Aerosol indirect effect in northeastern Atlantic estimated from geostationary satellite cloud retrievals.

Due to the wide range of spatiotemporal scales of aerosol-cloud interactions, large uncertainties remain in the estimated aerosol indirect effect and the associated radiative forcing. In this study, we used Meteosat-11 cloud retrievals with a 3-km spatial resolution and a half-hourly temporal resolution from 2018 to 2021 in July to quantify the responses of cloud liquid water path (LWP), cloud albedo, and cloud fraction (CF) due to perturbations in cloud droplet number concentration (Nd) for low-liquid clouds over the northeast Atlantic region (33-43°N, 23-33°W).

We compared the cloud susceptibilities estimated using the half-hourly data with the ones using the 4-yr mean relationships. Our results indicate that cloud adjustments are larger in the half-hourly data than the climate-mean perspective for all three cloud properties. The LWP susceptibilities are between -0.5 to -0.65 using the half-hourly data, while the mean LWP decreases more slowly with Nd, and the susceptibilities are between -0.1 to -0.34. Similarly, cloud albedo and CF increase with Nd in the half-hourly data with susceptibilities between 0.05-0.15 and 0.5-0.75, respectively. In contrast, the mean albedo and CF are almost constant or increase slightly for different Nd bins.

Cloud susceptibilities exhibit large diurnal variations and a consistent diurnal trend with the retrieved cloud properties over this region. For example, the domain mean LWP is largest in the morning, decreases to its minimum at ~13:30 LT, and increases again in the evening. The diurnal variation of the LWP susceptibility shows a similar "U-shaped" pattern with a minimum value of -0.60 at ~13:30 LT and larger values of -0.45 and -0.25 at ~8:00 LT and 18:00 LT, respectively. Correspondingly, the albedo susceptibility is largest (+0.13) in the morning, reaches a minimum of zero at 13:30 LT, and increases to 0.05 before sunset. Therefore, the cloud susceptibility estimated from a polar-orbiting satellite with the overpass time at ~13:30 LT is at the diurnal minimum and might under-estimate cloud LWP and albedo responses to Nd perturbations.

Acknowledgement: This work is supported by the ASR Program for the Office of Science of the U.S. DOE. This work was performed under the auspices of the U.S. DOE by LLNL under contract DE-AC52-07NA27344. LLNL-ABS-821839