Thermodynamics of Pyrocumulus: A Conceptual Study

In favorable atmospheric conditions, fires can produce pyrocumulonimbus cloud (pyroCb) in the form of deep convective columns resembling conventional thunderstorms, which may be accompanied by strong inflow, dangerous downbursts, and lightning strikes that can produce dangerous changes in fire behav...

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Bibliographic Details
Published in:Monthly weather review 2018-08, Vol.146 (8), p.2579-2598
Main Authors: Tory, Kevin J., Thurston, William, Kepert, Jeffrey D.
Format: Article
Language:English
Subjects:
Atmospheric boundary layer
Atmospheric conditions
Buoyancy
Clouds
Computer simulation
Condensates
Condensation
Convection
Dilution
Downbursts
Environmental conditions
Environmental factors
Fire behavior
Fire plumes
Fires
Forest & brush fires
Free convection
Gases
Height
Humidity
Induction heating
Inflow
Lifting condensation level
Lightning
Lightning strikes
Meteorology
Mixed layer
Moisture
Oceanic eddies
Plumes
Ratios
Research centers
Saturation
Stratosphere
Temperature
Temperature effects
Thermodynamics
Thunderstorms
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Summary:In favorable atmospheric conditions, fires can produce pyrocumulonimbus cloud (pyroCb) in the form of deep convective columns resembling conventional thunderstorms, which may be accompanied by strong inflow, dangerous downbursts, and lightning strikes that can produce dangerous changes in fire behavior. PyroCb formation conditions are not well understood and are difficult to forecast. This paper presents a theoretical study of the thermodynamics of fire plumes to better understand the influence of a range of factors on plume condensation. Plume gases are considered to be undiluted at the fire source and approach 100% dilution at the plume top (neutral buoyancy). Plume condensation height changes are considered for this full range of dilution and for a given set of factors that include environmental temperature and humidity, fire temperature, and fire-moisture-to-heat ratios. The condensation heights are calculated and plotted as saturation point (SP) curves on thermodynamic diagrams. The position and slope of the SP curves provide insight into how plume condensation is affected by the environment thermodynamics and ratios of fire heat to moisture production. Plume temperature traces from large-eddy model simulations added to the diagrams provide additional insight into plume condensation heights and plume buoyancy at condensation. SP curves added to a mixed layer lifting condensation level on standard thermodynamic diagrams can be used to identify the minimum plume condensation height and buoyancy required for deep, moist, free convection to develop, which will aid pyroCb prediction.
ISSN:0027-0644
1520-0493
DOI:10.1175/MWR-D-17-0377.1