Stable Boundary Layer in Complex Terrain. Part II: Geometrical and Sheltering Effects on Mixing

Abstract

AbstractThe authors examine how terrain texture and topography influence nocturnal mixing rates. Local topographic curvature and site sheltering exhibit systematic influences on nocturnal heat and momentum fluxes and the near-surface potential temperature distribution. This influence is particularly evident in hilly terrain with patchy forested areas, typical of eastern North America and many other regions. Exposure to local obstacles, quantified using Fujita’s “transmission factor,” has its maximum influence on mixing during strong winds (>5 m s−1), whereas the effects of local terrain curvature dominate under weaker winds. Such complementary dominance conditions currently limit direct comparison of the two effects. Even with a limited network of 10 stations, it is clear that preferred regions for mixing can be identified in advance given knowledge of land cover and topography. When designing a network of surface stations to be deployed in heterogeneous terrain, one should consider site curvature, slope, and exposure in addition to spatial coverage.