Fire-Modified Meteorology in a Coupled Fire–Atmosphere Model

704 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 54 Fire-Modified Meteorology in a Coupled Fire–Atmosphere Model MIKA PEACE Bushfire Cooperative Research Centre, Melbourne, Victoria, and Applied Mathematics, Adelaide University, and Bureau of Meteorology, Adelaide, South Australia, Australia TRENT MATTNER AND GRAHAM MILLS Applied Mathematics, Adelaide University, South Australia, Australia JEFFREY KEPERT Bureau of Meteorology, Adelaide, South Australia, and Bushfire Cooperative Research Centre, Melbourne, Victoria, Australia LACHLAN MCCAW Department of Parks and Wildlife, Manjimup, Western Australia, Australia (Manuscript received 19 March 2014, in final form 27 November 2014) ABSTRACT The coupled fire–atmosphere model consisting of the Weather and Forecasting (WRF) Model coupled with the fire-spread model (SFIRE) module has been used to simulate a bushfire at D’Estrees Bay on Kangaroo Island, South Australia, in December 2007. Initial conditions for the simulations were provided by two global analyses: the GFS operational analysis and ERA-Interim. For each NWP initialization, the simulations were run with and without feedback from the fire to the atmospheric model. The focus of this study was examining how the energy fluxes from the simulated fire modified the local meteorological environment. With feedback enabled, the propagation speed of the sea-breeze frontal line was faster and vertical motion in the frontal zone was enhanced. For one of the initial conditions with feedback on, a vortex developed adjacent to the head fire and remained present for over 5 h of simulation time. The vortex was not present without fire–atmosphere feedback. The results show that the energy fluxes released by a fire can effect significant changes on the surrounding mesoscale atmosphere. This has implications for the appropriate use of weather parameters extracted from NWP and used in prediction for fire operations. These meteorological modifications also have implications for anticipating likely fire behavior. 1. Introduction D’Estrees fire relative to the three other fires burning nearby. On the MODIS satellite image of the previous The D’Estrees fire was one of several fires ignited by afternoon (not shown), the smoke plumes from each of lightning on Kangaroo Island in early December 2007. the four fires were similar in size and opacity. Figure 1 shows the island’s location, off the coast of The heightened fire activity over the D’Estrees fire southern Australia. Figure 2 shows the fire activity on was explored in the case study by Peace and Mills 8 December. Figure 2 shows four fires burning on the (2012), who proposed that the increased fire activity at island; however, the D’Estrees fire, at the southeast the D’Estrees fire on this day was due to it being located corner of the island, has a smoke plume that is very in a sea-breeze convergence zone, and that interactions different from the other three fires: the plume is much between the fire and atmosphere played an important larger and more opaque. This event is of interest be- role in producing the extensive smoke plume. This study cause the smoke indicates increased fire activity at the explores the fire and atmosphere interaction processes by examining how the energy released by the D’Estrees Corresponding author address: Mika Peace, Bureau of Meteo- rology South Australia Regional Office, 25 College Rd., Kent fire modified the surrounding atmosphere. To address Town, Adelaide, SA 5071, Australia. the subject of fire–atmosphere feedback by examining E-mail: [email protected] observational data is challenging because of the DOI: 10.1175/JAMC-D-14-0063.1 Ó 2015 American Meteorological Society Unauthenticated | Downloaded 10/06/21 01:06 PM UTC MARCH 2015 P E A C E E T A L . 705 FIG. 1. Map of southern South Australia showing location of nested domains used in the WRF simulations over Kangaroo Island. Color shading is topography (m). difficulties involved in making comprehensive meteo- The premise underlying this study is that a fire can rological observations at a fire ground and of making modify the dynamical structure of the surrounding at- controlled experiments. However, an alternative ap- mosphere. This is consistent with observations from fire proach is examining the results of high-resolution cou- grounds, which frequently report local weather in the pled fire–atmosphere simulations, as these provide an vicinity of the fire as different from prevailing conditions opportunity for examining how a fire alters the structure in the broader area. This modification of local meteo- of the surrounding atmosphere. The aim of the current rology due to the fire’s influence includes elevated study is to use numerical simulations to better un- temperatures, as well as variation in both speed and derstand fire and atmosphere interactions in the context direction of wind in the vicinity of the fire, along with of the D’Estrees fire. pyrocumulus (or pyrocumulonimbus) cloud formations. FIG. 2. NOAA MODIS Aqua satellite image from 1535 