Dual observational constraint of Arctic cloud-forming aerosols reveals structural error in UKESM

Clouds play a significant role in determining surface radiation budgets, with associated uncertainties largely stemming from aerosol-cloud interactions. Improving the representation of cloud-forming aerosols in Global Climate Models (GCMs) is essential for reducing these uncertainties, particularly in climatically significant regions such as the Arctic, where aerosol populations are expected to change in the future as Arctic Amplification processes unfold.

This study utilises observations of accumulation-mode (cloud-forming) aerosol size and number terms to evaluate and constrain a GCM two-dimensionally. Accumulation-mode mean diameter and number concentration were obtained from fitting log distribution functions to size-distribution observations of five ground-based Arctic sites. Size and number terms were used simultaneously to evaluate and constrain the UK Earth System Model (UKESM), coupled with UKCA (United Kingdom Chemistry and Aerosols) and the aerosol microphysical scheme GLOMAP (Global Model of Aerosol Processes), which demonstrated a consistent seasonal bias in simulated accumulation-mode aerosols across Arctic sites. To investigate the drivers of these biases, a Generalised Additive Model was applied to assess the relative importance of parameters within a Perturbed Parameter Ensemble. Here, we present the dominant parameters controlling simulated accumulation-mode size and number terms, and their spatial and temporal variation across the Arctic. Using a dual-constraint method, we identify optimal parameter ranges that yield observationally representative size distributions at each Arctic site.

 

Through inter-seasonal and -spatial application of constrained parameter ranges, we identify clear inconsistencies with little to no shared parameter space between winter-spring and summer-autumn months across all Arctic sites. This work identifies areas for future model development to further constrain physical processes and natural and anthropogenic emissions in the Arctic to remedy biases, and identifies a structural error in the representation of accumulation-mode aerosols in UKESM-UKCA-GLOMAP.