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 degrees 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 microm in diameter. This new relationship reduces unexplained variability in ice nuclei concentrations at a given temperature from approximately 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 approximately 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.

Fig. 1.

Schematic diagram of the effect of ice nuclei from various possible aerosol sources on midlevel precipitating clouds and cirrus ice clouds. The likely but uncertain change in the magnitude of the general cooling impact (blue arrows) of midlevel clouds and warming impact (red arrows) of high cirrus clouds in response to increases in the relative number concentrations of IN is indicated (see text for further description).

Fig. 2.

IN number concentration (at STP) active at water saturation or above vs. temperature. Projects (see SI Text) are WISP-94 (gray triangle), Alliance Icing Research Study—2 (X), AMAZE-08 (square), Cloud Layer Experiment-10/Canadian Cloudsat/CALIPSO Validation Project (open circle), Ice in Clouds Experiment—Layer Clouds (solid circle), Ice Nuclei SPECTroscopy-1 (–), Ice Nuclei SPECTroscopy-2 (diamond), Mixed-Phase Arctic Cloud Experiment (black triangle), and Pacific Dust Experiment (open triangle). Parameterizations described in the text are labeled and are plotted over the experimental measurement range on which they were based. The dashed gray line is a T-dependent fit to all data [0.117 exp(-0.125^∗(T_K - 273.2)); r² = 0.2].

Fig. 3.

Comparison of predicted vs. observed IN concentrations. In A, the points correspond to the Meyers et al. (12) relation calculated at water saturation. Other parameterizations are compared in the SI. In B, predictions are based on the parameterization put forth in this paper that depends on both n_aer,0.5 and T. Uncertainties (1 standard deviation) are shown on selected data points in B. Dotted lines outline a range of a factor of 2 about the 1∶1 line (solid line) in both panels.