Questions related to climate models

What is a climate model and how does it work?

A numerical climate model is a computer program building on mathematical descriptions of the relevant processes of the climate system. The model progressively calculates the evolution of the state of the climate system over long periods of time (e.g. decades to centuries) in short time steps (minutes to hours).
The models are formulated considering the governing physical processes for momentum, mass, energy and water conservation etc. These are represented by a set of differential equations that are solved for their time tendencies. These tendencies are subsequently added to the state of the system thereby generating a future state. From the future state new tendencies are calculated that, in turn, are used to derive yet another new state etc.
Regional climate models have been developed as a tool to improve horizontal resolution, and thereby representation of detailed regional and local processes. The EURO-CORDEX regional climate models operate on a computational grid covering parts of the North Atlantic and Europe. To run these regional models, information from the larger scale global climate system is taken as input from a global climate model at the lateral boundaries, typically 4-8 times every day. Also, sea-surface temperatures and sea-ice conditions to be used in the regional model are most often taken from the global climate model unless regional ocean models are included in the regional climate model system. In the case of coupled models, also the regional ocean model needs to take input from the global ocean model at its boundaries. For Europe, such coupled models exist for the Baltic Sea and the North Sea in northern Europe and for the Mediterranean in the south. Such coupled models are, however, not currently part of EURO-CORDEX, however some of them are available for the Med-CORDEX domain.

Which processes are included in a climate model?

Earth System Models (ESMs) include a large number of processes describing the atmosphere, ocean, cryosphere and biosphere and the interaction between them. Most CORDEX regional climate models are relatively simple in comparison focusing mainly on standard processes describing the atmosphere and its interaction with the land surface.
Development of fully coupled ESMs involving a wide range of components of the Earth system has been ongoing for several decades. Components of such models involve: atmospheric dynamics and physical processes, physical ocean models, glacier and land ice models, dynamic vegetation models, models of the biogeochemistry of the oceans, models of atmospheric aerosols and chemistry. ESMs can be run in different configurations depending on the questions to be addressed. Also, some regional climate models have been set up as regional ESMs, RESMs. Such RESMs are generally rare in a wider CORDEX perspective.
Users of regional climate change information should consider whether the RCMs include relevant processes for their purpose. General features for all CORDEX domains include: the relatively crude treatment of atmospheric aerosols in most RCMs and the inability to realistically simulate convective clouds/precipitation in these relatively coarse-scale models. Specifically, for different regions limitations in representing regional features like ocean areas may need attention. For instance, in Europe, the EURO-CORDEX RCMs generally do not include detailed treatment of the Baltic Sea and the Mediterranean, and as those are poorly resolved in GCMs this may have negative consequences on the results of the models.

What is the resolution used in climate models and why is it important?

In numerical climate modelling high resolution is beneficial for two main reasons:

1. high resolution means that land-sea distribution, land cover fraction and height of mountains is better represented and
2. more relevant processes can be resolved at high resolution compared to at a coarse resolution. The EURO-CORDEX RCMs are operated at 12.5 km grid spacing.

The relatively high resolution of EURO-CORDEX at 12.5 km grid spacing implies that mountain ranges in Europe are described in a fairly realistic way. An important consequence is for instance that simulated precipitation is higher on the windward side of the British Isles than on the leeward side and in Norway compared to Sweden, which is in line with observations. Another feature is a much more realistic distribution of precipitation in the Alps compared to coarse-scale global climate models (e.g. Torma et al., 2015). Also, high-intensity precipitation is more realistic compared to at coarser resolution (Olsson et al., 2015).
Still, at 12.5 km resolution – important processes involving convection are not resolved in models. This implies that convective clouds and associated precipitation, i.e. high-intensity convective storms, are not simulated adequately. A growing scientific literature indicates that higher-resolution, convection-permitting models with spatial resolution of just a few km may in a much more realistic way describe such events (Prein et al., 2013). Furthermore, such models indicate that climate change signals may differ compared to traditional coarse-scale climate models such as those employed in EURO-CORDEX (Kendon et al., 2014).
Users of information from EURO-CORDEX are recommended to consider whether the resolution in the climate models is adequate for producing the requested information. This may be different depending on geographical location, time of year and climate processes of interest. Users should not directly interpolate climate model information to higher resolution grid, if no additional information is taken into account. That might be misleading because the model is not able to provide information for such small scales.

Why simply not increase the model resolution if it is so important?

The problem is the strongly increasing need for computing power with finer resolution. A doubling of the resolution (e.g. from 20 to 10 km) leads to an eightfold increase in computing power.
A twofold increase in the resolution of the computational grid implies that the number of gridpoints increase by a factor of four (two dimensions). Added on top of this is also the temporal resolution as the time step needs to be shorter to avoid numerical instability. Consequently, a doubling of the resolution (or halving of the grid size) implies an eightfold increase in demand for compute power. In addition, also the need for storage of results increases.
Another issue with increasing resolution is that some processes may need to be completely reformulated as they become increasingly resolved. This is the case with convection that is parameterized at coarse resolution but explicitly treated in convection-permitting models. This is a limitation for several regional climate models used in EURO-CORDEX that are not developed for being applied at grid spacing finer than around 10 km.

References

Kendon, EJ, NM Roberts, HJ Fowler, MJ Roberts, SC Chan, and CA Senior (2014) Heavier summer downpours with climate change revealed by weather forecast resolution model, Nature Climate Change, 4, 570–576, doi:10.1038/nclimate2258

Olsson J, Berg P and Kawamura A (2015) Impact of RCM Spatial Resolution on the Reproduction of Local, Subdaily Precipitation. J. Hydrometeorol., 16, 534–547, doi:10.1175/jhm-d-14-0007.1.

Prein AF, Gobiet A, Suklitsch M, Truhetz H, Awan NK, Keuler K and Georgievski G (2013) Added value of convection permitting seasonal simulations. Clim. Dyn., 41, 2655–2677, doi:10.1007/s00382-013-1744-6.

Torma Cs, Giorgi F, and Coppola E (2015) Added value of regional climate modeling over areas characterized by complex terrain—Precipitation over the Alps, J. Geophys. Res. Atmos., 120, 3957– 3972. doi: 10.1002/2014JD022781.