Atmospheric aerosols play an important role in modulating properties of warm (ice-free) clouds. For the same liquid water path (LWP), an increase of aerosol concentration leads to the decrease of the cloud droplet size. This impacts cloud radiative properties and cloud capability to develop precipitation. The former is typically referred to as the Twomey or first aerosol indirect effect. However, recent satellite observation in highly polluted regions near the Indian subcontinent suggest a reversal of the Twomey effect, with extremely polluted clouds showing larger cloud droplets (larger droplet effective radii to be exact) than clouds that develop in less polluted environments. This has been referred to as the reversed Twomey effect. According to satellite observations, the reversal is seen in shallow clouds with relatively low liquid water path, below 100 g m². One possibility suggested in the literature is that competition for the water vapor during cloud droplet formation (i.e., activation of cloud condensation nuclei, CCN) leads to a situation where lower droplet concentrations are possible in higher CCN concentrations. This hypothesis is tested in series of numerical simulations using an adiabatic parcel model, two-dimensional kinematic (prescribed flow) model, and a three-dimensional cloud model applying Lagrangian particle-based microphysics. Simulated droplet spectra and the effective radius dependence of on LWP are compared for simulations with CCN spectra representing observed polluted and extremely polluted conditions. Overall, all simulations fail to show the reversed Twomey effect. Possible ways to reconcile satellite observations and model simulations will be discussed.

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