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1. Probing the Response of Tropical Deep Convection to Aerosol Perturbations Using Idealized Cloud-Resolving Simulations with Parameterized Large-Scale Dynamics

   https://doi.org/10.1175/JAS-D-18-0351.1  

   Anber, Usama M.; Wang, Shuguang; Gentine, Pierre; Jensen, Michael P. (September 2019, Journal of the Atmospheric Sciences)

   Abstract A framework is introduced to investigate the indirect effect of aerosol loading on tropical deep convection using three-dimensional limited-domain idealized cloud-system-resolving model simulations coupled with large-scale dynamics over fixed sea surface temperature. The large-scale circulation is parameterized using the spectral weak temperature gradient (WTG) approximation that utilizes the dominant balance between adiabatic cooling and diabatic heating in the tropics. The aerosol loading effect is examined by varying the number of cloud condensation nuclei (CCN) available to form cloud droplets in the two-moment bulk microphysics scheme over a wide range of environments from 30 to 5000 cm−3. The radiative heating is held at a constant prescribed rate in order to isolate the microphysical effects. Analyses are performed over the period after equilibrium is achieved between convection and the large-scale environment. Mean precipitation is found to decrease modestly and monotonically when the aerosol number concentration increases as convection gets weaker, despite the increase in cloud liquid water in the warm-rain region and ice crystals aloft. This reduction is traced down to the reduction in surface enthalpy fluxes as an energy source to the atmospheric column induced by the coupling of the large-scale motion, though the gross moist stability remains constant. Increasing CCN concentration leads to 1) a cooler free troposphere because of a reduction in the diabatic heating and 2) a warmer boundary layer because of suppressed evaporative cooling. This dipole temperature structure is associated with anomalously descending large-scale vertical motion above the boundary layer and ascending motion at lower levels. Sensitivity tests suggest that changes in convection and mean precipitation are unlikely to be caused by the impact of aerosols on cloud droplets and microphysical properties but rather by accounting for the feedback from convective adjustment with the large-scale dynamics. Furthermore, a simple scaling argument is derived based on the vertically integrated moist static energy budget, which enables estimation of changes in precipitation given known changes in surfaces enthalpy fluxes and the constant gross moist stability. The impact on cloud hydrometeors and microphysical properties is also examined, and it is consistent with the macrophysical picture. 

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2. Ion-induced cloud modulation through new particle formation and runaway cloud condensation nuclei production

   https://doi.org/10.1093/oxfclm/kgae018  

   Chandrakar, Kamal Kant; Cantrell, Will; Shaw, Raymond A (September 2024, Oxford Open Climate Change)

   Abstract Cloud formation in the Pi Convection–Cloud Chamber is achieved via ionization in humid conditions, without the injection of aerosol particles to serve as cloud condensation nuclei (CCN). Abundant ions, turbulence, and supersaturated water vapor combine to produce new particles, which grow to become CCN sized and eventually are activated to form clouds. Coupling between the new particle formation and cloud droplets causes predator-prey type oscillations in aerosol and droplet concentrations under turbulent conditions. Leading terms are identified in the budgets for Aitken and accumulation mode aerosols and for cloud droplets. The cloud coupling is proposed to be a result of cloud-induced runaway CCN production through aerosol scavenging. The experiments suggest potential applications to marine cloud brightening, in which ions rather than sea-salt aerosols are generated. 

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3. Microphysics regimes due to haze–cloud interactions: cloud oscillation and cloud collapse

   https://doi.org/10.5194/acp-25-3785-2025  

   Yang, Fan; Sadi, Hamed Fahandezh; Shaw, Raymond A; Hoffmann, Fabian; Hou, Pei; Wang, Aaron; Ovchinnikov, Mikhail (January 2025, Atmospheric Chemistry and Physics)

