A compressible, nonhydrostatic model is employed to examine the effect of a vertically sheared crossflow on buoyant plumes arising from intense heat sources based at the surface. Specifically, the generation and evolution of vortical structures associated with the interaction of the plume with the crossflow are investigated for a range of the controlling parameters (i.e., intensity of the heat source; magnitude of the shear of the crossflow). Two types of plume are examined: (i) initially two-dimensional plumes associated with line heat sources; and (ii) three-dimensional plumes associated with isolated, axisymmetric heat sources. Of particular interest in this regard is the interaction between the bouyancy-generated vorticity of the plume with the vorticity in the crossflow.

Plumes associated with line heat sources in the presence of a crossflow are shown to be unstable to perturbations in the along-line (i.e., crosswise) direction, leading to an array of vortices of alternating sign that are oriented in the cross-line (i.e., streamwise) direction. This instability apparently leads to a transition of the plume from its initial two-dimensional structure to a three-dimensional one, particularly far from the heat source. Plumes associated with axisymmetric heat sources in the presence of a crossflow are dominated by an embedded counter-rotating vortex pair, which typically results in the bifurcation of the plume into two separate cores above the heat source. The development of these vortical features in the two types of plumes is examined, as is the dependence of this development on the heat source intensity and the crossflow shear. The relevance of the results of these simulations to buoyant plumes arising from intense heat sources such as wildland fires is discussed, and the implications for coupled atmosphere-fire behavior are presented.