Beyond the injection height: Understanding the plume top height behaviour of wildfire-induced plumes.

Wildfires are increasingly showing pyro-cloud formation, thereby transitioning into a state with unpredictable fire behaviour. Although a multitude of factors determine the ability of any plume to create pyro-clouds, one of the essential components is the ability of dry convective plumes to reach altitudes where condensation can occur. Hence, to predict the onset of pyro-cloud formation, it is essential to understand what factors govern the plume top height of dry convective plumes.

However, previous research predominantly focused on the injection height to improve smoke pollution predictions. Our research extends beyond the plume injection height, investigating the physics that govern the plume top height. As observations of convective wildfires are scarce due to the dangerous measurement conditions, we use MicroHH to create high-resolution (~20 m) large eddy simulations of convective wildfire plumes. Our preliminary analyses, in which we varied the fire intensity, revealed three distinct plume regimes dictated by the interaction between heating by the fire and atmospheric stratification:

• ABL-Plumes: plumes that cannot overshoot the atmospheric boundary layer (ABL), resulting in a plume top equal to the ABL top.
• Overshooting Plumes: Plumes that overshoot into the free troposphere, but with an injection height equal to the ABL top.
• Free Tropospheric Plumes: Plumes that both overshoot and inject into the free troposphere, meaning that both the plume top height and the injection height exceed the ABL top.

With our study, we aim to answer two questions. First, how do the scaling relationships for maximum plume top height evolve as a plume transitions from an Overshooting to a Free Tropospheric plume? Second, for the Free Tropospheric plumes, do the plume top height and injection height share the same scaling relationships, or do distinct physical processes govern the plume top height?

For example, we know from a previous study that the injection height of free tropospheric plumes scales with fire intensity to the power of 0.35. If both the plume top height and injection height follow the same scaling, this provides a unified physical explanation for plume rise. Alternatively, a different scaling suggests that additional physical processes govern the overshoot beyond the injection height. To explore these physics, we will vary the fire intensity across a range of realistic atmospheric conditions by modifying the boundary-layer height, ambient wind speed, capping inversion strength, and free-tropospheric lapse rate. Ultimately, our goal is to condense the plume dynamics derived from our 3D large eddy simulations into a simplified (adiabatic) parcel model to explain the scaling behaviours and regime transitions of dry convective wildfire plumes.