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Precipitation
Model capabilities for producing precipitation are revealed in Figures
18 and 19. Ratios of calculated to observed values are used for
precipitation because of the wide difference in precipitation for different
parts of the domain. As for temperature, the values in the upper left
panel give the observed values for each grid point. The first 4-panel
graph gives comparisons of 1x CO2 results with the observed climate.
Perfect simulations of the present climate would give 1.0 at each point,
since ratios are being plotted. Large percentage errors are produced by
the models for precipitation. A value of 2.0 indicates the model produces
twice the observed precipitation (100% error). Errors of 50 to 100% are
common, and much larger values are observed at many grid points. Such
results do not instill confidence in global climate model capability for
simulating precipitation. The graph for the ratio of 2x CO2 precipitation
to the models' 1x CO2 values gives values of 1.0 or less at most grid
points. Because of the large biases of the previous graph, there is little
justification for using these precipitation results in assessing impacts of
climate change.
The next unit discusses transient climate simulations, which give more realistic simulations of present climate and, hopefully, also future climates.
