Enhancements in Cloud Condensation Nuclei Activity From Turbulent Fluctuations in Supersaturation

Abstract

The effect of aerosols on the properties of clouds is a large source of uncertainty in predictions of weather and climate. These aerosol‐cloud interactions depend critically on the ability of aerosol particles to form cloud droplets. A challenge in modeling aerosol‐cloud interactions is the representation of interactions between turbulence and cloud microphysics. Turbulent mixing leads to small‐scale fluctuations in water vapor and temperature that are unresolved in large‐scale atmospheric models. To quantify the impact of turbulent fluctuations on cloud condensation nuclei (CCN) activation, we used a high‐resolution Large Eddy Simulation of a convective cloud chamber to drive particle‐based cloud microphysics simulations. We show small‐scale fluctuations strongly impact CCN activity. Once activated, the relatively long timescales of evaporation compared to fluctuations causes droplets to persist in subsaturated regions, which further increases droplet concentrations.

Relative humidity measurements of a thin-film humidity sensor in condensing conditions in the temperature range from −40 °C to 5 °C

“…Although WVP exceeding the saturation vapour pressure (i.e. 100% RH) would normally result in condensation, a metastable state known as supersaturation frequently occurs in the free atmosphere due to a lack of condensation nuclei [10][11][12]. Furthermore, the phenomenon of supercooling, where water vapour condenses into liquid water even at temperatures below 0 degrees Celsius, is also common in the upper-air atmosphere [13][14][15].…”