The Convective-To-Total Precipitation Ratio and the Drizzling Bias in Climate Models

Overestimation of precipitation frequency and duration while underestimating intensity, that is, the drizzling bias, has been a long-standing problem of global climate models. Here we explore this issue from the perspective of precipitation partitioning. We found that most models in the Climate Model Intercomparison Project Phase 5 (CMIP5) have high convective-to-total precipitation (PC/PR) ratios in low latitudes. Convective precipitation has higher frequency and longer duration but lower intensity than non-convective precipitation in many models. As a result, the high PC/PR ratio contributes to the drizzling bias over low latitudes. The PC/PR ratio and associated drizzling bias increase as model resolution coarsens from 0.5 degrees to 2.0 degrees, but the resolution's effect weakens as the grid spacing increases from 2.0 degrees to 3.0 degrees. Some of the CMIP6 models show reduced drizzling bias associated with decreased PC/PR ratio. Thus, more reasonable precipitation partitioning, along with finer model resolution should alleviate the drizzling bias within current climate models.

Automated summary

The overestimation of precipitation frequency and duration, coupled with the underestimation of intensity, known as the drizzling bias, is a long-standing issue in global climate models. By exploring this problem from the perspective of precipitation partitioning, it was found that high convective-to-total precipitation ratios in low latitudes contribute to the drizzling bias. Some CMIP6 models show reduced drizzling bias with decreased convective precipitation ratios, suggesting that more reasonable precipitation partitioning and finer model resolution could alleviate this bias.