Journal Article

Aircraft Observations Reveal the Relationship Between Cumulus Entrainment Rate and Aerosol Loading

Zhu, Lei, Lu, Chunsong, Wu, Xianghua, Li, Junjun, Wang, Yuan, Wu, Shang, He, Xin, Chen, Jingyi

2024

Overview

The influence of entrainment, a key process characterized by the entrainment rate in cumulus parameterization, on aerosol‐cloud interactions has been widely recognized. However, despite qualitative links established between entrainment and aerosol loading, a quantitative relationship based on observational evidence remains elusive. This study utilizes aircraft observations of cumulus clouds during two field campaigns to determine the quantitative relationship between entrainment rate and aerosol loading. In both campaigns, the entrainment rate is negatively correlated with aerosol loading. It is speculated that increased aerosol loading enhances cloud edge droplet evaporation, which leads to increased buoyancy and vertical velocity within the cloud, thereby reducing the entrainment rate. Further analysis shows that the response of entrainment rate to aerosol perturbations is more significant in smaller cumulus clouds with weak buoyancy and less pronounced under opposite conditions. These findings shed new light on improving the description of aerosol‐cloud interactions in cumulus parameterizations. Plain Language Summary Clouds play a crucial role in regulating Earth's climate, and understanding how they form and evolve is important for accurate weather and climate predictions. One key process affecting cloud development is entrainment, where drier air from outside the cloud mixes into the cloud, influencing its growth. Scientists know that aerosols can impact entrainment, but the exact relationship hasn't been clear. This study uses observations from research aircraft flown through cumulus clouds to quantify the relationship between entrainment and aerosol. The results show that aerosol concentration is negatively correlated with entrainment rate, meaning less dry air mixes into the cloud. We speculate that this happens because the increased aerosols enhance the evaporation of cloud droplets at the cloud edges, making the cloud more buoyant and less likely to mix with the surrounding air. Interestingly, this decline in entrainment rate to aerosol is stronger in smaller and weaker clouds. These findings provide valuable insights into how aerosols influence cloud development and can help improve the representation of aerosol‐cloud interactions in climate models, leading to more accurate climate projections. Key Points The relationship between cumulus entrainment rate and aerosol loading is quantified based on observation for the first time The plausible physical mechanism linking entrainment rate to aerosol loading has been elucidated A steeper decline in entrainment rate with increasing aerosol loading is observed in small and weakly buoyant cumulus clouds

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Aerosol concentrations / Aerosols / aerosol‐cloud interactions / Air / Air entrainment / Aircraft / Aircraft observations / Buoyancy / Climate models / Climate prediction / Cloud development / Cloud droplets / Cloud parameterization / Clouds / Cumulus clouds / Droplets / Dry air / Entrainment / entrainment rate / Evaporation / Evaporation rate / Mixtures / Parameterization / Qualitative analysis / Research aircraft / Vertical loads / Vertical velocities