A Study of Aerosol Effects on Warm Ice Clouds

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The main objective of this proposal is to study aerosol effects on warm ice clouds. These are mixed-phase clouds that contain both ice particles and supercooled liquid droplets, and can occur between 0C and -40C. Whether a cloud is composed of ice or liquid is crucial for determining how much of the incoming radiation from the sun it reflects back to space and therefore crucial for climate prediction. The PI is interested in how dust aerosols and smoke from biomass burning or other anthropogenic aerosols acting as ice nuclei influence the microphysical and radiative properties of these clouds as well as the extent to which precipitation formation processes are altered. To achieve this goal an aerosol model has been coupled with the National Center for Atmospheric Research (NCAR) General Circulation Model (GCM), Community Atmospheric Model (CAM). As part of the PI's present research, she has implemented ice crystal mixing ratio and number concentration prognostic equations in the NCAR CAM in order to study aerosol-cirrus-climate effects, based on a new parameterization for the effects of dust, soot, and sulfur aerosols on these cold clouds. This study will extend this parameterization to treat deposition nucleation and contact nucleation in warm ice clouds (T -40C). Data developed during the forthcoming Ice in Clouds Experiments (ICE) will be used to guide the development of the parameterization. A cloud parcel model with size-resolved microphysics will be used to both simulate conditions during ICE and to develop the ice nucleation parameterization. This parameterization will then be included in the ice phase microphysical processes in the CAM model. CAM coupled to the IMPACT aerosol model will be used for simulations for the present-day and pre-industrial climate to assess the effects of anthropogenic aerosols. The PI will also examine the possibility of trends observed in dust concentration and their effect on ice clouds and climate. Available data from satellite and lidar observations will be used to evaluate her results.In addition to the project outlined above, the PI plans to continue her collaboration in the European-based Quantify Project. Quantify is a European-funded project under the Sixth Framework Programme with 40 partners and 8 separate work packages. The main goal of QUANTIFY is to quantify the climate impact of global and European transport systems for the present situation and for several scenarios of future development. The PI's main effort in Quantify will be to compare the predictions of her coupled aerosol/climate model (the IMPACT model coupled to the NCAR CAM model) with ice microphysics to the predictions of a cloud-resolving model from the Danish Meteorological Institute. The intellectual merit of this project lays in its contribution to an understanding of the possible role of both dust and anthropogenic ice nuclei in indirect effects on climate. The broader merits of the project lie in its contribution to the science that is useful for policy decisions. Moreover, graduate students will participate in the research, so that this project will contribute to graduate training. Additionally, during March 2007, Penner will be teaching the "clouds and precipitation" course at Michigan. Real time data from the ICE experiments planned for this time frame will be used in an end-of-term project to acquaint students with the role of field experiments in understanding ice nucleation phenomena.