Modelling Earth’s atmosphere: What are Climate Models and how they work

The figure shows the temporal evolution of tropical (20°S-20°N) total ozone column anomalies. Different simulations have been carried out with the Chemistry-Climate Model EMAC aiming to study the influence of different model set-ups (e.g. climate forcing, different vertical resolution). NASA and ESA data sets which are based on satellite-based observations are shown for comparison with model data. For example, the dark green line (i.e. EMAC RC2, up to the year 2100) is showing the result of the reference simulation for the future where the ozone projection is based on the RCP6.0 climate scenario. Here the sea surface temperature field is fixed, i.e. the ocean is not fully interacting with the atmosphere. The green line (i.e. EMAC RC2-oce) is showing the results of a simulation with otherwise identical conditions, except for that in this simulation a deep-ocean model is interactively coupled to the atmosphere. In the last phase of these two simulations (about after 2060) an obvious difference occurs with respect to the tropical ozone trend. It points toward the specific role of the tropical sea-surface temperature for atmospheric processes. Further experiments are being carried out to explain this difference.

Numerical models are useful for investigations of the composition and the thermal and dynamical structure of Earth’s atmosphere. They allow evaluation of different processes and mechanisms as well as feedbacks. We can understand a lot of how the Earth’s atmosphere works by looking at the discrepancies between observations and results derived from model simulations. Projections of the future development of atmospheric dynamics and chemistry are typically based on scenario simulations and sensitivity studies.

Atmospheric General Circulation Models (AGCMs) are three-dimensional models of large-scale physical, radiative, and dynamical processes in the atmosphere. They are used for simulations over years and decades; they have a typical spatial resolution of a few hundred kilometres. An AGCM is used to study changes in natural variability of the atmosphere and for investigations of climate effects of radiatively active trace gases (greenhouse gases) and aerosols (natural and anthropogenic), along with their interactions and feedbacks. Usually, AGCM calculations employ prescribed concentrations of greenhouse gases, e.g., carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs), and ozone (O3). Such atmospheric models can simulate directly changes of water vapour (H2O) concentrations due to the hydrological cycle (i.e., the cycle of water throughout the Earth system). Sea surface temperature and sea ice cover are usually prescribed. This means that the atmospheric model used datasets for these two variables and that neither a chemistry model nor an ocean model is interactively coupled to an AGCM.

When an AGCM is interactively coupled to an ocean model, it’s commonly referred to as an Atmosphere-Ocean General Circulation Model (AOGCM) or a climate model and is used for climate change research. Climate models may also include other feedback processes (e.g., carbon cycle, interaction with the biosphere): these are so-called Earth-System Models.

An AGCM or climate model that is interactively coupled to a detailed chemistry module is called a Chemistry-Climate Model (CCM). In a CCM, the simulated concentrations of the radiatively active gases are used in the calculations of net heating rates. Changes in the abundance of these gases due to chemistry and advection influence heating rates and, consequently, variables describing atmospheric dynamics such as temperature and wind. This gives rise to a dynamical-chemical coupling in which the chemistry influences the dynamics (via radiative heating) and vice versa (via temperature and advection).

This kind of climate model, which is interactively coupled with both the deep-ocean and atmospheric chemistry, is used in the same way supporting climate change assessment reports, like the Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC) or the Scientific Assessment of Ozone Depletion of the United Nations Environment Program (UNEP) and the World Meteorological Organization (WMO). The big advantage is that a uniform and consistent set of model simulations are now available for looking at predictions of climate change, but also at the possible future evolution of the chemical composition of the Earth climate system. This allows for the development of more robust mitigation and adaptation strategies for the Earth climate and the ozone layer.

At the DLR Institute of Atmospheric Physics we study how to predict the future evolution of the stratospheric ozone layer, by comparing simulation results derived either from simulations considering an interactively coupled deep-ocean or simulations where the sea surface temperature and sea ice cover are prescribed from a previous simulation with the climate model HadGEM2 from the United Kingdom Met Office. Preliminary results indicate that there are distinct differences between the experiments with prescribed sea surface temperatures and sea ice cover and the coupled simulations, with respect to the future evolution of the tropical ozone layer in particular in the tropics (see figure caption for details). Possible feedback mechanisms are currently analysed in more detail to understand the interactions between ocean and atmosphere.