The Study of Mechanisms of Aerosol Indirect E ffects on Glaciated Clouds

Aerosols directly interact with radiation through their scattering, absorption and emission of long- and short-wave radiation. In addition, their changes in loading and chemistry modify clouds' micro- and macro-physical properties thereby altering the radiative properties of clouds and consequently the radiation budget of the Earth. How clouds will respond in future to changes in aerosol loading and chemistry remains the greatest source of uncertainty in climate prediction as was concluded in the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC) (Solomon et al., 2007). Thus, the treatment of clouds in climate models should be improved so as to reduce these uncertainties. This is achievable through a subtle understanding of aerosol indirect effects especially on glaciated clouds due to their large temporal and spatial extent mostly in the form of cirrus. This modelling study will investigate the different and salient mechanisms by which changes in aerosols, both in number concentration and chemical composition will afect the optical properties of clouds. The main objectives will be to (1) explore the cloud micro-physical and dynamical mechanisms for cold-cloud indirect effects on the meso-scale (focusing mainly on glaciation, thermodynamic and riming indirect effects, (Lohmann and Feichter, 2005)), from anthropogenic soluble and insoluble aerosols, (2) identify salient processes of ice initiation and (3) identify the important feedbacks associated with these indirect effects and if possible be able to quantify their respective radiative forcing. This shall be done by way of sensitivity tests using the Weather Research and Forecasting (WRF) model with a modified version of Phillips et al. (2009) semi-double moment bin- micro-physics scheme which has a semi-prognostic aerosol treatment, currently treating six different aerosol species assuming internal and external mixing. Two tropical maritime and continental cases shall be simulated; the (Tropical Warm Pool International Cloud Experiment, TWP-ICE and Cloud and Land Surface Interaction Campaign ,CLASIC). Significant progress has already been made; the micro-physics scheme has been improved to accurately represent the aerosol and precipitation distributions and bin-micro-physics is now being applied to all coagulation processes. In addition, ice morphology is also being treated so as to accurately represent the sticking efficiencies in ice crystal processes. The TWPICE case has been simulated and validated whilst the CLASIC simulation is ongoing. The main clouds' optical properties of interest to this study are droplet and crystal mean sizes, droplet and crystal number concentrations and liquid and ice water content; optical depth, lifetime and spatial extent will also be considered. Thefindings of this study will generally improve the understanding of aerosol indirect effects and representation of aerosols and clouds in Global Climate Models, (GCMs).

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