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Content archived on 2024-05-21

Parameterization of the aerosol indirect climatic effect


Problems to be solved
Reliable predictions of climatic change are impossible unless the magnitude and distribution of the anthropogenic perturbations of the climate can be quantified. The radiative effects of greenhouse gases are well understood; but the indirect effects of anthropogenic aerosols, that operate by altering cloud properties, are potentially significant and are very uncertain. Scientific objectives and approach The project aims to address these problems directly, using the unrivalled data sets of co-located observations of aerosols, in cloud properties and radiative fluxes obtained during the ACE-2 field campaign. The measurements will be compared with cloud properties and radiative fluxes simulated by several climate models, with the objective of rigorously assessing the models and developing and testing more realistic schemes for representing aerosol-cloud-radiation interactions in climate models. The results of the ACE-2 Cloudy-column experiment will be examined and extended to the scales that are significant for GCM parameterisations, namely 100 km in space and 1/2 hour in time. The quality of the closure, that has been evaluated at the scale of the physical processes, will be evaluated at the GCM scale. The assessment will be focused on the four processes which are identified as the most important for the aerosol indirect effect (AIE), namely aerosol activation, microphysics/radiation interaction, drizzle formation and feedback, and cloud dynamics and homogeneity. The variables used for describing the physical processes will be examined in term of large-scale statistics and the corresponding large-scale variables will be defined. Special attention will be given to non-linear processes, which cannot be parameterised with the mean value and the standard deviation of the variables, but rather by tail of the distributions, such as vertical velocity for the CCN activation process or droplet concentration for the onset of precipitation. Various solutions will be proposed for the modellers to determine which ones are predictable. The values of these large-scale variables will be calculated for each case study. Novel parameterisations based on large-scale variables will be developed and tested versus the observations. The results of the data analysis, of satellite image processing and initialisation fields extracted from the ECMWF analysis will be merged to form the data set for the models. Expected impacts The results will contribute to narrowing the large range of uncertainly in model simulations of the indirect effects of anthropogenic aerosols, thereby facilitating more reliable predictions of climatic change.

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EU contribution
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42,Avenue G. Coriolis 42

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Participants (8)