by Judith Curry
A new paper describing the latest version of the NCAR climate model has just been published at the Journal of Climate. This is the version of the model that is being used for the IPCC AR5.
The Community Climate System Model Version 4
Peter R. Gent, Gokhan Danabasoglu1, Leo J. Donner, Marika M. Holland, Elizabeth C. Hunke, Steve R. Jayne, David M. Lawrence1Richard B. Neale1, Philip J. Rasch, Mariana Vertenstein1 Patrick H. Worley6, Zong-Liang Yang, and Minghua Zhang
The fourth version of the Community Climate System Model (CCSM4) was recently completed and released to the climate community. This paper describes developments to all CCSM components, and documents fully coupled pre-industrial control runs compared to the previous version, CCSM3. Using the standard atmosphere and land resolution of 1◦ results in the sea surface temperature biases in the major upwelling regions being comparable to the 1.4◦ resolution CCSM3. Two changes to the deep convection scheme in the atmosphere component result in CCSM4 producing El Nino/Southern Oscillation variability with a much more realistic frequency distribution than CCSM3, although the amplitude is too large compared to observations. They also improve the Madden-Julian Oscillation, and the frequency distribution of tropical precipitation. A new overflow parameterization in the ocean component leads to an improved simulation of the Gulf Stream path and the North Atlantic Ocean meridional overturning circulation. Changes to CCSM4 land component lead to a much improved annual cycle of water storage, especially in the tropics. The CCSM4 sea ice component uses much more realistic albedos than CCSM3, and for several reasons the Arctic sea ice concentration is improved in CCSM4. An ensemble of 20th century simulations produces a pretty good match to the observed September Arctic sea ice extent from 1979 to 2005. The CCSM4 ensemble mean increase in globally-averaged surface temperature between 1850 and 2005 is larger than the observed increase by about 0.4◦C. This is consistent with the fact that CCSM4 does not include a representation of the indirect effects of aerosols, although other factors may come into play. The CCSM4 still has significant biases, such as the mean precipitation distribution in the tropical Pacific Ocean, too much low cloud in the Arctic, and the latitudinal distributions of short-wave and long-wave cloud forcings.
Journal of Climate 2011 ; e-View
The solar output anomaly timeseries is described in Lean et al. (2005), and is added to the 1360.9 W m−2 used in the 1850 Control run. The CCSM4 volcanic activity is included by a timeseries of varying aerosol op- tical depths, exactly as in CCSM3 (Ammann et al. 2003). The CO2 and other greenhouse gases (methane and nitrous oxide) are specified as in the IPCC 3rd Assessment Report. Atmosphere aerosol burden (sulphate, organic carbon and sea salt), aerosol deposition (black carbon and dust) onto snow and nitrogen deposition also vary with time. The burdens and deposition rates were obtained from a 20th century run with the CCSM chemistry component active, that is forced with prescribed historical emissions (Lamarque et al. 2010).
All four components are final- ized independently by the respective working groups using stand-alone runs, such as AMIP integrations and runs of the individual ocean, land or sea ice components forced by atmospheric observations. Once the components are cou- pled, then the only parameter settings that are usually al- lowed to change are the sea ice albedos and a single param- eter in the atmosphere component. This is the relative hu- midity threshold above which low clouds are formed, and it is used to balance the coupled model at the TOA. A few 100 year coupled runs are required to find the best values for these parameters based on the Arctic sea ice thickness and a good TOA heat balance.
There are several possibilities for the differences between the models and reality. Neither model includes the indirect effects of aerosols, which has cooled the earth some- what over the 20th century. This implies that both models should warm faster than the observations, and the fact that CCSM3 did not do so suggests that probably the cooling ef- fect of volcanoes is too strong in that model. Volcanoes are implemented in exactly the same way in both models, and Fig. 12 clearly shows the quite large temperature response to large eruptions that is not reflected in the observations. This could possibly be a problem with the temperature re- construction, which has only sparse data in the early part of the record, and a temperature drop might show up better using data just over land. However, there are other possi- bilities for model errors, such as a poor representation of the direct effect of aerosols, or the climate sensitivity is in- correct. In addition, the heat uptake by the ocean maybe too small, although Gent et al. (2006) show that CCSM3 heat uptake is larger than observations suggest, and uptake of chlorofluorocarbon-11 agrees well with observations. It is very difficult to say definitively which of these possibilities causes CCSM4 too large surface temperature increase over the 20th century. [JC comment: no mention of decadal scale variability, PDO, etc. in discussing descrepancies with observations; they still assume that everything can be explained by external forcing]
The transient climate sensitivity of the 1◦ version is 1.60◦C compared to 1.50◦C for CCSM3 T85 version (Kiehl et al. 2006). The CCSM4 1◦ version equilibrium climate sensitivity due to a doubling of CO2 is 3.0◦ ± 0.1◦C (Bitz et al. 2011 submitted to J. Climate), whereas CCSM3 T85 version sensitivity is 2.7◦ ± 0.1◦C (Kiehl et al. 2006). The reasons for this increase in CCSM4 equilibrium climate sensitivity are analysed in detail in Bitz et al. (2011 submitted to J. Climate).
The second conclusion is that CCSM4 still has signifi- cant biases that need to be worked on and improved. Fig- ure 5 shows that the improvements to deep convection in CAM4 have not eliminated the double ITCZ problem, even in the 1◦ version. Gent et al. (2010) show that the 0.5◦ resolution version of CCSM3.5 also had a double ITCZ, so that just increasing atmosphere resolution may not elimi- nate the double ITCZ; further parameterization improve- ments are almost certainly required. The CCSM4 still has biases compared to observations in the latitudinal distribution of both the shortwave and longwave cloud forcing (not shown). Unfortunately, these biases do not get smaller when higher horizontal resolution of 0.5◦ is used in the atmosphere component, because the cloud distribution in CCSM4 is not sufficiently accurate compared to observa- tions. There is still too much low cloud in the Arctic region, despite including the freeze-dry parameterization of Vavrus and Waliser (2008). Figure 11 shows room for improvement in the surface temperature over the continents, which has significant areas where the mean bias is > 2◦C compared to the Willmott and Matsuura (2000) observations.
The third conclusion is that the missing indirect effects of aerosols in CCSM4 is very likely a major factor causing the larger increase in globally-averaged surface tempera- ture over the 20th century than in observations shown in Fig. 12. However, there are other possibilities for this too large increase, such as a poor representation of the di- rect effect of aerosols, ocean heat uptake is too small, or the model climate sensitivity is too large. The absence of the aerosol indirect effects means that projections of future temperature rise due to increased CO2 and other greenhouse gases will be larger than if CCSM4 did include the aerosol indirect effects. These last two conclusions clearly point out the necessity of an improved atmosphere component that includes a better representation of cloud physics and aerosols that allows the feedback of the indirect effects of aerosols. A new version of CAM that includes these processes, and other improved parameterizations, has been under development for some time, and is ready to be in- corporated into CCSM. Results using this new atmosphere component will be documented in the very near future.