Improvements to an Empirical Parameterization of Heterogeneous Ice Nucleation and Its Comparison with Observations

Abstract

framework for an empirical parameterization (EP) of heterogeneous nucleation of ice crystals by multiple species of aerosol material in clouds was proposed in a 2008 paper by the authors. The present paper reports improvements to specification of a few of its empirical parameters. These include temperatures for onset of freezing, baseline surface areas of aerosol observed in field campaigns over Colorado, and new parameters for properties of black carbon, such as surface hydrophilicity and organic coatings. The EP?s third group of ice nucleus (IN) aerosols is redefined as that of primary biological aerosol particles (PBAPs), replacing insoluble organic aerosols. A fourth group of IN is introduced?namely, soluble organic aerosols.The new EP predicts IN concentrations that agree well with aircraft data from selected traverses of shallow wave clouds observed in five flights (1, 3, 4, 6, and 12) of the 2007 Ice in Clouds Experiment?Layer Clouds (ICE-L). Selected traverses were confined to temperatures between about ?25° and ?29°C in layer cloud without homogeneously nucleated ice from aloft. Some of the wave clouds were affected by carbonaceous aerosols from biomass burning and by dust from dry lakebeds and elsewhere. The EP predicts a trend between number concentrations of heterogeneously nucleated ice crystals and apparent black carbon among the five wave clouds, observed by aircraft in ICE-L. It is predicted in terms of IN activity of black carbon.The EP?s predictions are consistent with laboratory and field observations not used in its construction, for black carbon, dust, primary biological aerosols, and soluble organics. The EP?s prediction of biological ice nucleation is validated using coincident field observations of PBAP IN and PBAPs in Colorado.