| Substantial uncertainty still exists regarding the distribution of radiative energy within the global climate system, and its representation in General Circulation Models.
Compared to a comprehensive set of surface observations, the majority of the GCMs participating in the latest IPCC forth assessment report (AR4) overestimate the surface insolation, by 6 Wm-2 on average, while the bias is smaller at the TOA (Wild 2008). This is in line with an analysis of 20 earlier GCMs participating in the Atmospheric Model Intercomparison Project AMIP II and suggests that the GCM atmospheres are still overly transparent for solar radiation (Wild 2005). Based on observational clear-sky climatologies at worldwide distributed anchor sites from the Baseline Surface Radiation Network (BSRN) and the Atmospheric Radiation Measurement Program (ARM) it is shown, that the surface insolation is also overestimated under cloud-free conditions in many GCMs with comparatively low atmospheric clear-sky solar absorption (around 60 Wm-2 in the global mean) (Wild et al. 2006). This identifies an overly transparent cloud-free atmosphere as a key error source for the long known problem of excessive surface insolation in GCMs. However, there are now several models participating in IPCC-AR4 with higher atmospheric clear-sky absorption (70 Wm-2 and up, globally averaged) and more realistic aerosol treatment, which are in excellent agreement with the observational clear-sky climatologies. This underlines the progress made in radiative transfer modeling as well as in the observation and diagnosis of solar radiation under cloudless atmospheres (Wild 2008).
The most difficult component to model in the longwave is the downward longwave flux at the surface. Accordingly, large discrepancies exist in the global means of this component in the GCMs, both under all sky and clear sky conditions. A comparison with available observations from GEBA and BSRN suggests that the IPCC AR4 GCMs tend to underestimate the longwave downward flux (Wild et al. 2001, Wild 2008).
Selected related references:
Wild, M., 2008: Shortwave and longwave surface radiation budgets in GCMs: a review based on the IPCC-AR4/CMIP3 models. Tellus, 60, 932 - 945. doi: 10.1111/j.1600-0870.2008.00342.x
Wild, M., Long, C.N., and Ohmura, A., 2006: Evaluation of clear-sky solar fluxes in GCMs participating in AMIP and IPCC-AR4 from a surface perspective. J. Geophys. Res., 111, D01104, doi:10.1029/2005JD006118.
Wild, M., and Roeckner, E., 2006: Radiative fluxes in ECHAM5. J. Climate, 19, 3792-3809.
Wild, M., 2005: Solar radiation budgets in atmospheric model intercomparisons from a surface perspective. Geophys. Res. Lett., 32, L07704, doi:10.1029/ 2005GL022421.
Wild, M., Ohmura, A., Gilgen, H., Morcrette, J.J., and Slingo, A., 2001: Downward longwave radiation in General Circulation Models. J. Climate, 14, 3227-3239.
Wild, M., Ohmura, A., Gilgen, H., Roeckner, E., Giorgetta, M., and Morcrette, J.J., 1998: The disposition of radiative energy in the global climate system: GCM versus observational estimates. Climate Dynamics, 14, 853-869. |