Analysis of extreme fire weather during catastrophic wildfires in Croatia and Australia

The main focus of this study was to investigate fire weather of two catastrophic wildfires, one from Croatia, which is a part of one of the world’s most fire prone areas – the Mediterranean basin and other from Tasmania, a part of well-known fire continent – Australia. The Croatian case study included the Split wildfire in July 2017, the most severe wildfire in history given the size and unexpected fire behavior, which produced the downslope fire runs into the densely populated area while the Australian case study included the Forcett-Dunalley wildfire in January 2013, which caused vast destruction, rapid fire spread and generated firestorm in form of pyroCb, the first on record in Tasmania. Meteorological analyses of wildfires were preceded by their reconstruction and definition of the most severe burn periods in order to associate atmospheric conditions and fire behavior in detail. The study draws similarities between dynamic atmospheric processes and mechanisms that occurred in chosen wildfire cases and implemented a coupled fire-atmosphere model for the first time in Croatia. The study also contains comprehensive literature review of fire weather meteorology together with past and future climate influencing fire risk at the Adriatic coast in Croatia and in southeast Australia, including Tasmania. The research has showed that both wildfires were wind driven from the ignition due to wildfires’ locations situated in the area of tight pressure gradient which resulted in strong gusty surface wind. In the case of Split, the ignition of the wildfire coincided with an episode of strong downslope bura windstorm, while in Dunalley case wildfire occurred at the peak of the heatwave and right before the cold front passage. The antecedent conditions in both cases included the drier and warmer-than-average periods in months’ prior the wildfires, which contributed to continued drying out fuels in the area and had an impact on fire danger rating. In particular, the FWI reached its annual maximum exactly on the day of the Split wildfire, while in the case of Dunalley FFDI reached ‘catastrophic’ category and got close to the all-time state record. Important findings on atmospheric dynamics in the Split case of included long amplitude and shortwave upper-level trough, which caused the cool and dry air outbreak and produced a deep northeasterly bura flow. Upper-level features of the deep bura flow included hydraulic jump, dry air subsidence and low-level jet (LLJ). This research is the first known to present spatial distribution of LLJ. The explosive pyroCb development in the case of the Dunalley was triggered by the highly unstable atmosphere and the line of convergence over the wildfire’s area in the hours prior to the cold front passage. Also, combustion processes within the escalated wildfire further enhanced instability and contributed to the blow-up up to 12 km height, in spite of the strong jet stream at the tropopause. Enhanced fuel consumption in combination with complex local topography is found to play an important role in the total fire escalation in the Split case as well.