Reconciling Satellite–Model Discrepancies in Aerosol–Cloud Interactions Using Near-LES Simulations of Marine Boundary Layer Clouds

Abstract. Aerosol–cloud interactions (ACI) remain the largest source of uncertainty in estimates of anthropogenic radiative forcing, largely due to inconsistent cloud liquid water path (LWP) responses to aerosol perturbations between observations and models. To reconcile this discrepancy, we conducted a series of simulations at near large-eddy scale (~200 m) driven by realistic meteorology over the Eastern North Atlantic, and evaluated LWP susceptibility, precipitation processes, and boundary layer thermodynamics using satellite and ground-based observations from the Atmospheric Radiation Measurement program.

Simulated LWP responses show strong dependence on cloud state. Non-precipitating thin clouds exhibit a modest LWP decrease (mean susceptibility = −0.13) due to enhanced turbulent mixing and evaporation. In contrast, non-precipitating thick clouds show the largest model–observation mismatch, with simulated LWP susceptibilities significantly more positive than observed (+0.32 vs. −0.69). This discrepancy stems from excessive precipitation linked to underestimated entrainment, overactive accretion, and overly broad drop size distributions in polluted clouds. While our high-resolution setup avoids the excessive drizzling common in coarse models and captures key regime transitions, these biases persist—highlighting the need for improved representation of cloud-top processes, precipitation, and aerosol effects in numerical models, which cannot be fully resolved by increasing model resolution alone.

Furthermore, misrepresented moisture inversions in reanalysis, which is used for the model initial and boundary conditions, introduce a moist bias in cloud-top relative humidity, amplifying positive LWP susceptibility. Our results also suggest that large negative cloud droplet number– LWP relationships in observations may reflect internal cloud processes rather than true ACI effects.