Revisiting Aerosol–Cloud Interactions From Weekly Cycles.

Weekly cycles (WCs) in cloud properties have been reported and linked to aerosol effects. Yet the extent to which human activities contribute to their occurrence remains unclear. Here, we revisit aerosol–cloud interactions from the WCs over central Europe using long‐term satellite and reanalysis data. Significant WCs in aerosol and cloud droplet number concentration (Nd) are detected with minima/maxima on Monday/Friday, indicating a clear signal of the Twomey effect. Notably, Nd–to–aerosol sensitivity from WCs is found to decrease at larger aerosol concentrations, confirming the nonlinear behavior of the aerosol–Nd relation (in log–log space) reported previously, but from a distinct perspective. Nevertheless, no discernible WCs in liquid water path are found. The pronounced WCs in cloud cover are demonstrated to be driven by natural variability. Our results indicate that the WCs offer a useful pathway for investigating the Twomey effect, but are not as effective for detecting cloud adjustments. Plain Language Summary: Aerosol–cloud interactions are the largest uncertainty in the anthropogenic forcing of climate. Specifically, an increase in aerosols increases cloud droplet number concentration (the Twomey effect), which further changes liquid water path and cloud cover (cloud adjustments), ultimately alters radiations. Weekly cycles would be a useful tool for such study, assuming no 7‐day periodicity in meteorological dynamics. In this study, we revisit aerosol–cloud interactions from weekly cycles using long‐term satellite observations and reanalysis data. The analysis is restricted to central Europe—a region with strong weekly cycles in anthropogenic emissions. We find significant weekly cycles in aerosol and cloud droplet number concentration with minima/maxima on Monday/Friday. Importantly, the sensitivity of cloud droplet number concentration to aerosol is found to decrease in polluted conditions, confirming the behavior of the nonlinear cloud response to aerosol as reported previously. It is demonstrated that the weekly cycle in liquid water path is negligible; though a pronounced cycle in cloud cover detected, it is predominately caused by natural variability. Hence, caution is warranted when attributing observed weekly cycle in cloud cover to aerosol effects. Conclusively, weekly cycles are useful for detecting the Twomey effect but less effective for cloud adjustments. Key Points: The analysis of weekly cycles reveals a reduced sensitivity of cloud droplet number concentration to aerosol at larger aerosol loadingThe strong weekly cycle in cloud cover is mainly a result of natural variability rather than being attributed to aerosol effectsWeekly cycles offer a useful pathway to investigating the Twomey effect, but are not as effective for detecting cloud adjustments [ABSTRACT FROM AUTHOR]