The Convective-To-Total Precipitation Ratio and the “Drizzling” Bias in Climate Models
Scientists at the University of California Los Angeles and the University at Albany, SUNY have investigated why precipitation occurs more frequently and lasts longer, but with lower intensity in global climate models than in the real world. They explored this longstanding problem from the perspective of precipitation partitioning, that is, how total precipitation is divided into convective and non-convective components in models. They found that more realistic precipitation partitioning and smaller grid spacing should help reduce the “drizzling” bias.
The study traces the long-standing “drizzling” bias to the physics parameterizations in multiple Climate Model Intercomparison Project Phase 5 (CMIP5) models. The “drizzling” bias impedes realistic representation of precipitation characteristics, as well as projections of future hydrologic extremes. The study shows that the bias can be reduced by more realistic partitioning of total precipitation into its convective and large-scale components, through improving the physics parameterization. The convective-to-total precipitation ratio can be used as a metric for future model development and assessment.
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 a 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° to 2.0°, but the resolution's effect weakens as the grid spacing increases from 2.0° to 3.0°. 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.