Global Mean Precipitation and Precipitation Bias in 1-degree and 2-degree CCSM4 Simulations

Biases still remain in climate models. One in particular is the so-called double Intertropical Convergence Zone, or ITCZ, bias in precipitation. In general, coupled climate models show excessive tropical precipitation, which through ocean-atmosphere coupling, leads to less precipitation in the equatorial Pacific. A drier equatorial bias splits the wetter tropical Pacific into northern and southern bands, giving rise to the apparent double ITCZ.

Comparing the bias for the 1-degree and 2-degree versions of the model, we can see that increasing the model resolution has only a small effect on the bias. Biases like the double ITCZ motivate researchers to improve the parameterization schemes that lead to excessive tropical precipitation.

Precipitation Bias for GFS 96-120 hr Forecasts, June-August 2010, Relative to CAM-OPI Precipitation from Rain Gauge Observations and Satellite Estimates

Note that NWP models produce similar biases. This graphic shows biases produced by GFS 4-5 day forecasts for June through August, 2010. Even over this brief period, a double ITCZ is apparent, along with excess precipitation in the Himalaya. These and other features are similar to those we saw in long-term simulations by climate models.