Ccn Activation in Homogeneous Isotropic Turbulence: Response to Particle Characteristics and Environmental Conditions

Abstract

Direct Numerical Simulation (DNS) of a turbulent cloud parcel is presented where particles evolve in response to the local values in supersaturation (s) led by turbulent fluctuations. A pseudo-spectral DNS is modified to incorporate Cloud Condensation Nuclei (CCN) microphysics and a droplet growth equation based on single-parameter hygroscopicity by -Köhler theory that works for the whole range of warm cloud particles, from un-activated haze to activated cloud droplets, that grow by condensation. The Lagrangian microphysics applied ensures that each particle experiences a supersaturation (fluctuations added to the mean) corresponding to its near-particle gas phase surroundings, which differs from the uniform parcel view where an average value of supersaturation,   is applied to every particle in the domain. We show that the activation of CCN and its subsequent evolution into a cloud droplet is well simulated by the modified growth equation in the DNS. In addition to this, a non-turbulent Lagrangian parcel model with similar mean thermodynamics and CCN properties is applied to distinguish the role of turbulence. It is shown that the turbulent fluctuations impact the activation of CCN and resulting cloud droplet number concentration. The cloud microphysical properties are then analysed in a series of DNS experiments with contrasting thermodynamic and CCN properties. The DNS results agree with our general understanding of the aerosol activation process and provide insights on the impact of the numerous variables such as turbulence, size distribution characteristics, chemical composition of CCN, magnitude of updraft, as well as the subset of their various combinations, that can affect the activation and droplet growth processes. It is argued that incorporating turbulent fluctuations in simulating the CCN activation is well reasoned and a constructive way forward.

Keywords: Turbulent cloud parcel, DNS, Supersaturation fluctuations, CCN activation