Abstract
A fire-spotting parameterization was developed for the WRF-Fire
component of the WRF model version 4.0.1. The parameterization uses a
Lagrangian particle transport framework and is coupled to the fire
component of the WRF-ARW model as an independent Fortran module. When
fires are active, the fire-spotting module identifies areas at risk of
fire spotting by modeling transport and physical processes of individual
firebrands released from fire locations. Firebrands are released at
varying heights, from locations with higher emission potential, defined
as a function of fire rate of spread and fuel load. Firebrands are
transported with the atmospheric flow, and physical properties
(temperature, mass, and terminal velocity) are updated at the default
model timestep. The particles may either burnout before settling or
deposit at a grid point when carried below a specified height threshold.
The number and spatial distribution of deposited firebrands correspond
to the flow-dependent risk component of new fire ignitions due to fire
spotting. The flow-dependent component is then combined with the risk
associated with local fuel properties (load and moisture) to yield the
fire spotting spatial likelihood. In this presentation, the
fire-spotting parameterization is assessed through a qualitative
analysis of wildfires in Colorado. Uncertainties in fire ignition
observations, used to initialize fires in the WRF-Fire model, often
limit the ability to accurately model fire area, which in turn controls
the firebrands’ emission location. Limited spotting observations are
also a challenge to an objective verification of the module skill. We
expect that the most recent remote sensing products will improve the
representation of surface properties and accuracy of ignition parameters
for WRF-Fire, which will directly transfer to the fire-spotting module
capability. Direct enhancements to the parameterization may be
incorporated into the module as laboratory experiments and field
campaigns provide data to improve our ability to model firebrands’
initial properties (e.g. firebrand size and ejection height) and
physical processes (burnout and terminal velocity).