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Aerosol–cloud interactions are explored using 1 km simulations of a case study ofpredominantly closed-cell SE Pacific stratocumulus clouds. The simulationsinclude realistic meteorology along with newly implemented cloud microphysicsand sub-grid cloud schemes. The model was critically assessed againstobservations of liquid water path (LWP), broadband fluxes, cloud fraction(fc), droplet number concentrations (Nd), thermodynamic profiles, and radarreflectivities. Aerosol loading sensitivity tests showed that at low aerosol loadings,changes to aerosol affected shortwave fluxes equally through changes to cloudmacrophysical characteristics (LWP, fc) and cloud albedo changesdue solely to Nd changes. However, at high aerosol loadings, only the Nd albedochange was important. Evidence was also provided to show that a treatment ofsub-grid clouds is as important as order of magnitude changes in aerosolloading for the accurate simulation of stratocumulus at this grid resolution. Overall, the control model demonstrated a credible ability to reproduceobservations, suggesting that many of the important physical processes foraccurately simulating these clouds are represented within the model andgiving some confidence in the predictions of the model concerningstratocumulus and the impact of aerosol. For example, the control run wasable to reproduce the shape and magnitude of the observed diurnal cycle ofdomain mean LWP to within  ∼  10 g m−2 for the nighttime, butwith an overestimate for the daytime of up to 30 g m−2. Thelatter was attributed to the uniform aerosol fields imposed on the model,which meant that the model failed to include the low-Nd modethat was observed further offshore, preventing the LWP removal throughprecipitation that likely occurred in reality. The boundary layer was too lowby around 260 m, which was attributed to the driving global model analysis.The shapes and sizes of the observed bands of clouds and open-cell-likeregions of low areal cloud cover were qualitatively captured. The daytimefc frequency distribution was reproduced to within Δfc = 0.04 for fc >  ∼ 0.7 as was the domain meannighttime fc (at a single time) to withinΔfc = 0.02. Frequency distributions of shortwavetop-of-the-atmosphere (TOA) fluxes from the satellite were well represented bythe model, with only a slight underestimate of the mean by 15 %; this wasattributed to near–shore aerosol concentrations that were too low for theparticular times of the satellite overpasses. TOA long-wave flux distributionswere close to those from the satellite with agreement of the mean value to within0.4 %. From comparisons of Nd distributions to those fromthe satellite, it was found that the Nd mode from the model agreedwith the higher of the two observed modes to within  ∼  15 %

atmospheric chemistry and physics

The relative importance of macrophysical and cloud albedo changes for aerosol-induced radiative effects in closed-cell stratocumulus: insight from the modelling of a case study