The ADREA-I code is a three-dimensional transient analytical tool that can provide atmospheric boundary layer and dispersion analysis at the mesoscale and microscale levels, under any atmospheric stability conditions and with any ground complexity, and it is particularly suitable for large topographical disturbances and consequently large atmospheric altitudes. The code, in response to the need to treat large topographical disturbances with the highest spatial and temporal numerical resolution possible, introduces new features in the description of anomalous topography, turbulent diffusion coefficients, and numerical approach. Applications carried out within the framework of validation and demonstration studies of ADREA-I give reasonable results.