Cloud-aerosol interactions in fields of organizing shallow cumulus

Despite decades of studies, cloud-aerosol interactions remain a notoriously difficult topic. An especially important regime are trade-wind cumulus clouds, which often barely precipitate. Many of the postulated cloud-aerosol interactions involve precipitation to limit for instance the cloud size or the cloud life time. However, if the precipitation exceeds a certain threshold, it will feedback on the cloud field through cold pools and mesoscale organization. Such mesoscale responses have mostly been ignored so far in the discussion of aerosol indirect effects.

In this study, we use high resolution (25m in all directions), large domain (50km width), long duration (48hr) Large-Eddy Simulations to work towards quantification of the aerosol indirect effects through the lifetime, size, and depth of cumulus clouds. Six idealized simulations are run with the Rain In Cumulus over the Ocean (RICO) intercomparison case as the baseline, and cloud droplet concentration and cloud layer relative humidity as parameters. Using an in-house developed cloud tracking algorithm, we are able to directly calculate the cloud life time and cloud size distributions. As it turns out, the occurrence and timing of the mesoscale organization plays a crucial role in assessing the aerosol indirect effects for these fields of cumulus clouds. When comparing the different simulations, we find that all cases transition from non-precipitating, randomly distributed, small shallow cumulus to a well-organized field of precipitating clouds, However, the pathway towards organization differs significantly between the different cases, with the moist, pristine simulations organizing much faster than the dry, polluted simulations. An accurate assessment of the changes in cloud lifetime and size can only be made when distinguishing between the organized and the randomly distributed situation.