Monthly and 3-hourly precipitation data from 20th century climate simulations by the newest generation of eighteen coupled climate system models are analyzed and compared with available observations. The characteristics examined include mean spatial patterns, intraseasonal to interannual and ENSO-related variability, convective versus stratiform precipitation ratio, precipitation frequency and intensity for different precipitation categories, and the diurnal cycle. Although most models reproduce the observed broad patterns of precipitation amount and year-to-year variability, models without flux corrections still show an unrealistic double ITCZ pattern over the tropical Pacific, whereas the flux-corrected models, especially MRI-CGCM2.3.2a, produce realistic rainfall patterns at low latitudes. As in previous generations of coupled models, the rainfall double ITCZs are related to westward expansion of the cold tongue of sea surface temperature that is observed only over the equatorial eastern Pacific but extends to the central Pacific in the 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 the TRMM satellite observations. The biases in the convective versus stratiform precipitation ratio are linked to unrealistically strong coupling of tropical convection to local sea surface temperature (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 contribution (to total precipitation) and frequency for moderate precipitation (10-20 mm/day), but underestimate the contribution and frequency for heavy (>20 mm/day) precipitation and overestimate them for light (<10 mm/day) precipitation. The newest generation of coupled models still rains too frequently, mostly within the 1-10 mm/day category. Precipitation intensity over the storm tracks around the eastern coasts of Asia and North America has intensity comparable to that in the ITCZ (10-12 mm/day) 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 the 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 world's coupled climate models.

Full Article: http://www.cgd.ucar.edu/cas/adai/papers/Dai-cmep-paper.pdf

Last Updated: 2007-02-27