Aerosols and Cloud-Aerosol Interactions in the Energy Exascale Earth System Model (E3SM) Version 1

Tropospheric aerosols as short-lived climate forcers have large spatio-temporal variability. It is critical to have accurate global three-dimensional distributions of aerosols in order to fully assess their radiative and microphysical impacts in climate models. Many current global aerosol-climate models have large biases in the prediction of aerosol distributions and its radiative forcing. The U.S. Department of Energy E3SM model (Version 1 to be released in July 2018), which was branched from the Community Earth System Model (CESM1.3), was designed to run at a low and high horizontal resolution of 1° (30 spectral elements) and 0.25° (120 spectral elements), respectively, and 72 layers in the vertical for both dynamics and physics, with 14 layers below 850 hPa and the model top at 0.1 hPa. Several new treatments to the representations of aerosols and cloud-aerosol interactions have been implemented in E3SMv1, including emissions, new aerosol particle formation, explicit aging of carbonaceous aerosol species, wet scavenging processes (i.e., aerosol activation, cloud processing, resuspension, and wet removal), ice nucleation, and deposition of light-absorbing particles to snowpack and sea ice. Stand-alone atmosphere (with data ocean) simulations show that aerosol residence times and spatial distributions, impacts on clouds and precipitation, and deposition onto snow and ice surfaces have changed significantly due to the new treatments, compared to the original model (E3SMv0/CESM1.3). Some of the advanced treatments of aerosols that lead to better spatial distributions in E3SMv1 also give a stronger aerosol shortwave indirect forcing, which has been found to relate to uncertainties in aerosol-cloud parameterizations. We will discuss these new features of aerosols and their impact on clouds and radiation, as well as further model developments needed for a more accurate representation of aerosol effects in the Earth system.