Saturday, March 26, 2011

An Introduction to NOAA's Climate Model CM2.1

Source: http://www.acespace.org/
The IPCC (Intergovernmental Panel on Climate Change) used climate models to conclude that developed nations should reduce  carbon emissions in order to mitigate anthropogenic climate change. But what exactly are those models? The IPCC uses about a dozen different models, listed in table 8.1 of the 4th Assessment Report.

One of the models (no. 12 in table 8.1) comes from the U.S. National Oceanic and Atmospheric Administration (NOAA), or to be more precise, from NOAA's General Fluid Dynamics Laboratory (GFDL). We'll investigate this model, called CM2.1.
Source: noaa.gov

CM2.1

The model is called CM2.1. GFDL also developed CM2.0, which uses a different atmospheric core, among other things, and is now developing CM3 for use in the next IPCC assessment report.

Source: http://www.gfdl.noaa.gov
The model has two broad categories of output:
  1. Atmospheric and land model output
  2. Ocean and sea ice model out.

Model Output

The output of the model available to the public on the GFDL data portal is specifically for use in the IPCC 4th Assessment Report. As such, it follows strict conventions set by the IPCC so that models from different organizations can be easily compared.

Technically, the model output is a collection of hundreds of files--one file per experiment, per time series, per time span, per variable. The following summary comes largely from the document "Requirements for IPCC Standard Output Contributed to the PCMDI Archive."

Experiments

The IPCC specifies 12 experiments, along with their short-hand and full names.
  • PIcntrl (i.e., the pre-industrial control experiment)
  • PDcntrl (i.e., the present-day control experiment)
  • 20C3M (i.e., the climate of the 20th Century experiment (20C3M))
  • Commit (i.e., the committed climate change experiment)
  • SRESA2 (i.e., the SRES A2 experiment)
  • SRESA1B (i.e., the 720 ppm stabilization experiment (SRES A1B))
  • SRESB1 (i.e., the 550 ppm stabilization experiment (SRES B1))
  • 1%_to2x (i.e., the 1%/year CO2 increase experiment (to doubling))
  • 1%_to4x (i.e., the 1%/year CO2 increase experiment (to quadrupling))
  • Slabcntl (i.e., the slab ocean control experiment)
  • 2xCO2 (i.e., the 2xCO2 equilibrium experiment)
  • AMIP (i.e., the AMIP experiment)

Variables

The IPCC defines official variable names to make comparisons across different organizations' models easier. GFDL uses these variable names. Some examples:

CF standard_name output variable name units
air_temperature tas K

moisture_content_of_soil_layer

mrsos
kg m-2
soil_moisture_content mrso kg m-2
surface_downward_eastward_stress tauu Pa
surface_downward_northward_stress tauv Pa

Time series

CM2.1 outputs data in four time series:
  1. Every 3 hours
  2. Daily
  3. Monthly
  4. Yearly

Time span

Each experiment run has a start time an an end time. The IPCC standardizes these as well, and they depend on the experiment and reporting time series. For example, the experiment "1%/year CO2 increase experiment (to doubling)" has three different time spans depending on whether the data is output as daily, monthly, or annual values.

For monthly data, IPCC specifies that this experiment should include the ~70 years it takes for a 1% annual increase in CO2 to result in a doubling, plus an additional 150 years, during which period the CO2 should be held constant. This experiment, then, runs for 220 years. The IPCC provides a complete table of experiments and their corresponding time spans.

Experiments

The GFDL CM2.1 model can output all 12 IPCC-defined experiments listed above. IPCC looks closely at climate change, which is related to atmospheric CO2, among other greenhouse gases. The model can output atmospheric CO2, so a convenient way to compare the 12 different experiments is to compare how they predict CO2.

Here's a graphical representation of the different experiments, each started in the year 1860. The vertical axis shows atmospheric CO2 concentration, and the horizontal axis represents time. Notice that each experiment has a defined end time.
Source: http://data1.gfdl.noaa.gov/CM2.X/CM2.0/data/cm2.0_data.html#schematic

1PctTo2X

The 1%/year CO2 increase (to doubling) is the solid brown line. It begins at 1860 levels of CO2 and increases the atmospheric CO2 concentration by 1% every year. After 70 years (about 1930), it has doubled, at which point the CO2 concentration is held constant while other variables (not shown here) are allowed to move.

Under this experiment, one could then look at other variables besides atmospheric CO2, such as surface temperature or sea ice, to see how they behave under this scenario. 

Here's the official description of the experiment called "a 1%/year CO2 increase experiment (to doubling)":
Initial conditions for this experiment were taken from 1 January of year 1 of the 1860 control model experiment named CM2.1U_Control-1860_D4. In the CM2.1U-D4_1PctTo2X_I1 experiment atmospheric CO2 levels were prescribed to increase from their initial mixing ratio level of 286.05 ppmv at a compounded rate of +1 percent per year until year 70 (the point of doubling). CO2 levels were held constant at 572.11 ppmv from year 71 through the end of the 220 year long experiment. For the entire 220 year duration of the experiment, all non-CO2 forcing agents (CH4, N2O, halons, tropospheric and stratospheric O3, tropospheric sulfates, black and organic carbon, dust, sea salt, solar irradiance, and the distribution of land cover types) were held constant at values representative of year 1860.

Conclusion

The IPCC uses many models in its assessment reports on climate change. NOAA's GFDL develops one of these models, called CM2.1. This model contains 12 experiments representing different climate scenarios, with each experiment executed over several different time spans. These experiments produce hundreds of NetCDF files, each one representing one variable from the experiment.

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