Abstract. We report on the first multi-year km-scale global coupled simulations using ECMWF’s Integrated Forecasting System (IFS) coupled to both the NEMO and FESOM ocean-sea ice models, as part of the Horizon 2020 Next Generation Earth Modelling Systems (nextGEMS) project. We focus mainly on the two unprecedented IFS-FESOM coupled setups, with an atmospheric resolution of 2.8 km and 4.4 km, respectively, and the same spatially varying ocean resolution that reaches locally below 5 km grid-spacing. This is enabled by a refactored ocean model code that allows for more efficient coupled simulations with IFS in a single-executable setup, employing hybrid parallelisation with MPI and OpenMP. A number of shortcomings in the original NWP-focussed model configurations were identified and mitigated over several cycles collaboratively by the modelling centres, academia, and the wider nextGEMS community. The main improvements are (i) better conservation properties of the coupled model system in terms of water and energy balance, which benefit also ECMWF’s operational 9 km IFS-NEMO model, (ii) a realistic top-of-the-atmosphere (TOA) radiation balance throughout the year, (iii) improved intense precipitation characteristics, and (iv) eddy-resolving features in large parts of the mid- and high-latitude oceans (finer than 5 km grid-spacing) to resolve mesoscale eddies and sea ice leads. New developments made at ECMWF for a better representation of snow and land use, including a dedicated scheme for urban areas, were also tested on multi-year timescales. We provide first examples of significant advances in the realism and thus opportunities of these km-scale simulations, such as a clear imprint of resolved Arctic sea ice leads on atmospheric temperature, impacts of km-scale urban areas on the diurnal temperature cycle in cities, and better propagation and symmetry characteristics of the Madden-Julian Oscillation.
Results are presented from a two-dimensional model of the stratosphere that simulates the seasonal movement of ozone by both wind and eddy transport, and contains all the chemistry known to be important. The calculated reductions in ozone due to NO2 injection from a fleet of supersonic transports are compared with the zonally averaged results of a three-dimensional model for a similar episode of injection. The agreement is good in the northern hemisphere, but is not as good in the southern hemisphere. Both sets of calculations show a strong corridor effect in that the predicted ozone depletions are largest to the north of the flight corridor for aircraft operating in the northern hemisphere.
Thermo-chemical convection in the Earth's mantle is thought to significantly affect the topography of the lithosphere. However, the comparison of such dynamic topography as obtained from convection models with the observed, non-isostatic topography remains complicated, both because of uncertainties about crustal structure and mantle flow estimates. Here, we focus on the latter and evaluate the role of lateral and radial viscosity variations for topography estimates. We report the magnitude of dynamic topography and uplift rates from a set of numerical computations of mantle convection in regional sections of a spherical annulus using the finite element software CitcomS. We strive to establish scaling laws of dynamic topography and uplift rate as a function of the rheology and Rayleigh number. We test both Newtonian and non-Newtonian rheologies with temperature-dependent viscosities. The dimensions of the Stokes equation suggest that both the uplift rate and the dynamic topography can be described by the Frank-Kamenetskii parameter for temperature dependent viscosity, the average viscosity, and the Rayleigh number, but with different exponents. We test the validity of this approach.
