Philip Cunningham

Coherent vertical structures in numerical simulations of buoyant plumes from wildland fires

The structure and dynamics of buoyant plumes arising from surface-based heat sources in a vertically sheared ambient atmospheric flow are examined via simulations of a three-dimensional, compressible numerical model. Simple circular heat sources and asymmetric elliptical ring heat sources that are representative of wildland fires of moderate intensity are considered. Several different coherent vortical structures that dominate the plume structure and evolution are evident in the simulations, and these structures correspond well with those observed in plumes from wildland fires. For the circular source, these structures include: (i) a counter-rotating vortex pair aligned with the plume trajectory that is associated with a bifurcation of the plume, (ii) transverse shear-layer vortices on the upstream face of the plume, and (iii) vertically oriented wake vortices that form periodically with alternating sign on either side of the downstream edge of the plume base. For the elliptical ring source, a streamwise counter-rotating vortex pair is apparent on each flank, and a transverse horizontal vortex is observed above the head of the source. In all simulations the plume cross section is represented poorly by a self-similar Gaussian distribution.

Vorticity-Based Detection of Tropical Cyclogenesis

Abstract Ocean wind vectors from the SeaWinds scatterometer aboard the Quick Scatterometer (QuikSCAT) satellite and Geostationary Operational Environmental Satellite (GOES) imagery are used to develop an objective technique that can detect and monitor tropical disturbances associated with the early stages of tropical cyclogenesis in the Atlantic basin. The technique is based on identification of surface vorticity and wind speed signatures that exceed certain threshold magnitudes, with vorticity averaged over an appropriate spatial scale. The threshold values applied herein are determined from the precursors of 15 tropical cyclones during the 1999–2004 Atlantic Ocean hurricane seasons using research-quality QuikSCAT data. The choice of these thresholds is complicated by the lack of suitable validation data. The combination of GOES and QuikSCAT data is used to track the tropical disturbances that are precursors to the 15 tropical cyclones. This combination of data can be used to test detection but is not as...

Interaction between a wildfire and the sea-breeze front

Florida experiences sea breezes, lake breezes, and bay breezes almost every day during the year, and there are frequently complex interactions between many of these breezes. Given the often-rapid changes in temperature, humidity and wind speed that accompany these breezes, most wildfires and prescribed fires in Florida are affected in some way by their interaction with these circulations. In this paper, we explore the interaction between sea breezes and wildland fires from both an observational and an idealized modeling perspective. The progression of the sea-breeze front and its interaction with the smoke plume from a 26,000 acre wildfire are tracked using a variety of data sources including surface and upper-air observations as well as NEXRAD radar imagery. Idealized numerical simulations of a thermally buoyant plume interacting with a density current are performed in an effort to enhance our understanding of the dynamics of the interaction between sea breeze circulations and the convective column of the fire. Our observational analysis and idealized modeling results suggest that the arrival of a sea breeze front induces a temporary, but significant, increase in fire intensity. This intensification precedes the arrival of the sea-breeze front at the location of the fire, such that the fire intensity is at a maximum at the time of, and slightly after, the passage of the front, and decreases gradually thereafter as cooler and moister air behind the front arrive.

High-resolution numerical models for smoke transport in plumes from wildland fires

A high-resolution large-eddy simulation (LES) model is employed to examine the fundamental structure and dynamics of buoyant plumes arising from heat sources representative of wildland fires. Herein we describe several aspects of the mean properties of the simulated plumes. Mean plume trajectories are apparently well described by the traditional two-thirds law for plume rise; however, the spatial structure of the mean plume is significantly different from the Gaussian distributions typically assumed in simple plume models. This discrepancy arises from the fact that entrainment properties of a buoyant plume in a cross wind are significantly different from those of a buoyant plume in the absence of a cross wind, a result of the interaction of the buoyancy-generated vorticity in the plume with the vorticity in the ambient wind. The depth of the crosswind shear layer at the surface also appears to play a role in both the horizontal and vertical spread of the plume boundaries downwind, and in particular the increase in horizontal spread acts to increase the departure from a Gaussian distribution seen in the plume cross sections.

Large-Eddy Simulations of Air Flow and Turbulence within and around Low-Aspect-Ratio Cylindrical Open-Top Chambers

AbstractThe flow around cylindrical open-top chambers (OTCs) with aspect ratios (i.e., height-to-diameter ratios) much less than unity is investigated using a large-eddy simulation (LES) model. The solid structures are represented using the immersed boundary method, and the ambient flow in which the OTCs are embedded is representative of a turbulent atmospheric boundary layer. Results from the LES model show that the flow inside OTCs depends strongly on the height of the chamber wall. In particular, as chamber height increases the flow impinging on the upstream wall is deflected more in the vertical direction, a stronger recirculation flow develops inside the chamber, turbulence intensities are greater, and there is stronger vertical transport and mixing within the OTC, even at or near the ground. For low wall heights (i.e., very low aspect ratios), however, the flow impinging on the OTC is only diverted weakly in the vertical direction; aside from a small recirculation zone inside the OTC near the upstre...