#18 Aerosol distributions and forcing

Abstract

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. Several new treatments to the representations of aerosols and cloud-aerosol interactions have been implemented in the ACME model (version 1), including emissions, new aerosol particle formation, explicit aging of carbonaceous aerosol species, wet scavenging processes (i.e., aerosol activation, cloud processing, 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 (ACMEv0/CESM1.3). The revised treatment to precipitation scavenging alone leads to a 25% reduction in global mean cloud droplet number concentration and a 10% reduction in liquid water path. However, the advanced treatments of aerosols that lead to better spatial distributions in ACMEv1, along with the recent advances in other components of the model and the required model tuning for top-of-the-atmosphere energy balance, give a very strong aerosol shortwave indirect forcing (-3.6 W m^-2). We have explored why the ACMEv1 model gave such a strong indirect forcing, and we have managed to reduce it to a reasonable range (less than 2 W m^-2) by further tuning of aerosol-cloud parameterizations. We will discuss these new features of aerosols and their impact on aerosol radiative forcing, as well as further model developments needed for a more accurate representation of aerosol-cloud interactions.