The first Special Observation Period of the HyMeX campaign took place during autumn 2012. A specific observational network was set up in the south of France with the aim of better understanding the role and impact of complex terrain on the horizontal and vertical structure of rainfall and the associated microphysical processes. The event, referred to as IOP7a, which occurred on 26 September 2012, is studied in this article. A first analysis, based on two reference parameters of the drop-size distribution (DSD), shows that the topography of the region has an impact on the rainfall structure at the fine scale and that rainfall intensity also has an influence on the DSD parameters. A more detailed study, based on the evolution of the shape of the DSD and of the liquid water content (LWC) within different rainfall intensities, reveals that the coalescence mechanism is of significant importance for different rainfall regimes and for different topographic areas. As we get closer to the ground, the modification of the DSD shows that other microphysical and dynamical processes (such as evaporation, break-up, turbulence in the presence of vertical wind) might compete with the coalescence mechanism or enhance it. The respective strengths of these processes depend on the local environment; coalescence seems enhanced by a layer with higher LWC as the local terrain becomes more complex, and evaporation appears more important above flatter terrain. Thus, both the orography and the rainfall intensity can impact the rainfall structure and the associated microphysical and dynamical processes.