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...

This paper describes the design and performance of a photoacoustic aerosol absorption spectrometer (PAS) built for operation on a research aircraft platform. The PAS instrument is capable of measuring dry absorption at 659 nm, 532 nm, and 404 nm, and absorption enhancement due to coatings at 532 nm and 404 nm. The measurement accuracy for all channels is < = 10% and inflight 1 Hz sensitivities lie within the range of 0.5-1.5 Mm −1 . PAS measurements of calibrated absorbing aerosol samples are shown to be consistent with measurements made by a previous generation single channel photo-acoustic instrument. Aircraft data collected during a recent field campaign in California are used to demonstrate the capabilities of the PAS. In combination with an aircraft cavity ring down aerosol extinction spectrometer described in a companion paper, the new PAS instrument provides a sensitive airborne in-situ characterization of aerosol optics.

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