Contrasting Influences of Recent Aerosol Changes on Clouds and Precipitation in Europe and East Asia

Over the last few decades, aerosol loadings have increased greatly over Southeast Asia, while Europe and North America have experienced huge reductions. Previous studies have suggested that these changes may have influenced the temperature trends as well as precipitation patterns due to the direct and semidirect aerosol effects. Here, an Earth system model with parameterized aerosol-radiation and aerosol-cloud interactions is used to investigate changes in cloud properties and precipitation between 1975 and 2005. This is done globally as well as for the two focus areas Europe and East Asia. Despite systematic changes in cloud droplet number concentration and cloud droplet size, changes in stratiform precipitation are less clear. In both regions there is a dominance of autoconversion over liquid water accretion as the primary precipitation release mechanism, which alone should imply a strong sensitivity to changes in cloud droplet size. However, in these areas liquid water paths are relatively low and background concentrations are high, which produce low simulated precipitation susceptibilities. High susceptibilities are instead found over remote ocean regions, in agreement with expectations. For convective precipitation, both regions show statistically significant changes that are consistent with oppositely signed changes in direct aerosol forcing over Europe and East Asia, respectively.