Impact of cloud-base turbulence on CCN activation: Single-size CCN

This paper examines the impact of cloud-base turbulence on activation of cloud condensation nuclei (CCN). Following our previous studies, we contrast activation within a nonturbulent adiabatic parcel and an adiabatic parcel filled with turbulence. The latter is simulated by applying a forced implicit large-eddy simulation within a triply periodic computational domain of 64(3) m(3). We consider two monodisperse CCN. Small CCN have a dry radius of 0.01 mu m and a corresponding activation (critical) radius and critical supersaturation of 0.6 mu m and 1.3%, respectively. Large CCN have a dry radius of 0.2 mu m and feature activation radius of 5.4 mu m and critical supersaturation 0.15%. CCN are assumed in 200-cm(-3) concentration in all cases. Mean cloud-base updraft velocities of 0.33, 1, and 3 m s(-1) are considered. In the nonturbulent parcel, all CCN are activated and lead to a monodisperse droplet size distribution above the cloud base, with practically the same droplet size in all simulations. In contrast, turbulence can lead to activation of only a fraction of all CCN with a nonzero spectral width above the cloud base, of the order of 1 mu m, especially in the case of small CCN and weak mean cloud-base ascent. We compare our results to studies of the turbulent single-size CCN activation in the Pi chamber. Sensitivity simulations that apply a smaller turbulence intensity, smaller computational domain, and modified initial conditions document the impact of specific modeling assumptions. The simulations call for a more realistic high-resolution modeling of turbulent cloud-base activation.