Purpose:

to document the existence and discover causes behind erratic plume behavior such as plume descent far from prescribed burns.

 

Problem

The prescribed burning method of mass ignition usually refers to setting a fire along the boundaries of the plot to be burned and flying a helicopter over the interior of the site to drop dozens of "ping pong" balls filled with chemicals that ignite when mixed and exposed to the air. This method develops a strong convective column of smoke over the fire.

This convection column carries smoke in through the mixed layer of air above ground into a stable transport layer. Smoke confined to the transport layer drifts away far above sensitive targets located on the ground. Instances have been reported of urban areas being smoked up some 40- 60 miles downwind from mass ignition burns. One can infer that smoke transported above the mixing layer re-enters the mixing layer some distance downwind from the burns and is transported back to the ground in high concentrations. Dr. Achtemeier has named the process "plume collapse".

 

Method

A set of airborne missions must be conducted to observe mass ignition smoke plume behavior. These will be photographic missions designed to document the life histories of the smoke plumes within the ambient meteorological environment. The purpose of the missions will be to gain answers to the following questions:

Does plume collapse happen?

Observers in the field speak of smoke "coming down". Is this just a perception of observers with perspective limited from the ground or is plume collapse a real phenomenon? The primary purpose of the aircraft mission will be to document plume collapse in action.

What causes plume collapse?

a) Plume sedimentation. Sedimentation velocities cause smoke to sink back into the mixed layer. Larger soot particles, which are more easily seen from the ground, may settle out of the transport layer into the mixing layer and return to the ground. Smaller particles including PM2.5 remain within the transport layer.

b) Radiative cooling. Radiative cooling of the particulates within the smoke plume cools the surrounding air causing the plume to subside back into the mixed layer. This process occurs at night and may be responsible for plume descent after dark.

c) Growth of the mixed layer. Mass ignition burns typically are begun in late morning and run through much of the afternoon. However, during the day solar heating deepens and warms the mixing layer. Thus the mixing layer should be deeper late in the afternoon in comparison with its depth in the morning. Therefore, the top of the mixing layer (the mixing height) may grow above a plume initially trapped in the transport layer, allowing for the plume to be transported back to the ground.

d) Clear air convection. Convection in clear air is what causes smoke plumes to loop. Clear air convection is hard to observe and harder to measure. Clear air convection can be envisioned as a field of vertically oriented three-dimensional circulations that drift with the mean wind of the layer containing the circulations. The hypothesis is that the intense convection of the mass ignition smoke plume destroys clear air convection, leaving a relatively undisturbed wake within the mixed layer downwind from the plume. Vertical circulations redevelop some distance downwind and circulate the plume to the ground.

e) The urban heat island. Increased absorption of solar energy by buildings and paved surfaces in urban areas raises the surface temperature over cities––the well–known heat island effect. The warmer temperatures over the cities heat the mixing layer and locally deepen the mixing layer over cities. Smoke plumes confined to the transport layer may enter the deeper mixing layer over cities and be brought to the ground over urban areas.

f) Isentropic advection. Isentropic advection is a well-documented meteorological process. Air does not move horizontally, but it is transported along surfaces of constant potential or isentropic surfaces. Thus smoke trapped in the transport layer will move along isentropic surfaces. In the event the isentropic surfaces intersect the top of the mixing layer, smoke returns to the mixing layer and hence to the ground. In some instances, particularly at night, isentropic surfaces may intersect the ground. Then smoke in great concentration can be brought directly to the ground from the transport layer.