What Is the Contribution of Convergence Zones to Global Precipitation? Assessing Observations and Climate Models Biases

Convergence zones (CZs) are known drivers of precipitation regimes from regional to planetary scales. However, there is a scarcity of accounts of the contribution of CZs to the global precipitation. In this study, we build upon a recently developed Lagrangian diagnostic to attribute precipitation to CZ events in observations and simulations submitted to the Coupled Model Intercomparison Project 6 (CMIP6). Observed CZs are identified using ERA5 reanalysis wind and attributed precipitation from observational products based on satellite estimates and rain gauges. We estimate that approximately 54% (51%-59%, depending on the precipitation product) of global precipitation falls over CZs; in some regions, such as the Intertropical Convergence Zone (ITCZ) and subtropical monsoon regions, this proportion is greater than 60%. All CMIP6 simulations analyzed here attribute about 10% more precipitation to CZ events than what the observations suggest. To investigate this overestimation, we decompose the precipitation error in terms of frequency and intensity of CZ precipitation and find that all models present a substantial positive bias in the frequency of CZ precipitation, suggesting that climate models trigger precipitation too easily in regions of airmass confluence; such positive frequency biases in CZ precipitation help explaining well-known biases in climate models, such as the double-ITCZ in the Pacific. We also find that models with better mass conservation present an apportionment of CZ precipitation closest to the observational estimates, demonstrating the relevance of mass conservation in advection schemes. Rain is crucial for human and animal livelihoods. In the Earth's atmosphere, rain often occurs in organized cloud bands of hundreds or thousands of kilometers lengthwise. This type of organized rain is usually denominated as occurring within a convergence zone (CZ). In this study, we use a recently developed method to identify these CZs and apply it to observational data to find out how much of Earth's rain falls in regions under the influence of CZs (approximately 54%). We repeat this exercise in climate model data and find that all models overestimate rain in regions under the influence of CZs by about 10%. Our results point to the importance of CZs for the global hydrological cycle and ways in which climate models need improvement to better simulate CZ rainfall. Approximately 54% of global precipitation falls over convergence zones Climate models overestimate this ratio by approx. 10% Conservative moisture advection schemes seem to alleviate this overestimation