Impacts of Suppressing Excessive Light Rain on Aerosol Radiative Effects and Health Risks

Global climate models (GCMs) have been used widely to study radiative forcing and health risks of aerosols. A recent study using two GCMs found that light rain plays a dominant role in controlling aerosol loading. However, too much light rain and too little heavy rain is a longstanding bias in GCMs. It is unclear how much light rain affects aerosol-cloud-radiation interactions and health risks from air pollution. Here we show that, with the correction of the rainfall intensity spectrum in the National Center for Atmospheric Research Community Atmosphere Model version 5.3 by introducing a stochastic deep convection scheme, the reduced frequency of light rain (1-20 mm d(-1)) results in changes of aerosol direct radiative effects (DRE) of up to -0.5 +/- 0.03 W/m(2) and aerosol cloud radiative effects (CRE) of up to -0.9 +/- 0.03 W/m(2). The total (CRE + DRE) radiative effects of light rain-mediated aerosol changes exceed the present-day anthropogenic forcing of aerosols relative to preindustrial levels from the Coupled Model Intercomparison Project (CMIP5&6) models. However, the correction of the rainfall intensity spectrum has little effect on anthropogenic aerosol forcing (defined as the radiative perturbation due to changes in aerosol concentrations between the industrial era and preindustrial levels). Due to increased exposure to fine particulates (PM2.5), the estimated global total premature mortality is much higher than previously estimated, by 300,000 +/- 60,000 deaths per year, and is more severe in populous regions such as India and China. The findings in this study highlight the need to understand uncertainties in radiative effects and health risks of aerosols due to simulation biases of precipitation in GCMs.

Automated summary

This study shows that correcting the rainfall intensity spectrum in global climate models can alter the radiative effects of aerosols and result in an increase in global premature mortality. However, this correction has little impact on anthropogenic aerosol forcing.