Aerosol particles are an important component of the atmosphere and are of great concern in the case of an accidental release of radioactive particles. To better understand ground contamination due to a particle release in the vicinity of an accident, it is important to study the particle scavenging associated with rain. To achieve this objective during this thesis, an experiment at the microphysical scale (BERGAME experiment) was developed and the modelling of the washout was improved at mesoscale. The model used was DESCAM : a microphysics model that describes the bin size distributions for aerosol particles and hydrometeors and calculates their evolution due to microphysical cloud processes. The BERGAME experiment was designed and built to measure the collection efficiencies which disagree with theoretical calculation. Using an optical method, it was shown that those discrepancies were due to rear capture. A new model of collection efficiencies for the 2 mm drops was proposed that takes into account for big raindrops the turbulent circulation occurring downstream increasing significantly the capture of small particles. The corrected collection efficiencies were implemented in the DESCAM model. By this way, significant modifications of the modelling were observed and studied. This is an important step toward an accurate modelling of the ground contamination in atmospheric dispersion models.
