Evaluating Precipitation Distributions at Regional Scales: A Benchmarking Framework and Application to CMIP 5 and 6 Models.

A framework for quantifying precipitation distributions at regional scales is presented and applied to CMIP 5 and 6 models. We employ the IPCC AR6 climate reference regions over land and propose refinements to the oceanic regions based on the homogeneity of precipitation distribution characteristics. The homogeneous regions are identified as heavy, moderate, and light precipitating areas by K-means clustering of IMERG precipitation frequency and amount distributions. With the global domain partitioned into 62 regions, including 46 land and 16 ocean regions, we apply 10 established precipitation distribution metrics. The collection includes metrics focused on the maximum peak, lower 10th percentile, and upper 90th percentile in precipitation amount and frequency distributions, the similarity between observed and modeled frequency distributions, an unevenness measure based on cumulative amount, average total intensity on all days with precipitation, and number of precipitating days each year. We apply our framework to 25 CMIP5 and 41 CMIP6 models, and 6 observation-based products of daily precipitation. Our results indicate that many CMIP 5 and 6 models substantially overestimate the observed light precipitation amount and frequency as well as the number of precipitating days, especially over mid-latitude regions outside of some land regions in the Americas and Eurasia. Improvement from CMIP 5 to 6 is shown in some regions, especially in mid-latitude regions, but it is not evident globally, and over the tropics most metrics point toward over degradation. [ABSTRACT FROM AUTHOR]