A Specific Large-Scale Pressure Gradient Forcing for Computation of Realistic 3D Wind Fields Over a Canopy at Stand Scale

Abstract

15Turbulent flows over and within forest canopies have recently been modeled with success 16using Large Eddy Simulations (LES). Validation exercises against experimental data suggest 17 that models can be applied with a high degree of confidence for many applications, 18 mechanical and physiological plant/atmosphere interaction analysis, seed or pollen dispersal, 19wildfire spread and firebrand transport, or investigation of causes of eddy-covariance 20 technique bias. Long distances required for shear-induced turbulence to equilibrate, result in 21 the widespread use of cyclic boundary conditions in LES atmospheric boundary layer studies. 22Vegetation drag dissipates air momentum in the atmosphere, but equilibrium is often achieved 23 through compensatory momentum source, supplied by macro-scale pressure gradient forcing. 24Unfortunately, both classical Ekman balance or simple spatially-constant pressure gradient 25 techniques for implementing this forcing have major drawbacks in the context of cyclic 26 boundary conditions for the applications listed above. Among them, it is difficult to specify 27 aspects of the mean velocity profile such as a specific desired wind velocity and direction at a 28 reference height. In the present paper, we propose a new technique for capturing the effects of 29 a large-scale pressure gradient force (LSPGF) that can be used at stand scale and enables 30 simulation of realistic and specifiable wind fields. Several variants of this LSPGF are 31 developed and analyzed here and validated against experimental data. Although this LSPGF 32 technique is developed in the context of HIGRAD/FIRETEC wildfire simulations, LSPGF 33 can be used for any LES wind modeling application aimed at generating detailed stand-scale 34 wind fields with resolved turbulence and shear profiles consistent with vegetation structure in 35 the boundary layer. 36 37 Keyword Ekman balance -Forest canopy -Large-eddy simulation -Large-scale pressure 38 gradient -Streaks 39 40 41

Sensitivity of Grass Fires Burning in Marginal Conditions to Atmospheric Turbulence

“…Vegetation drag continuously dissipates momentum in the domain. A dynamic macro‐scale pressure gradient forcing is applied to compensate for this drag, enabling simulations of realistic wind fields (Pimont et al., 2018). We allow 60 min, which is equivalent to ∼9 km of fetch, for turbulence to develop that is consistent with both upper‐level winds and surface fuel structure.…”