By: T.M. Giannaros, G. Papavasileiou
On June 29, 2021, Canada recorded its highest ever temperature (49.6 ºC, Lytton, British Columbia), as the western coast of North America experienced an unprecedented heatwave. The heatwave itself was associated with the persistence of an abnormal high-pressure ridge over Canada’s and the US’s west coast. Prolonged warm and dry conditions increased landscape flammability to above critical levels, eventually allowing for the ignition of several wildfires (June 30, 2021). These wildfires ignited and began propagating in an environment conducive to extreme fire behavior. Indeed, several pyrocumulus (PyroCu) and pyrocumulonimbus (PyroCb) clouds were observed, indicating extreme fire behavior (Images 1, 2). Such types of clouds are formed by convection induced by the intense heating of the atmosphere caused by wildfires.
What is pyroconvection, and how does it relate to extreme fire behavior?
The term pyroconvection describes the capacity of the atmosphere to form PyroCu or/and PyroCb clouds. Pyroconvection itself is directly related to the heat and moisture fluxes of a wildfire. A parcel of air that is warmer and moister than its surrounding environment is less dense and subject to an upward buoyant force. A wildfire releases significant amounts of heat and moisture that ultimately force the air above it to begin rising rapidly. As the hot air rises, cooler air from around the wildfire is drawn in to compensate for the void. The cooler air provides additional oxygen that intensifies combustion, enhances the upward flow of hot air, and eventually creates a dynamic feedback loop. If the hot air rises sufficiently high, water vapor condenses and pyro-clouds begin forming. While this circulation can occur in any wildfire, PyroCu and PyroCb clouds will only form if the atmosphere is unstable enough to enhance vertical motions. The strong, decreasing temperature gradient of an unstable atmosphere allows for sustaining and enhancing the vertical movement of the rising hot air.
Pyroconvection, as manifested by the formation of pyroclouds, indicates that fire suppression capacity may be overwhelmed by unpredictable changes in fire behavior. PyroCb clouds may generate lightning, setting up new ignitions that may lead to firefighting traps. When present, pyroconvection also poses a significant threat to aerial firefighting means, which may have to cope with strong updrafts that limit their operational capacity. Last, processes related to the occurrence of pyroconvection can significantly intensify the surface winds driving fire spread, also modifying their spatial variations.
Is pyroconvection the new normal in wildfire activity?
Pyroconvection is not something new. However, pyroconvective wildfires, such as the British Columbia ones, are becoming a globally increasing concern. Recent research suggests that their frequency, extent, and impacts may be rising due to changes in atmospheric conditions associated with climate change. As global air temperature increases, future atmospheric conditions are projected to promote instability and hence favor pyroconvection. Along with robust evidence of longer and more intense droughts, which precondition fuels by increasing their flammability, it becomes clear that pyroconvective wildfires will be more likely in the future. In fact, we are already witnessing this.
Have we witnessed pyroconvection in Greece?
Evidence of pyroconvective wildfires in Greece is mostly concentrated in recent years. It is unclear if this is related to the extended use of social media, which allows for the easier and quicker exchange of information, or to a robust change in the atmospheric conditions that favor pyroconvection.
Pyroconvection was observed in: (1) the 2006 Chalkidiki wildfire, (2) the 2017 Kalamos wildfire (Image 3), (3) the 2020 Kechries wildfire (Image 4), and (4) the 2021 Schinos wildfire (Image 5).