Numerical Simulation of Atmospheric Boundary Layer Flow Over Battlefield-scale Complex Terrain: Surface Fluxes From Resolved and Subgrid Scales

: The large-eddy simulation (LES) tool has been used to study the transient characteristics of turbulent mixing in atmospheric boundary layer(ABL) flows over canonical topographies representative of complex, urban environments. Specifically, distributions of uniform height, staggered cubes are considered with heights equal to one quarter and one eighth of the ABL depth. An immersed boundary method has been implemented within the LES, to represent the presence of the cubes; this enables the computational domain to capture the classical roughness sublayer and aloft inertial layer. With this, time series of fluctuating streamwise and vertical velocity is recorded over the ABL depth at various positions. Thus, the code has been used to record datasets that would more typically be recorded with micrometeorological instrumentation in field campaigns. However the unique spatial and temporal accessibility offered via simulations facilitates novel analysis that has led to new physical insights regarding the transient characteristics of roughness sublayer dynamics. It has been shown periods of momentum excess(deficit) in the inertial layer precede excitation(subdual) of cube-scale coherent vortices in the roughness sublayer, and a physics-based predictive model for the observed advective lag has been developed.