In this study, we compare convection characteristics in three models that are at the forefront of global km-scale modelling, the ICON model developed by the Max Planck Institute for Meteorology (MPI-M) and German Weather Service (DWD), the IFS developed by the European Centre for Medium-Range Weather Forecasts (ECMWF), and the NICAM model developed by the University of Tokyo, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the National Institute for Environmental Studies (NIES). For IFS and ICON, we analyse 1-year coupled simulations at 4.4 and 5 km resolution, respectively, which stem from Cycle 3 of the H2020 Next Generation Earth Modelling Systems (nextGEMS) project. For NICAM, we analyse a 1-year AMIP-type simulation at 3.5 km resolution. Convection schemes have been switched off in ICON and NICAM, while in the IFS the deep convection scheme’s cloud base mass flux is strongly reduced.  Modelling convection at km-scale resolutions is both exciting and challenging because some important processes are already resolved at these scales (e.g., deep convection) but other important processes remain under-resolved (e.g., mixing of grid-scale updrafts with their environment). Thus, we analyse in this study what common issues exist in ICON, IFS and NICAM with respect to the convection characteristics in the tropics, in what respects all models do well and where there are substantial inter-model differences. Specifically, we analyse local convection characteristics and show that compared to satellite observations, the models tend to overestimate precipitation intensity (NICAM and ICON), while they underestimate precipitation cell size and precipitation duration. We study mesoscale organisation by using different organisation metrics and show that the models tend to underestimate organisation, even though they all consistently show that when organisation is enhanced, heavy precipitation is enhanced as well. We also investigate moisture-convection relationships and show that the models generally do not moisten enough during a convective event compared to ERA5 reanalysis data. Consistently, the sensitivity of lower-tropospheric moisture variations to the life cycle of deep convection over ocean looks too weak in ICON and IFS. Finally, we look at land-ocean differences of the convection characteristics and show that while all models capture the diurnal cycle of precipitation over ocean well, there are some substantial differences over land, even though biases are not consistent between the models. Over coastal regions of the Maritime Continent, ICON has too strong mean precipitation and a too strong diurnal cycle, whereas IFS overall underestimates both, connected to a too weak propagation of convection onto the ocean during nighttime, potentially connected to too weak cold pools. Meanwhile, NICAM has more realistic convection characteristics in these coastal regions.
General circulation models show that as the surface temperature increases, the convective anvil clouds shrink. By analyzing radiative-convective equilibrium simulations, we show that this behavior is rooted in basic energetic and thermodynamic properties of the atmosphere: As the climate warms, the clouds rise and remain at nearly the same temperature, but find themselves in a more stable atmosphere; this enhanced stability reduces the convective outflow in the upper troposphere and decreases the anvil cloud fraction. By warming the troposphere and increasing the upper-tropospheric stability, the clustering of deep convection also reduces the convective outflow and the anvil cloud fraction. When clouds are radiatively active, this robust coupling between temperature, high clouds, and circulation exerts a positive feedback on convective aggregation and favors the maintenance of strongly aggregated atmospheric states at high temperatures. This stability iris mechanism likely contributes to the narrowing of rainy areas as the climate warms. Whether or not it influences climate sensitivity requires further investigation.
The ultimate drivers of convection - radiation, tropospheric humidity and surface fluxes - are altered both by the large-scale circulation and by convection itself. A quantity to which all drivers of convection contribute is moist static energy (MSE). Therefore, both a variance analysis of the MSE budget and an analysis of gross moist stability help understanding the interaction of precipitating convection with the large-scale environment. In addition, this method provides insights concerning the impact of convective aggregation on this coupling. The interaction is analyzed with a general circulation model as a starting point, but a model intercomparison study validating the general circulation model with large-eddy simulations is planned. The interaction of precipitating convection with the large-scale environment is investigated by studying the influence of surface temperature and convection scheme on the MSE variance budget in a radiative-convective equilibrium version of ECHAM6. Different fixed surface temperatures lead to different specific humidities and cause a change of longwave emission height temperature. Subsidence fraction, a large-scale property that is important for radiative transfer, increases with surface temperature, indicating an increase of convective aggregation. MSE variance increases with increasing surface temperature because more water vapor in the atmosphere allows for more MSE fluctuations. In addition, the advection term in the MSE variance budget turns from a sink to a source of MSE variance. The Tiedtke and Nordeng convection scheme show similar dependencies on surface temperature, though deep convection is forming more rapidly with the Tiedtke convection scheme. The further analysis will focus on understanding cloud-radiation and moisture-convection feedbacks within the hierarchy of models, before effective coupling parameters will be derived from cloud resolving models. These will in turn be related to assumptions used to parameterize convection in large-scale models.
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