Impacts of giant cloud condensation nuclei on precipitation formation in marine low clouds

Abstract

Precipitation in low clouds strongly affects cloud responses to aerosol perturbations, thereby impacting aerosol radiative forcing of climate, a significant but indeterminate contribution to uncertainty in future warming. The onset of drizzle formation in marine low clouds may be influenced by the presence of coarse-mode sea salt aerosols with dry diameters exceeding 1 um. These particles can, under some circumstances, act as giant cloud condensation nuclei (GCCN) that benefit from rapid condensational growth to a size that initiates collision-coalescence. Aircraft-based in-situ observations of cloud microphysical properties and aerosol size distributions from the Aerosol-Cloud-Meteorology Interactions Over the Western Atlantic Experiment (ACTIVATE) are analyzed from January to June 2022. ACTIVATE focused on making detailed in situ microphysical measurements of aerosols and clouds using a low-flying research aircraft. The western Atlantic region is ideal for this study due to its location in the midlatitudes, where diverse cloud types can be observed under varying meteorological conditions. Unlike many aerosol-cloud interaction studies focused on subtropical regions, ACTIVATE’s emphasis on the midlatitudes provides a unique opportunity to examine aerosol impacts on precipitation formation in an area characterized by complex cloud structures and dynamic atmospheric processes. Observations from a cloud aerosol spectrometer (CAS) and a cloud droplet probe (CDP) in clear air from just below the cloud base are used to quantify size distributions of haze droplets. Clear-sky conditions for accurate interpretation of the size distributions are identified by strictly filtering for liquid water content below 0.0025 g/m³ to remove contamination from cloud droplets and total precipitation number concentration below 100 particles per cubic meter (#/m³), as measured by a two-dimensional stereo probe. These thresholds ensure that the size distributions measure haze droplets without cloud or drizzle droplet contamination. Haze droplet size distributions reveal modest correlations with near surface-level wind speeds when averaged together into wind speed bins, a result consistent with other recent studies. This is seen by a strong correlation coefficient between wind speed and total concentration of R =0.90 for the CAS and R =0.84 for the CDP. And consequently, I conclude, as near surface wind speed increases, the production of giant cloud condensation nuclei increases. Observed GCCN distributions and cloud thicknesses are used to drive simulations with an explicit microphysics parcel model, exploring the conditions under which the observed range of GCCN induces a first-order impact on precipitation rates. Analysis of droplet size distributions using a 1-dimensional kinematic super-droplet cloud model (PySDM) demonstrates that even the addition of a small number of GCCN accelerates the onset of drizzle and increases drizzle rate in the marine boundary layer. This effect was analyzed under conditions with liquid water paths similar to those in the thicker parts of low cloud fields over the Western Atlantic region. Similarly, analysis with observationally derived rainwater content as a function of total concentration and liquid water content for high and low GCCN concentrations reveals an increased ratio of condensate in precipitation drops to that in cloud drops for flights with higher measured concentrations of GCCN. These results suggest that GCCN can meaningfully influence precipitation in marine low clouds.