• GG 312: Global Climate Change and Environmental Impacts (Fall 2002)

     

    Chapter 4: Climate Model Evaluation

    4.1. Introduction
    4.2. What is Model Evaluation?
    4.3. Atmosphere
    4.4. Ocean
    4.5. Sea Ice
    4.6. Land Surface
    4.7. Climate of the 20th Century
    4.8. Phenomena
    4.9. Extreme Events
    4.10. Assessment

    4.1. Introduction

    The coupled climate system consists of the atmosphere, oceans, cryosphere and the land surface (deserts, vegetation, orography, etc). The climate models studied here include a three dimensional representation and interaction of these components on a global time-dependent basis. These models when run in conjunction with specified chemical composition of the atmosphere and surface charactertistics, provide the current basis for understanding and simulating the climate system and its future changes.

    Note that the simulation of the coupled climate system requires integrations of over 100 to a 1000 simulated years; therefore, computational costs restrict model complexity and resolution in space and time.

    Another point of interest is the large range of time scales at which processes in the various components of the climate system operate. For instance, the time scales for the atmosphere are of the order of weeks; for the land surface, a season, and, for the deep ocean, 100 to 1000 of years. This makes climate model simulations of future change tricky.

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    4.2. What is Model Evaluation

    Model evaluation is assessing the suitability of models (in particular coupled atmosphere-ocean general circulation models) for use in climate change projection and in detection and attribution studies.

  • We concentrate on the variables and time scales that are important for this task.
  • Models are evaluated against observations and differences between models are explored using information from a number of systematic model intercomparisons.
  • Even if a model is assessed as performing credibly when simulating the present climate, this does not necessarily guarantee that the response to a perturbation remains credible. Therefore, we also assess the performance of the models in simulating the climate over the 20th century and for selected palaeoclimates.

    4.3. Atmosphere

    Coupled climate models simulate mean atmospheric fields with reasonable accuracy, with the exception of clouds and some related hydrologic processes. Problems in the simulation of clouds and upper tropospheric humidity, however, remain worrisome because the associated processes account for most of the uncertainty in climate model simulations of anthropogenic change.

    Figure 4.1: December-January-February climatological surface air temperature in K simulated by the CMIP1 model control runs.
    Figure 4.2: December-January-February climatological precipitation in mm per day (top) simulated by CMIP1 model control runs.
    Figure 4.3: Zonally averaged December-January-February total cloudiness simulated by 10 AMIP1 models (a) and by revised versions of the same 10 models (b). The solid black line gives observed data from the International Satellite Cloud Climatology Project (ISCCP).

    4.4. Ocean

    Considerable progress has been made since IPCC-1995 in the realism of the ocean component of climate models. Models now exist which simultaneously maintain realistic poleward heat transports, surface temperatures and thermocline structure. However, there are still a number of processes which are poorly resolved or represented, for example convection.

    Ocean Circulation and Climate

    Figure 4.4: Zonal mean air and sea temperature 'errors' in °C (defined here as the difference from the initial model state, which was derived from observations), for three different coupled models. The models differ only in the parameterisation of lateral mixing used in the ocean component. The different mixing schemes produce different rates of heat transport between middle and high latitudes, especially in the Southern Hemisphere. The atmosphere must adjust in order to radiate the correct amount of heat to space at high latitudes, and this adjustment results in temperature differences at all levels of the atmosphere.

    4.5. Sea Ice

    The sea-ice simulations of 15 global coupled models contributed to CMIP-1 are summarised in Table 4.1 which provides a comparison of ice extent, defined as the area enclosed by the ice edge, for winter and summer seasons in each hemisphere. The last row of the Table provides an observed estimate based on satellite data covering the period 1978-87.

    Figure 4.5provides a visual presentation of the range in simulated ice extent, and was constructed as follows. For each model listed in Table 4.1, a 1/0 mask is produced to indicate presence or absence of ice. The 15 masks were averaged for each hemisphere and season and the percentage of models that had sea ice at each grid point was calculated. There is a large range in the ability of models to simulate the position of the ice edge and its seasonal cycle, particularly in the Southern Hemisphere.

    Table 4.1: Coupled model simulations for December, January, February (DJF) and June, July, August (JJA) of sea-ice cover (columns 2-5) and snow cover (106 km2 columns 6 and 7).
    Figure 4.5: Illustration of the range of sea-ice extent in CMIP-1 model simulations listed in Table 4.1: Northern Hemisphere, DJF (left) and Southern Hemisphere, JJA (right). For each model, a 1/0 mask is produced to indicate presence or absence of ice. The masks were averaged for each hemisphere and season. The 0.5 contour therefore delineates the region for which at least half of the models produced sea-ice. The 0.1 contour indicates the region outside of which only 10% of models produced ice, while the 0.9 contour indicates that region inside of which only 10% of models did not produce ice. The observed boundaries are averages over 1961-1990 period.

    4.6. Land Surface

    Uncertainty in land surface processes, coupled with uncertainty in parameter data combines, at this time, to limit the confidence we have in the simulated regional impacts of increasing CO2. In general, the evidence suggests that the uncertainty is largely restricted to surface quantities (i.e., the large-scale climate changes simulated by coupled climate models are probably relatively insensitive to land surface processes). Our uncertainty derives from difficulties in the modelling of snow, evapotranspiration and below ground processes.

    Figure 4.6: An uncertainty ratio for 10-degree latitude bands; (a) control simulations; (b) difference between the control and doubled greenhouse gas simulations. E is for evaporation, P is precipitation, Tscr is screen temperature, cld is the percent cloud cover and Sn is the net short wave radiation at the surface. The units on the Y-axis are dimensionless. An asterisk means the value is statistically significant at 95% and a diamond at 90%.

