Numerical Simulation of the Meteorological Conditions Above a Grassfire on a Slope

In this study we utilize the Advanced Regional Prediction System (ARPS) model to examine the meteorological conditions above a grassfire on a steep (~30%) slope. This work is one part of a two-part modeling effort to simulate smoke behavior associated with a prescribed grassfire at a U.S. Environmental Protection Agency (EPA) Superfund site near Palmerton, PA. In this presentation, we focus on the simulated meteorological conditions likely to influence smoke behavior predictions, including mean and turbulent flow in the vicinity of the fire. The topography in the vicinity of the burn site consisted of a west-southwest – east-northeast oriented ridgeline, with the burn site located on the north-facing slope. Vegetation consisted primarily of dried grasses with scattered defoliated trees; areas of exposed shale rock were also present.

The ARPS model is run with fine grid spacing [O(10 m) horizontal; O(2 m) vertical] to resolve many scales of turbulence associated with the fire, as well as resolve the background turbulent state. Specific meteorological variables examined include mean wind speed, wind direction, temperature, and turbulent kinetic energy (TKE). For all variables, comparison is made to measurements taken during the prescribed fire, and strengths and weaknesses of the model simulation are noted. The effect of the fire on the degree of anisotropy in the turbulent flow is examined, and the relative contributions of TKE generation mechanisms (vertical wind shear vs. buoyancy) before and after the fire develops are discussed.