Study of mechanisms of aerosol indirect effects on glaciated clouds

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Description

Aerosol-cloud interactions account for some of the greatest uncertainties in global models’ simulation of climate change. Aerosol indirect effects on glaciated clouds are particularly uncertain, partly because of the diversity of physical mechanisms for initiation of ice. For example, an offline inter-comparison of various schemes of heterogeneous ice nucleation from the literature revealed five orders of magnitude of difference in predicted ice concentrations at -30°C (Phillips et al. 2008). It is thought that insoluble aerosols, by nucleating extra ice heterogeneously, can alter mixed-phase clouds’ precipitation production (Phillips et al. 2003), lifetime, and potentially also, their phase. Conversely, extra soluble aerosols might alter the warm rain process in convective clouds, altering raindrop-freezing and supercooled cloud-liquid aloft, as well as homogeneous freezing and cirrus properties. Such influences, among others, may alter the properties and lifetime of mesoscale cloud systems, altering their impacts on the radiation budget. We have a model of the aerosol-cloud interaction with “double-moment” bulk microphysics, semi-prognostic treatment of multiple aerosol species, and representation of the known pathways for ice initiation (Phillips et al. 2009). In this current first year of the ASR (ARM) project, the model is being enhanced. Relations for the size- and temperature-dependence of ice-particle morphology, partly predicted by a more detailed (spectral) model, are being incorporated into it. During subsequent years of the project, ARM cases from the Cloud and Land Surface Interaction Campaign (CLASIC; Oklahoma, 2007) and Tropical Warm Pool-International Cloud Experiment (TWP-ICE; Pacific Ocean, 2006) will be simulated by the model, with validation against ARM aircraft and satellite and ground-based observations. Sensitivity studies will elucidate the relative roles of mechanisms for the indirect effect from anthropogenic soluble and insoluble aerosol on glaciated clouds. Such process-level insights are needed if global models are to be improved by the community. In this presentation, our ongoing model enhancement will be described.