Regimes of dry and moist convective plumes above forest fires: Idealized numerical simulations and dimensional analysis

Forest fires have a profound impact on atmospheric circulations due primarily to the large temperature anomalies produced by the fire. The fundamental dynamics through which a forest fire and its environment interact to yield different plume regimes is still not well understood. This study investigates the impact of wind speed and critical level height on dry convection above a prescribed heat source. This is done using the Advanced Regional Prediction System (ARPS) model in its two-dimensional form with an imposed 400 K soil potential temperature perturbation.

The result of these experiments is the identification of three modes of convective plumes. The first, termed multicell convective plumes is analogous to multicell convection generated from squall line cold pools in the moist atmosphere. The second mode, a deep wave mode consists of disturbances with wavelengths of 7-10 km, and results from the multicell plumes perturbing the dynamically unstable shear flow centered at the critical level. The third mode, termed intense fire plume, has stronger updrafts than the multicell mode and is marked by quasi-stationary movement and substantial low-level inflow and upper-level outflow. The presence of a critical level is shown to be crucial to development of both the deep wave and intense plume modes. The intense fire plume mode is most consistent with the so-called fire storm, or conflagration phenomenon, in which strong updrafts and low-level indrafts can produce mesocyclones and tornadic fire-whirls capable of significant damage.

Additionally, fully three-dimensional experiments were performed to evaluate the importance of the critical level in the behavior and dynamics of a three-dimensional plume. Preliminary results are also presented from current work investigating the dynamics of moist convective plumes and the dependence of plume behavior and fire spread on relevant control parameters.