The Ember Geometry
# The Ember Geometry
Wildfire spread models focus on two mechanisms: direct flame contact at the fire perimeter and long-range spotting, where burning embers are lofted by convective plumes and land far ahead of the main fire. The spotting models treat ember transport as a plume-driven, convective process — embers rise, travel through the atmosphere, and fall at distances determined by wind speed and plume dynamics.
Near-surface ember transport — ember wash — is a third mechanism that follows fundamentally different physics. Embers roll, bounce, and saltate along the ground surface, driven by surface winds rather than convective plumes. The transport is geometric rather than convective: embers spread radially from the fire perimeter along the ground, with distances determined by surface roughness, ember size, and near-surface wind speed rather than by plume height and atmospheric stability.
The distinction matters for prediction. Plume-driven spotting produces sparse, long-range ignitions — a few embers landing far ahead. Ember wash produces dense, short-range ignitions — many embers spreading along the ground surface in a pattern determined by terrain geometry. The two mechanisms produce qualitatively different fire growth patterns: spotting creates isolated secondary fires that may or may not merge with the main fire; ember wash creates a continuous expansion of the fire perimeter driven by surface-level transport.
The structural observation: conventional models mispredict fire expansion in ember-wash-dominated regimes because they model the wrong transport mechanism. The ember is the same physical object — burning material moving through space — but the transport physics is entirely different depending on whether it travels through the atmosphere or along the ground. The prediction error is not in the model parameters but in the model class.
