Forecasting fire-generated thunderstorms

In this training session recorded in November 2019, Dr Kevin Tory (Bureau of Meteorology) introduces a tool to aid in the forecasting of fire-generated thunderstorms or pyrocumulonimbus (pyroCb).

The tool is built around a concept termed the PyroCb Firepower Threshold (PFT), which seeks to identify the minimum firepower required to generate pyroCb for a given atmospheric environment.

The video includes a presentation that introduces the PFT, explains how it works and gives examples of PFT usage. It provides valuable insight into plume dynamics, and a deeper understanding of the types of environments that support or suppress pyroCb formation. The video also includes instructions on how to calculate the PFT manually from atmospheric soundings on a thermodynamic diagram.

What is a pyroCb forecast?

Pyrocumulonimbus (pyroCb) formation has much in common with conventional thunderstorms. Both require warm humid air to be lifted into an unstable layer above. A sufficiently large fire provides this lift and it boosts the temperature and humidity of the lifted air. While meteorologists have developed great skill in forecasting conventional thunderstorms, pyroCb prediction remains a substantial challenge, because of the difficulty in anticipating the temperature and humidity boost and lift for a given fire. The PFT offers a method to diagnose the minimum firepower required for pyroCb development, which can be presented as forecast maps to help forecasters identify how the atmospheric support for pyroCb formation varies in space and time.

Introducing a pyroCb prediction tool

Pyrocumulonimbus (fire-induced thunderstorms, pyroCb) are associated with unpredictable changes in fire intensity, spread rates and direction, enhanced ember transport and lightning ignitions. Conventional thunderstorm threats such as downbursts, hail, lightning, and tornadoes may also be present.

In favourable atmospheric conditions, suitably large and hot fires can produce pyroCb cloud in the form of deep convective columns with many similarities to conventional thunderstorms. They may be accompanied by strong inflow, dangerous downbursts and lightning strikes, which 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.

Bushfire and Natural Hazards CRC research has developed a method for determining how favourable the atmospheric environment is for pyroCb development. This method is combined with a plume-rise model (originally developed for pollutant dispersion prediction) to determine how much heat a fire must produce 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’. A theoretical minimum firepower required for pyroCb to develop in a given atmospheric environment is calculated, 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. Preliminary studies have shown that the PFT can vary substantially from day to day, and that days that favour pyroCb formation do not necessarily favour large-hot fires. A PFT-flag is also under development that identifies when both pyroCb and large-hot fires are favourable.

A series of CRC studies beginning with a little ‘blue-sky’ research into the thermodynamics of smoke plumes, led to the ability to identify potential condensation heights in plumes, and the minimum plume buoyancy required for plumes to freely convect to the electrification level. With this knowledge, equations for a theoretical minimum firepower required for pyroCb formation were derived using the Briggs plume model (PyroCb Firepower Threshold, PFT).

The work has culminated in the development of a diagnostic that seeks to determine when the atmosphere is conducive to both deep plume development and large, hot fires (PFT-flag). Originally designed as a flag to alert users when to examine the PFT, the PFT-flag may prove to be a more valuable prediction tool than the PFT itself.

It was developed and tuned to identify the atmospheric conditions corresponding to two pyroCb events at opposite ends of the pyroCb spectrum. The first (Sir Ivan fire, NSW 2017) occurred in catastrophic fire weather conditions when pyroCb formation conditions were not especially favourable. The second (Licola fire, Victoria 2019) occurred in much milder fire weather conditions when plume formation conditions were considerably more favourable.

The PFT-flag has now been applied to more than 20 cases covering multiple days and time periods. While no rigorous performance assessment has yet been made the tool appears to be working surprisingly well. It not only identifies days of pyroCb occurrence, but also reproduces the diurnal variation in pyroCb threat, plus variations in threat associated with atmospheric features such as troughs, fronts and sea-breezes.

Both the PFT and PFT-flag are under continued development with real-time testing during southern Australian fire seasons.

Read the full conference paper on the Pyrocumulonimbus Firepower Threshold here. Download the full non-peer reviewed research proceedings from the Bushfire and Natural Hazards CRC Research Forum here.

To learn more about the research Dr Tory and his team are doing with the Bushfire and Natural Hazards CRC visit the Improved predictions of severe weather to reduce community impact project page.