   Abstract. It is known that aqueous haze particles can be activated into cloud droplets in a supersaturated environment. However, haze–cloud interactions have not been fully explored, partly because haze particles are not represented in most cloud-resolving models. Here, we conduct a series of large-eddy simulations (LESs) 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 the cloud is in a slow microphysics regime at low aerosol injection rates, where the cloud responds slowly to an environmental change and droplet deactivation is negligible. The cloud is in a fast microphysics regime at moderate aerosol injection rates, where the cloud responds quickly to an environmental change and haze–cloud interactions are important. More interestingly, two more microphysics regimes are observed at high aerosol injection rates due to haze–cloud interactions. Cloud oscillation is driven by the oscillation of the mean supersaturation around the critical supersaturation of aerosol due to haze–cloud interactions. Cloud collapse happens under weaker forcing of supersaturation where the chamber transfers cloud droplets to haze particles efficiently, leading to a significant decrease (collapse) in cloud droplet number concentration. One special case of cloud collapse is the haze-only regime. It occurs at extremely high aerosol injection rates, where droplet activation is inhibited, and the sedimentation of haze particles is balanced by the aerosol injection rate. Our results suggest that haze particles and their interactions with cloud droplets should be considered, especially in polluted conditions. 

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4. Fast and slow microphysics regimes in a minimalist model of cloudy Rayleigh-Bénard convection

   https://doi.org/10.1103/PhysRevResearch.5.043018  

   Shaw, Raymond A; Thomas, Subin; Prabhakaran, Prasanth; Cantrell, Will; Ovchinnikov, Mikhail; Yang, Fan (October 2023, Physical Review Research)

   A minimalist model of microphysical properties in cloudy Rayleigh-Bénard convection is developed based on mass and number balances for cloud droplets growing by vapor condensation. The model is relevant to a turbulent mixed-layer in which a steady forcing of supersaturation can be defined, e.g., a model of the cloudy boundary layer or a convection-cloud chamber. The model assumes steady injection of aerosol particles that are activated to form cloud droplets, and the removal of cloud droplets through sedimentation. Simplifying assumptions include the consideration of mean properties in steady state, neglect of coalescence growth, and no detailed representation of the droplet size distribution. Closed-form expressions for cloud droplet radius, number concentration, and liquid water content are derived. Limits of fast and slow microphysics, compared to the turbulent mixing time scale, are explored, and resulting expressions for the scaling of microphysical properties in fast and slow regimes are obtained. Scaling of microphysics with layer thickness is also explored, suggesting that liquid water content and cloud droplet number concentration increase, and mean droplet radius decreases with increasing layer thickness. Finally, the analytical model is shown to compare favorably to solutions of the fully-coupled set of governing ordinary differential equations that describe the system, and the predicted power law for liquid water mixing ratio versus droplet activation rate is observed to be consistent with measurements from the Pi convection-cloud chamber. 

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5. The Effect of Turbulence, Gravity, and Noncontinuum Hydrodynamic Interactions on the Drop Size Distribution in Clouds

   https://doi.org/10.1175/JAS-D-24-0278.1  

   Dhanasekaran, Johnson; Koch, Donald L (August 2025, Journal of the Atmospheric Sciences)

   Abstract The evolution of micrometer-sized droplets in clouds is studied with focus on the “size-gap” regime of 15–40-μm radii, where condensation and differential sedimentation are least effective in promoting growth. This bottleneck leads to inaccurate growth models, and turbulence can potentially rectify disagreement with in situ cloud measurements. The role of turbulent collisions, mixing of droplets, and water vapor fluctuations in crossing the size gap has been analyzed in detail. Collisions driven by the coupled effects of turbulent shear and differential sedimentation are shown to grow drizzle sized droplets. Growth is also promoted by turbulence-induced water vapor fluctuations, which maintain polydispersity during the initial-condensation-driven growth and facilitate subsequent growth by differential-sedimentation-driven coalescence. The collision rate of droplets is strongly influenced by noncontinuum hydrodynamics, and so the size evolution beyond the condensation regime is found to be very sensitive to the mean-free path of air. Turbulence-induced inertial clustering leads to a moderate enhancement in the growth rate, but the intermittency of the turbulent shear rate does not change the coalescence rate significantly. The coupled influence of all these phenomena is evaluated by evolving a large number of droplets within an adiabatically rising parcel of air using a Monte Carlo scheme that captures turbulent intermittency and mixing. Significance StatementThis study is directed toward improving descriptions of the microphysical determinants of the time for rain formation in clouds. Existing models predict significantly longer times than the tens of minutes observed in warm clouds. There is a growing body of evidence that turbulence plays a key role in resolving this discrepancy. We incorporate accurate turbulent collision dynamics and assess the interplay of the various underlying physical factors facilitating growth to rain-sized droplets. Our study, in addition to providing important insight into cloud microphysics, will pave the path to the next generation of large-scale rain cloud evolution studies. 

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