  This study investigates the role of topography-induced turbulence, generated
by an idealized urban region, in the transport of firebrands and risk of
spotting. Flight dispersion, deposition, and smoldering state of tens of
thousands of individual mass and size-changing firebrands were investigated in
the atmospheric boundary layer turbulence, which was obtained using Large-eddy
simulations. Firebrands were assumed to be smoldering spherical particles of
Stokes numbers ranging from 30 to 175. Results indicate that the presence of
urban topography significantly affects the firebrand flight behavior, landing
distribution, and risk of spotting. Compared to a case with flat topography,
horizontal dispersions of the smallest size firebrands were significantly
enhanced when urban topography was presented, while the largest firebrands
landed closer to each other and closer to the release point. Consequently, a
notably different and more compact spotting risk map was achieved. Within the
urban boundary layer turbulence, firebrands had shorter flight and smoldering
times in comparison with the flat case. As a result, firebrands landed with
larger temperatures, which contributed to a higher risk of spotting in the
presence of urban topography.
