Preprints
https://doi.org/10.5194/nhess-2022-116
https://doi.org/10.5194/nhess-2022-116
 
04 May 2022
04 May 2022
Status: this preprint is currently under review for the journal NHESS.

The 2017 Split wildfire in Croatia: Evolution and the role of meteorological conditions

Ivana Čavlina Tomašević1,2,3, Kevin K. W. Cheung4, Višnjica Vučetić2, Paul Fox-Hughes5, Kristian Horvath2, Maja Telišman Prtenjak3, Paul J. Beggs1, Barbara Malečić3, and Velimir Milić2 Ivana Čavlina Tomašević et al.
  • 1School of Natural Sciences, Faculty of Science and Engineering, Macquarie University, Sydney, NSW 2109, Australia
  • 2Croatian Meteorological and Hydrological Service, Ravnice 48, 10 000 Zagreb, Croatia
  • 3Department of Geophysics, Faculty of Science, University of Zagreb, Horvatovac 95, 10 000 Zagreb, Croatia
  • 4E3 -Complexity Consultant, Eastwood, NSW 2122, Australia
  • 5Bureau of Meteorology, Level 5, 111 Macquarie St, Hobart, TAS 7001, Australia

Abstract. The Split wildfire in July 2017, which was one of the most severe wildfires in Croatian history of this World Heritage site, is the focus in this study. The Split fire is a good example of wildfire-urban interface, with unexpected fire behavior including rapid downslope spread to the coastal populated area. Thus, it is critical to clarify the meteorological conditions behind the fire event, those that have limited the effectiveness of firefighting operations and the rapid escalation and expansion of the fire zones within thirty hours.

First, the Split fire propagation was reconstructed using radio logs, interviews with firefighters and pilots involved in the intervention, eye-witness statements, digital photographs from fire detection cameras, media and firefighting monthly journal. Four phases of fire development have been identified. Then, weather observations and numerical simulations using an enhanced-resolution operational model are utilized to analyze the dynamics in each phase of the fire runs. The synoptic background of the event includes large surface pressure gradient between the Azores anticyclone accompanied by cold front and a cyclone over southeastern Balkan Peninsula. At the upper level, there was a deep shortwave trough extending from the Baltic Sea to the Adriatic Sea, which developed into a cut-off low.

Such synoptic conditions have resulted in the annual maximum of Fire Weather Index and the highest monthly severity rating for July in the period 1981–2020. Combined with topography, they also provoke locally the formation of the strong northeasterly bura wind along the Adriatic coast. During the fire event, wind gust of nearly 25 m s-1 occurred. Low level jet (LLJ) has also been formally identified during an extended period, with a peak prior to the fire event possessing wind speed of over 21 m s-1 at a height of 600–700 m. Analysis of the upper-level jet also reveals that there was a deep tropospheric bura, which has facilitated the subsidence of dry air from the upper troposphere. In the mid to lower level, gravity wave breaking and turbulence mixing (as in the hydraulic jump theory) in the downslope bura wind further enabled the rapid drying at the surface.

Low level jet and strong downslope wind such as the bura are known to be related to many severe wildfire events worldwide, besides the antecedent hot and dry weather conditions and fuel loads. As has been demonstrated in this study, numerical guidance that indicates the spatial and temporal occurrence of low level jet is highly implicative to explain the Split fire evolution from the ignition potential to its extinguishment stage. Thus, in addition to the conventional fire weather indexes, such products are able to improve fire weather behavior forecast and in general more effective decision-making in fire management.

Ivana Čavlina Tomašević et al.

Status: open (until 15 Jun 2022)

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Ivana Čavlina Tomašević et al.

Ivana Čavlina Tomašević et al.

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Short summary
One of the most severe and impactful urban wildfire events in Croatia history has been reconstructed and analysed. The study identified some important meteorological influences related to the event: the synoptic conditions of the Azores anticyclone, cold front and upper-level shortwave trough all led to the highest fire weather index in 2017. Low-level jet, locally known as bura wind that can be explained by hydraulic jump theory, was the dynamic trigger of the event.
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