Pyrocumulonimbus Firepower Threshold: A pyrocumulonimbus prediction tool.
In favourable atmospheric conditions, suitably large and hot fires can produce pyrocumulonimbus (pyroCb) cloud in the form of deep convective columns with many similarities to conventional thunderstorms, which may be accompanied by strong inflow, dangerous downbursts and lightning strikes. These in turn may enhance fire spread rates and fire intensity, cause sudden changes in fire spread direction, and the lightning may ignite additional fires. Dangerous pyroCb conditions are not well understood and can be very difficult to forecast. In recent BNHCRC research, a method for determining how favourable the atmospheric environment is for pyroCb development was developed. This method is combined with a plume-rise model (originally developed for pollutant dispersion prediction) to determine how hot a fire must be for pyroCb to develop in a given atmospheric environment. More specifically, this fire heat is the rate at which heat enters the fire plume (which has units of power), often termed the "power of the fire" or "firepower". Thus, we identify a theoretical minimum firepower required for pyroCb development in a given atmospheric environment, termed the Pyrocumulonimbus Firepower Threshold (PFT). Forecast spatial plots of PFT are being trialled that provide an indication of how the favourability of the atmosphere for pyroCb development varies in space and time over typical weather forecast periods. It is anticipated that such plots will provide useful guidance for fire weather forecasters and fire agencies. A number of case studies will be presented to demonstrate the value of using PFT for pyroCb prediction.