Sound impact of road and railway infrastructures are more and more severely regulated by European laws: acceptable thresholds in emission and reception are decreasing. This implies to develop propagation models able to take many phenomena into account at the same time (meteorology, uneven ground, impedances discontinuities...). The parabolic equation (PE) is one of the numerical methods used for sound propagation simulation in complex outdoor situations. It neglects backscattering. Even if this assumption is effective in many configurations, it does not allow to use PE for studies of acoustic wave propagation between a source and a receiver when an obstacle (rigid barrier, building) is located just before the source, or just behind the receiver. In those cases, energy reflected by obstacle is not negligible and results obtained with PE may be incorrect. This paper aims at presenting a new method able to integrate backscattering in GFPE (Green’s Function Parabolic Equation method). In this approach a complementary Kirchhoff approximation is used by setting to zero the sound pressure above the vertical obstacle. Thus, new configuration as the multi-reflections can be studied with this new method. In order to point out the role played by backscattering, we first study a barrier located just behind a source. Then, comparison with BEM (Boundary Element Method) calculations is presented in the case of a simple reflection in homogeneous and inhomogeneous atmosphere. A more complex road traffic noise configuration made with two parallel barriers and meteorological effects is also studied. Results show that the complementary Kirchhoff approach seems to be promising.