 are used to test the complete suite of parameterizations. Model and observed temperatures at all depths are shown to agree very well into September, after which systematic advective cooling in the ocean produces expected differences. It is argued that this cooling and a steady salt advection into the model are needed to balance the net annual surface heating and freshwater input. With these advections, good multiyear simulations of temperature and salinity can be achieved. These results and KPP simulations of the diurnal cycle at the Long-Term Upper Ocean Study (LOTUS) site are compared with the results of other models. It is demonstrated that the KPP model exchanges properties between the mixed layer and thermocline in a manner consistent with observations, and at least as well or better than alternatives.

Oceanic vertical mixing: a review and a model with a nonlocal boundary layer parameterization

Soil, through its various functions, plays a vital role in the Earth's ecosystems and provides multiple ecosystem services to humanity. Pedotransfer functions (PTFs) are simple to complex knowledge rules that relate available soil information to soil properties and variables that are needed to parameterize soil processes. In this paper, we review the existing PTFs and document the new generation of PTFs developed in the different disciplines of Earth system science. To meet the methodological challenges for a successful application in Earth system modeling, we emphasize that PTF development has to go hand in hand with suitable extrapolation and upscaling techniques such that the PTFs correctly represent the spatial heterogeneity of soils. PTFs should encompass the variability of the estimated soil property or process, in such a way that the estimation of parameters allows for validation and can also confidently provide for extrapolation and upscaling purposes capturing the spatial variation in soils. Most actively pursued recent developments are related to parameterizations of solute transport, heat exchange, soil respiration and organic carbon content, root density and vegetation water uptake. Further challenges are to be addressed in parameterization of soil erosivity and land use change impacts at multiple scales. We argue that a comprehensive set of PTFs can be applied throughout a wide range of disciplines of Earth system science, with emphasis on land surface models. Novel sensing techniques provide a true breakthrough for this, yet further improvements are necessary for methods to deal with uncertainty and to validate applications at global scale.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1002/2017RG000581

Pedotransfer functions in Earth system science: challenges and perspectives

The aim of this article is to describe the reference configuration of the convection-permitting numerical weather prediction (NWP) model HARMONIE-AROME, which is used for operational short-range weather forecasts in Denmark, Estonia, Finland, Iceland, Ireland, Lithuania, the Netherlands, Norway, Spain, and Sweden. It is developed, maintained, and validated as part of the shared ALADIN–HIRLAM system by a collaboration of 26 countries in Europe and northern Africa on short-range mesoscale NWP. HARMONIE–AROME is based on the model AROME developed within the ALADIN consortium. Along with the joint modeling framework, AROME was implemented and utilized in both northern and southern European conditions by the above listed countries, and this activity has led to extensive updates to the model’s physical parameterizations. In this paper the authors present the differences in model dynamics and physical parameterizations compared with AROME, as well as important configuration choices of the reference, such...

/pdf/the-harmonie-arome-model-configuration-in-the-aladin-hirlam-23te6lgngt.pdf

The HARMONIE-AROME Model Configuration in the ALADIN-HIRLAM NWP System

. The European Centre for Medium-Range Weather Forecasts (ECMWF) recently
released the first 7-year segment of its latest atmospheric reanalysis: ERA-5
over the period 2010–2016. ERA-5 has important changes relative to the former ERA-Interim
atmospheric reanalysis including higher spatial and temporal resolutions as
well as a more recent model and data assimilation system. ERA-5 is foreseen
to replace ERA-Interim reanalysis and one of the main goals of this study is
to assess whether ERA-5 can enhance the simulation performances with respect
to ERA-Interim when it is used to force a land surface model (LSM). To that
end, both ERA-5 and ERA-Interim are used to force the ISBA (Interactions
between Soil, Biosphere, and Atmosphere) LSM fully coupled with the Total
Runoff Integrating Pathways (TRIP) scheme adapted for the CNRM (Centre
National de Recherches Meteorologiques) continental hydrological
system within the SURFEX (SURFace Externalisee) modelling platform of
Meteo-France. Simulations cover the 2010–2016 period at half a
degree spatial resolution. The ERA-5 impact on ISBA LSM relative to ERA-Interim is evaluated using
remote sensing and in situ observations covering a substantial part of the
land surface storage and fluxes over the continental US domain.
The remote sensing observations include (i) satellite-driven model
estimates of land evapotranspiration, (ii) upscaled ground-based observations
of gross primary production, (iii) satellite-derived estimates of surface
soil moisture and (iv) satellite-derived estimates of leaf area index (LAI).
The in situ observations cover (i) soil moisture, (ii) turbulent heat fluxes,
(iii) river discharges and (iv) snow depth. ERA-5 leads to a consistent
improvement over ERA-Interim as verified by the use of these eight
independent observations of different land status and of the model
simulations forced by ERA-5 when compared with ERA-Interim. This is
particularly evident for the land surface variables linked to the terrestrial
hydrological cycle, while variables linked to vegetation are less impacted.
Results also indicate that while precipitation provides, to a large extent,
improvements in surface fields (e.g. large improvement in the representation
of river discharge and snow depth), the other atmospheric variables play an
important role, contributing to the overall improvements. These results
highlight the importance of enhanced meteorological forcing quality provided
by the new ERA-5 reanalysis, which will pave the way for a new generation of
land-surface developments and applications.

/pdf/era-5-and-era-interim-driven-isba-land-surface-model-ufv9nandkw.pdf

ERA-5 and ERA-Interim driven ISBA land surface model simulations: which one performs better?

. SURFEX is a new externalized land and ocean surface platform that describes the surface fluxes and the evolution of four types of surfaces: nature, town, inland water and ocean. It is mostly based on pre-existing, well-validated scientific models that are continuously improved. The motivation for the building of SURFEX is to use strictly identical scientific models in a high range of applications in order to mutualise the research and development efforts. SURFEX can be run in offline mode (0-D or 2-D runs) or in coupled mode (from mesoscale models to numerical weather prediction and climate models). An assimilation mode is included for numerical weather prediction and monitoring. In addition to momentum, heat and water fluxes, SURFEX is able to simulate fluxes of carbon dioxide, chemical species, continental aerosols, sea salt and snow particles. The main principles of the organisation of the surface are described first. Then, a survey is made of the scientific module (including the coupling strategy). Finally, the main applications of the code are summarised. The validation work undertaken shows that replacing the pre-existing surface models by SURFEX in these applications is usually associated with improved skill, as the numerous scientific developments contained in this community code are used to good advantage.

/pdf/the-surfexv7-2-land-and-ocean-surface-platform-for-coupled-261aoq10jv.pdf

G. Kerdraon

Papers

The SURFEXv7.2 land and ocean surface platform for coupled or offline simulation of earth surface variables and fluxes

The SURFEXv7.2 externalized platform for the simulation of Earth surface variables and fluxes.