Predicting global atmospheric ice nuclei distributions and their impacts on climate

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.aerosol indirect effects | climate forcing | ice nucleation T he formation of ice in clouds is of vital importance to life on Earth, as ice formation is one of the key processes initiating precipitation. In addition, since ice nucleation is tied to the action of specific aerosol particles, natural and human impacts on ice nucleation in the atmosphere can lead to alteration of the energy and hydrological cycles (1). Ice nucleation in clouds occurs via two primary pathways: homogeneous freezing of liquid particles below about −36°C and heterogeneous ice nucleation, triggered by "ice nuclei" that possess surface properties favorable to lowering the energy barrier to crystallization. Once ice is formed, some circumstances may favor generation of ice from preexisting ice particles, or secondary ice formation (2). Heterogeneous ice nucleation remains an enigmatic topic involving multiple mechanistic processes (3) that sometimes defy ready investigation or description. Despite the lack of a complete understanding of heterogeneous ice formation processes, a variety of techniques have been developed and used to detect the presence of and quantify the number concentrations of atmospheric ice nuclei as a function of temperature (4). These measurements show that, although generally representing only 1 in 10 5 of ambient particles in the free troposphere (5), ice nuclei (IN) can nevertheless exert an influence on cold cloud microphysical processes that is disproportionate to their low number concentrations. For example, the concentrations of IN needed to explain observed precipitation rates range from as small as 10 −3 per standard liter at −10°C (6) to more typical estimates of a few IN per standard liter at −20°C (7).The sensitivity of precipitation initiation fro...

Radiative impact of mineral dust on monsoon precipitation variability over West Africa

“…Dust effects on convective cloud and precipitation are not represented in the convective parameterization. In addition, dust aerosols can only be activated to cloud condensation nuclei (CCN) in the Lin microphysics scheme in this study, but dust can also be activated as ice nuclei (IN) to influence clouds and precipitation (e.g., DeMott et al, 2003DeMott et al, , 2010. We intend to address these issues in future studies of dust effects to further elucidate the relative impacts of dust direct and indirect effects on monsoon precipitation.…”

Microbiome of the upper troposphere: Species composition and prevalence, effects of tropical storms, and atmospheric implications

“…Most of the airborne cells are viable (Fig. 1C) and are large enough (0.25-1 μm, SI Appendix, Table S2) to support IN activity (25). Given also that many species (especially Proteobacteria) are known to be efficient nuclei for the formation of water droplets and ice crystals, and that middle-toupper troposphere (e.g., low temperature) clouds can be affected by low-activity IN (26), airborne bacterial cells could influence cloud formation and precipitation more than previously thought.…”

Balloon-borne match measurements of midlatitude cirrus clouds

“…), cloud internal dynamics (Marsham and Dobbie, 2005;Fusina and Spichtinger, 2010) and background aerosols that are either liquid and freeze homogeneously (Hoyle et al, 2005;Spichtinger and Gierens, 2009a) or contain a solid and may act as heterogeneous ice nuclei (DeMott et al, 2010). Ice in the upper troposphere is known to nucleate via both pathways, homogeneous freezing of aqueous solution droplets (Koop et al, 2000) or heterogeneous nucleation on solid aerosol particles (DeMott et al, 2003).…”