On the Sensitivity of Wind and Temperature in the PBL and Roughness Sub-Layer to Canopy and Fire Properties

Smoke dispersion from wildland fires is a critical health and safety issue, impacting air quality and visibility across a broad range of space and time scales. Existing integrated smoke dispersion modeling systems (e.g., BlueSky), which are designed for prediction of smoke from multiple sources on a regional scale, do not have the resolution and physics parameterizations necessary to adequately reproduce smoke from low-intensity fires which can meander around the source and reside within forest canopies for an extended period of time. This study is part of a Joint Fire Science Program (JFSP) project with the goal of developing and validating modeling tools for predicting smoke dispersion from low-intensity fires.

As part of achieving this goal, the Advanced Regional Prediction System (ARPS) atmospheric model has been modified to allow simulation of flow through a multi-layer canopy. The effects of vegetation elements (e.g., branches, leaves) on drag, turbulence production/dissipation, radiation transfer, and the surface energy budget are accounted for through modifications to the ARPS model equations and physical parameterization schemes. The new version of ARPS is referred to herein as ARPS-CANOPY. To account for the first-order effects of a wildland fire, upward sensible heat fluxes are imposed within a fixed area of the model domain in a modified version of ARPS-CANOPY (i.e., ARPS-CANFIRE).

In a series of idealized experiments, we use ARPS-CANFIRE to examine the sensitivity of turbulent and mean flow in the roughness sub-layer and PBL to canopy, fire, and ambient atmospheric conditions. Specific parameters examined include canopy density, canopy morphology, and background wind speed. Finally, some discussion of the relevance of our findings to local smoke dispersion from low-intensity fires is provided.