Towards an improved understanding of the impact of clouds and precipitation on the representation of aerosols over the Boreal Forest in GCMs

Abstract. General circulation models (GCMs) face uncertainties in estimating Earth's radiative budget due to aerosol-cloud interactions (ACI). Accurate aerosol number size distributions are crucial for improving ACI representation in GCMs, requiring precise modelling of aerosol source and sink processes throughout their lifetime. This study employs a Lagrangian trajectory framework to analyse how clouds and precipitation influence aerosol lifecycles during transport in the boreal forest. A comparison of two GCMs, the United Kingdom Earth System Model (UKESM1) and ECHAM6.3-HAM2.3-MOZ1.0 with the SALSA2.0 aerosol module (ECHAM-SALSA), is conducted. An evaluation against in-situ observations and reanalysis-based trajectories is performed. Results show that overall aerosol-precipitation trends are similar between GCMs and observations. However, seasonal differences emerge: in summer, UKESM1 exhibits more efficient aerosol removal via precipitation than ECHAM-SALSA and observations, whereas in winter, the opposite is observed. These were found to coincide with differences in key variables controlling aerosol activation, such as sub-grid scale updraughts and number size distributions. For example, in winter the removal of the total aerosol mass in ECHAM-SALSA was stronger compared to UKESM1, coinciding with higher activated fractions during airmass transport, which, on the other hand, were likely due to the larger sub-grid scale updraughts in ECHAM-SALSA. For both GCMs, investigation of aqueous-phase chemical processing along the trajectories showed clear increase of SO₄ mass for cloud-processed air masses when compared to clear sky conditions, in-line with the observations. As expected, based on the model parametrizations, these increases in SO₄ were mostly distributed to the accumulation mode aerosols.