Boundary layer flow over forest canopies of variable density

Numerical models often represent the effects of forest canopies using a roughness length parametrisation, however theoretical and experimental work in recent years has demonstrated that interactions with the canopy can be important for heterogenous problems. The focus so far been on complex terrain, however heterogeneous canopies over flat terrain may also induce similar effects. Here an analytical model of boundary layer flow over a canopy of varying density is developed. The model demonstrates some important differences with existing analytical solutions for flow over a surface with variable roughness. The model also highlights the importance of including differences in displacement height. The analytical results are backed up with numerical simulations using BLASIUS (an idealised boundary layer flow model, originally developed at the UK Met Office). The implications of this work for model parametrisation of forest canopies are discussed.