Predicting global atmospheric ice nuclei distributions and their impacts on climate

TLDR

Knowledge of cloud and precipitation formation remains incomplete, yet global precipitation is largely produced by clouds containing ice. The study aims to relate concentrations of ice nuclei active in mixed‑phase clouds to temperature and particle number concentrations. The authors combine 14 years of field observations from diverse global sites to establish this relationship. The new relationship reduces unexplained variability in ice nuclei concentrations, and when applied to a global climate model it markedly alters cloud liquid and ice water distributions and produces a net cloud radiative forcing increase of ~1 W m⁻² per order‑of‑magnitude rise in ice nuclei, underscoring climate sensitivity to glaciation initiation.

Abstract

Knowledge of cloud and precipitation formation processes remains incomplete, yet global precipitation is predominantly produced by clouds containing the ice phase. Ice first forms in clouds warmer than -36 °C on particles termed ice nuclei. We combine observations from field studies over a 14-year period, from a variety of locations around the globe, to show that the concentrations of ice nuclei active in mixed-phase cloud conditions can be related to temperature and the number concentrations of particles larger than 0.5 μm in diameter. This new relationship reduces unexplained variability in ice nuclei concentrations at a given temperature from ∼10 3 to less than a factor of 10, with the remaining variability apparently due to variations in aerosol chemical composition or other factors. When implemented in a global climate model, the new parameterization strongly alters cloud liquid and ice water distributions compared to the simple, temperature-only parameterizations currently widely used. The revised treatment indicates a global net cloud radiative forcing increase of ∼1 W m -2 for each order of magnitude increase in ice nuclei concentrations, demonstrating the strong sensitivity of climate simulations to assumptions regarding the initiation of cloud glaciation.