Comparison of CMIP5 Cloud Statistics to Long-term ARM Data

Cloud occurrence and radiative forcing statistics derived from long-term, continuous ground observations from the ARM Southern Great Plains, Darwin, and Manus Island sites are compared to similar statistics derived from general circulation models submitted to CMIP5, the latest version of the Coupled Model Intercomparison Project, compiled in mid 2011. This comparison serves to characterize the representation of clouds in the models relative to what is observed. Quantities considered include radiation, the annual cycles of cloud cover, volumetric cloud fractional occurrence, solar and IR cloud forcing at the surface and TOA, and solar and IR atmospheric forcing. While results clearly demonstrate the fact that the models represent regional averages for each grid rather than a single geographical point over each ARM site, the results are able to highlight each model's strengths and weaknesses. In addition to comparing the long-term averaged data, time-series plots created for each month outline the models' ability to represent inter-seasonal and decadal variations, such as the effects of drought years and ENSO. Additionally, the comparisons show the models' differences in atmospheric physics in both the clear and cloudy sky, including cloud properties, leading to variations in cloud forcing with respect to the amount of clouds shown in the models. These comparisons show which numerical calculations in each model accurately represent the observed atmosphere at the ARM sites, and they also show where improvements to the models need to be made.