Microphysics regimes due to haze-cloud interactions: cloud oscillation and cloud collapse

Abstract. It is known that aqueous haze particles can be activated to cloud droplets in a supersaturated environment. However, haze-cloud interactions have not been fully explored because, among other things, haze particles are not represented in most cloud-resolving models. Here, we conduct a series of large-eddy simulations of a cloud in a convection chamber using a haze-capable Eulerian-based bin microphysics scheme to explore haze-cloud interactions over a wide range of aerosol injection rates. Results show that at low aerosol injection rates (i.e., clean conditions), the cloud exists in a slow microphysics regime where cloud response is slow compared to the environmental change and droplet deactivation is negligible. At moderate aerosol injection rates (i.e., polluted conditions), the cloud is in a fast microphysics regime where cloud response is fast compared to the environmental change and haze-cloud interactions are important. The increase in liquid water mixing ratio with aerosol injection rate agrees well with the scaling law predicted by a previous theoretical study of these two microphysics regimes. More interestingly, two other microphysics regimes are observed at high aerosol injection rates: cloud oscillation and cloud collapse. Cloud oscillation occurs as a result of competition between haze and cloud droplets that lead to synchronized droplet activation/deactivation, while cloud collapse happens under weaker forcing of supersaturation where the chamber transfers cloud droplet to haze particles efficiently, leading to a significant decrease (collapse) of cloud droplet number concentration. Results from a box model using a particle-based microphysics approach show similar transitions across microphysics regimes – from slow microphysics, to fast microphysics, and then to cloud oscillation – confirming that cloud oscillation arises from complex interactions between haze and cloud droplets in a turbulent cloud. One special case of cloud collapse leading to a haze-only regime, occurs at extremely high aerosol injection rates, where the sedimentation of haze particles is balanced by the aerosol injection rate, without cloud droplet activation contributing substantially. Our results suggest that haze particles and their interactions with the cloud should be considered especially in polluted conditions, like fog or clouds close to the source of intense natural and anthropogenic aerosol emissions, or in highly dissipated clouds when droplet deactivation is important.