    4.7. Climate of the 20th Century

    Several coupled models are able to reproduce the major trend in 20th century surface air temperature, when driven by historical radiative forcing scenarios corresponding to the 20th century (Figure 4.7). However, in these studies idealised scenarios of only sulphate radiative forcing have been used. One study that includes both the indirect and direct effects of sulphate aerosols, as well as changes in tropospheric ozone, suggests that the observed surface and tropospheric air temperature discrepancies since 1979 are reduced when stratospheric ozone depletion and stratospheric aerosols associated with the Pinatubo eruption are included (Figure 4.8). Systematic evaluation of 20th century AOGCM simulations for other trends found in observational fields, such as the reduction in diurnal temperature range over the 20th century and the associated increase in cloud coverage, have yet to be conducted.

    The inclusion of changes in solar irradiance and volcanic aerosols has improved the simulated variability found in several AOGCMs. In addition, some evaluation studies aimed at the reproduction of 20th century climate have suggested that changes in solar irradiance may be important to include in order to reproduce the warming in the early part of the century. Another study has suggested that this early warming can be solely explained as a consequence of natural climate variability. Taken together, we consider that there is an urgent need for a systematic 20th century climate intercomparison project with a standard set of forcings, including volcanic aerosols, changes in solar irradiance and land use, as well as a more realistic treatment of both the direct and indirect effects of a range of aerosols.

    Figure 4.7: Observed (Parker/Jones) and modelled global annual mean temperature anomalies (degrees C) from the 1961- 1990 climatological average. The control and three independent simulations with the same greenhouse gas plus aerosol forcing and slightly different initial conditions are shown. The three greenhouse gas plus aerosol simulations are labelled GHG+Asol1,GHG+Asol2 and GHGAsol3 respectively.
    Figure 4.8: Radiative forcing (Wm-2)since 1979 due to changes in stratospheric aerosols, ozone, greenhouse gases and solar irradiance. (b-d) Observed global annual mean surface, tropospheric and stratospheric temperature changes and GISS GCM simulations as the successive radiative forcings are cumulatively added one-by-one. The base period defining the zero mean observed temperature was 1979-1990 for the surface and the troposphere and 1984-1990 for the stratosphere.

    4.8. Phenomena

    Recent atmospheric models show improved performance in simulating many of the important phenomena by using better physical parametrisations and using higher resolutions both in the horizontal and in the vertical domain. An intercomparison of El Niņo simulations, one of the most important phenomena, has revealed an ability of coupled climate models to simulate the El Niņo-like SST variability in the tropical Pacific and associated changes in precipitation in the tropical monsoon regions, although the region of maximum SST variability is displaced further westward than in the observations (Figure 4.9).

    Figure 4.9: Simulated (March-May) and observed (March-April) averaged Nordeste (northeastern Brazil) rainfall indices for (a) the original AMIP simulations, (b) the revised AMIP simulations.

    4.9. Extreme Events

    Attention has been paid to the analysis of extreme events in climate model simulations. This increases our confidence that they may be better reproduced by high-resolution climate models in the future.

  • Evaluations indicate that climate models are more capable of reproducing the variability in maximum and minimum temperature in the global scale than the daily precipitation variability.
  • The ability of climate models to simulate extra-tropical storm tracks and storm frequency is encouraging.
  • It is worth noting that some high-resolution operational numerical weather prediction models have demonstrated reasonable ability in forecasting tropical cyclones.

    4.10. Assessment

    We regard the following as well established:

    • Incremental improvements in the performance of coupled models have occurred since IPCC-1995, resulting from advances in the modelling of the oceans, atmosphere and land surface, as well as improvements in the coupling of these components.
    • Coupled models can provide credible simulations of both the annual mean climate and the climatological seasonal cycle over broad continental scales for most variables of interest for climate change. Clouds and humidity remain sources of significant uncertainty but there have been incremental improvements in simulations of these quantities.
    • Several coupled models are able to reproduce the major trend in surface air temperature, when driven by radiative forcing scenarios corresponding to the 20th century. However, in these studies only idealised scenarios of only sulphate radiative forcing have been used.

    We regard the following as evolving:

    • Coupled model simulation of phenomena such as monsoons and the North Atlantic Oscillation has improved since IPCC-1995.
    • Analysis of, and confidence in, extreme events simulated within climate models is emerging, particularly for storm tracks and storm frequency.
    • The performance of coupled models in simulating ENSO has improved; however, the region of maximum SST variability is displaced westward and its strength is generally underestimated. When suitably initialised, some coupled models have had a degree of success in predicting ENSO events.
    • Our ability to increase confidence in the simulation of land surface quantities in coupled models is limited by the need for significant advances in the simulation of snow, liquid and frozen soil moisture (and their associated water and energy fluxes).
    • Some coupled models now include improved sea ice components, but they do not yield systematic improvements in the sea ice distributions. This may reflect the impact of errors in the simulated near surface wind fields, which offsets any improvement due to including sea ice motion.
    • The relative importance of increased resolution in coupled models remains to be evaluated systematically but many models show benefits from increased resolution.

    Our overall assessment: Coupled models have evolved and improved significantly since IPCC-1995. In general, they provide credible simulations of climate, at least down to sub-continental scales and over temporal scales from seasonal to decadal. The varying sets of strengths and weaknesses that models display leads us to conclude that no single model can be considered "best" and it is important to utilise results from a range of coupled models. We consider coupled models, as a class, to be suitable tools to provide useful projections of future climates.

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    Oct-21-2002
    rmyneni@bu.edu