Understanding the strong mass-loss mechanism is evolved stars is a milestone problem. Mass-loss largely impact the chemical and dust evolution of galaxies. One of the main physical ingredients to explain it is the vigorous convection. In fact, the surfaces of evolved stars are covered by very few and large convective structure that show strong dynamics evolving on very long timescales. To account for convection, the atmospheres of these stars are modeled using realist three-dimensional hydrodynamical simulations that solve the coupled equations of non-local radiation transport and hydrodynamics in the presence of a gravity field.

We present a tomographic method allowing to recover the velocity field at different optical depths in a stellar atmosphere. It is based on the computation of the contribution function to identify the depth of formation of spectral lines in order to construct numerical masks probing different optical depths. These masks are cross-correlated with observed spectra to extract information about the average shape of lines forming at a given optical depth and to derive the velocity field projected on the line of sight. The final aim of this project is to reconstruct spatially and spectrally velocity field in the atmospheres of these stars and to eventually link it to the levitation of the gas during mass-loss processes.