LT 8 Dec 2007. Red pixels show active fire area. Unauthenticated | Downloaded 10/06/21 01:06 PM UTC 706 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 54 The modification to local meteorology is driven by heat This paper documents the first of a series of three and water vapor released in the combustion process WRF and SFIRE simulations of Australian fire events. generating increased buoyancy in the near-fire environ- The two companion studies are of the Rocky River and ment. Potter (2005) describes interactions between a fire’s Layman fires. The meteorology of the events is described convection column and the surrounding atmosphere. in the case studies by Peace and Mills (2012) and Peace An extreme example of dynamical interactions be- et al. (2012). These three fires were selected because tween a fire and the surrounding atmosphere is the fire unusual fire behavior occurred at each one. Coupled tornado that developed at the 2003 Canberra bushfires. simulations have been used to examine the interactions The event is documented by McRae et al. (2013) and between the fire and surrounding environment and to test simulated in highly idealized form by Cunningham and the hypothesis developed in the meteorological case Reeder (2009). The D’Estrees fire simulated in this studies that fire–atmosphere interactions are a mecha- study is a much smaller fire than the Canberra event, nism for producing unexpected fire behavior. burning in more moderate meteorological conditions. We proceed by describing the WRF and SFIRE These simulations are an example of how a fire can model and the configuration used in our simulations, modify the local meteorology in a realistic (as opposed followed by the results. In the discussion we focus on the to idealized) atmosphere, and they provide insights into simulation results of development of a vortex and re- dynamic interaction processes that may occur during location of a sea-breeze wind change. To conclude, we a real event. discuss implications for fire weather forecasts and de- This study uses the coupled Weather Research and velopment of fire simulation models, as well as consid- Forecasting (WRF) Model and fire-spread model (SFIRE) ering some of the limitations of our approach. module, described in detail by Mandel et al. (2011).WRF and SFIRE couple the WRF Model with an implemen- 2. WRF and SFIRE tation of the Rothermel (1972) fire-spread equations. Coupled fire–atmosphere models have been used in Several models that integrate a fire simulation model a number of studies to show that dynamical feedback with an atmospheric model are in use, including Méso- processes, in particular the fire-modified winds, have an NH–‘‘ForeFire’’ (Filippi et al. 2011), ‘‘FIRETEC’’ important influence on how a fire perimeter evolves. (Linn et al. 2002), and Wildland-Urban Fire Dynamics Coupled simulations that examine the influence of Simulator (WFDS; Mell et al. 2010). The coupled model the fire-modified winds include Clark et al. [1996; using used in this study is WRF and SFIRE, chosen as it is ac- the Coupled Atmosphere–Wildland Fire–Environment cessible and well supported, and it provides the oppor- (CAWFE) model, the predecessor to WRF and SFIRE], tunity to examine in detail the meteorological dynamics Coen (2005), Coen et al. (2013), Coen and Riggan (2014), of a fire environment. The WRF Model, described by and Kochanski et al. (2013a), all using model releases of Skamarock et al. (2008),isanumericalweatherprediction WRF and SFIRE or wildland fire module WRF-Fire. model with a range of chemistry and physics extensions The focus of this study is somewhat different from and user-defined initialization options. WRF is widely previous coupled simulations, as the emphasis is on used for meteorological operational and research pur- examining how the fire modifies the surrounding meso- poses. The SFIRE model is a two-dimensional fire-spread scale atmosphere, rather than assessing how the in- model coupled to WRF, described by Mandel et al. (2011). clusion of fire–atmosphere feedback alters the predicted The simulations shown here were made with the most fire perimeter. The main objective of this study was to recent available model release, which at the time was WRF explore how the energy fluxes from the fire affected the and SFIRE, released by the SFIRE group and sourced in surrounding atmosphere. Our simulated fire spread was mid-2012 (the link that was used is no longer active; the intended to be a reasonable approximation of actual current version is https://github.com/jbeezley/wrf-fire). fire spread in order for the energy release to be realistic. A detailed description of how the fire model and at- However, we did not aim to reproduce observations of mospheric model interact is given in Mandel et al. fire perimeter or provide a detailed verification, since (2011). In summary, at each time step of the atmospheric observations of the event are very limited, thereby re- simulation, fire progression and fuel burnt are calculated stricting verification possibilities.

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