Aerosol–cloud interactions in Arctic mixed-phase clouds under dust size perturbations in a regional chemical weather modeling system (WRF-Chem)

This study investigates aerosol–cloud–radiation interactions over the Arctic using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) together with an aerosol-aware microphysics scheme. In the atmosphere, aerosols directly affect the radiation budget by absorbing and scattering solar radiation (Jin et al., 2014), and indirectly by modifying cloud albedo and lifetime (Li et al., 2018). Aerosols also act as nuclei for heterogeneous condensation and promote cloud droplet formation; therefore, changes in aerosol number concentration are expected to alter cloud droplet number and size distributions and, in turn, cloud properties (Ramanathan et al., 2001). Also, Aerosol size distribution can play an important role under high aerosol loadings, whereas aerosol composition tends to be much less important, except perhaps under very polluted conditions and low updraught velocities (McFiggans et al., 2006).

More recently, dust sources in the northern high latitudes have received increased attention (Meinander et al., 2022). high-latitude dust is defined as dust emitted from regions north of 50°N (Bullard et al., 2016). Observations and modeling studies suggest that high-latitude dust can act as an efficient ice-nucleating particle (INP), promoting the conversion of cloud droplets to ice crystals. This can strongly reduce the cloud’s liquid water content, lower its albedo, and make the underlying surface more exposed.

Aerosol size distribution strongly controls how many particles can activate as cloud condensation nuclei (CCN) and also affects aerosol optical properties. Therefore, changing particle size, even if all other model settings are kept the same, can change the CCN-active particle population. In mixed-phase clouds, ice formation depends on ice-nucleating particles, and mineral dust is an important source of these particles.  To isolate the role of particle size from confounding influences, we conduct one control simulation and a suite of sensitivity experiments in which dust mass is redistributed toward finer versus coarser size bins within a sectional (size-bin) aerosol representation, while keeping the remaining model configuration fixed. Preliminary analyses suggest that changing particle size alone leads to only small changes in cloud properties and radiation, likely because the results are mainly controlled by meteorology (e.g., vertical motion, moisture, and stability) or because the microphysics scheme does not strongly transfer aerosol changes into cloud optical properties and radiative fluxes. Accordingly, we advance a more targeted experimental methodology that explicitly separates CCN and INP pathways. The first step applies direct, controlled perturbations to CCN-relevant aerosol number to generate clean-to-polluted contrasts; the second step independently varies dust INP activity to isolate ice-nucleation pathways; and the framework is configured to distinguish direct radiative effects from indirect (microphysical) effects.

Finally, diagnostics are performed in a regime-based case by stratifying clouds by thermodynamic phase (liquid-dominated versus mixed-phase clouds) and by large-scale forcing (regions of ascent and moisture-flux convergence versus moisture divergence and subsidence). This approach is intended to identify conditions under which aerosol sensitivity is expected to be maximized and to facilitate evaluation against observational and satellite-derived products.