From Glossary of Meteorology
A region having irregular topography, such as mountains or coastlines.
Complex terrain can also include variations in land use, such as urban, rural, irrigated, and unirrigated. Complex terrain often generates local circulations, or modifies ambient synoptic weather features, to create unique local weather characteristics such as katabatic winds, anabatic clouds, and sea breezes. In regions of complex terrain, weather forecast models must have high resolution to reproduce numerically the terrain-induced weather features.