Precipitation characteristics in eighteen coupled climate models

The partitioning of the total variance of precipitation among intraseasonal, seasonal, and longer time scales is generally reproduced by the models, except over the western Pacific where the models fail to capture the large intraseasonal variations. Most models produce too much convective (over 95% of total precipitation) and too little stratiform precipitation over most of the low latitudes, in contrast to 45%-65% in convective form in the Tropical Rainfall Measuring Mission (TRMM) satellite observations.

The biases in the convective versus stratiform precipitation ratio are linked to the unrealistically strong coupling of tropical convection to local SST, which results in a positive correlation between the standard deviation of Niño-3.4 SST and the local convective-to-total precipitation ratio among the models. The models reproduce the percentage of the contribution (to total precipitation) and frequency for moderate precipitation (10-20 mm day⁻¹), but underestimate the contribution and frequency for heavy (>20 mm day⁻¹) and overestimate them for light (<10 mm day⁻¹) precipitation.

The newest generation of coupled models still rains too frequently, mostly within the 1-10 mm day⁻¹ category. Precipitation intensity over the storm tracks around the eastern coasts of Asia and North America is comparable to that in the ITCZ (10-12 mm day⁻¹) in the TRMM data, but it is much weaker in the models. The diurnal analysis suggests that warm-season convection still starts too early in these new models and occurs too frequently at reduced intensity in some of the models.

The results show that considerable improvements in precipitation simulations are still desirable for the latest generation of the world's coupled climate models.