— Simulated radar observations of the sea surface dynamics as used in the MODENA project, are based on an original methodology for sea states : the " groupy " wave model (GWM). Random wave fields can have very different modulations but nearly identical spectra. Nevertheless, the response of a floating object to waves depends strongly on the likelihood of large wave encounters. Sea surface fluxes also depend on wave breaking and air-flow separation, both being consequences of large-amplitude events. So wave group structure is one key description to simulate radar clutter under various environmental and instrumental configurations. The GWM builds on random distributions of wave groups and conditionally distributed breaking waves over these groups. Each wave group travels across the simulated area, and breaking waves appear dynamically on the wave crest at the rear of a group, propagating and disappearing at the front of this group. The generation of sea states follows a prescribed sea wave directional spectrum, and any breaking wave statistical distribution such as Λ(c)d c describing the total length of breaker per unit area and time with phase speed between c and c + d c. Accordingly, the group density per surface unit can lead to very different sea state structures, and the results will be discussed.