Near Real-Time Extracting Wildfire Spread Rate from Himawari-8 Satellite Data

Fire Spread Rate (FSR) can indicate how fast a fire is spreading, which is especially helpful for wildfire rescue and management. Historically, images obtained from sun-orbiting satellites such as Moderate Resolution Imaging Spectroradiometer (MODIS) were used to detect active fire and burned area at the large spatial scale. However, the daily revisit cycles make them inherently unable to extract FSR in near real-time (hourly or less). We argue that the Himawari-8, a next generation geostationary satellite with a 10-min temporal resolution and 0.5–2 km spatial resolution, may have the potential for near real-time FSR extraction. To that end, we propose a novel method (named H8-FSR) for near real-time FSR extraction based on the Himawari-8 data. The method first defines the centroid of the burned area as the fire center and then the near real-time FSR is extracted by timely computing the movement rate of the fire center. As a case study, the method was applied to the Esperance bushfire that broke out on 17 November, 2015, in Western Australia. Compared with the estimated FSR using the Commonwealth Scientific and Industrial Research Organization (CSIRO) Grassland Fire Spread (GFS) model, H8-FSR achieved favorable performance with a coefficient of determination (R2) of 0.54, mean bias error of –0.75 m/s, mean absolute percent error of 33.20% and root mean square error of 1.17 m/s, respectively. These results demonstrated that the Himawari-8 data are valuable for near real-time FSR extraction, and also suggested that the proposed method could be potentially applicable to other next generation geostationary satellite data.


Introduction
Wildfires are the essential factor in the formation and evolution of ecosystems [1,2].They shape the creation of a natural landscape, ensure the diversity and stability of the organism and change the biophysical characteristics of the soil [1,[3][4][5].Additionally, wildfires are the natural disaster that can cause severe economic, environmental and social losses [6][7][8][9][10].According to the Emergency Events Database (EM-DAT), the global annual average economic losses due to wildfires by 2015, reached 2677 million US dollars, and around six million people are affected worldwide, every year [11,12].These numbers highlight the importance of a quantitative understanding of wildfire behavior.Fire Spread Rate (FSR) is a fundamental parameter for estimating wildfire behavior, flame height, flame length, fire line intensity, duration of the fire, etc. [13][14][15][16].Wildfires typically spread forward with a rapid FSR in areas with dense, unbroken vegetation.Estimating near real-time FSR during wildfires can provide timely useful information for fire monitoring and management and thus improve the efficiency of operational firefighting [17][18][19].
Several methods have been applied to estimate FSR in the last few decades.These methods can be divided into two categories: traditional (e.g., laboratory or field observation, fire spread model estimation) and remotely sensing based methods (e.g., extraction from satellite and airborne images).The field or laboratory experiments generally estimate FSR by using ocular observations, visible or infrared spectrum images and thermocouple instruments [20].These methods are supposed to be highly accurate.On the other hand, they also have the disadvantage of being site-specific, low efficiency, and therefore impossible to be extended to larger scales [20].Fire spread models can be further classified into two categories: empirical statistical and physical models [21,22].Empirical statistical models make use of observed data generated from the field or laboratory experiments to establish the relationship between FSR and fuel-, topographical-, and meteorological-related factors [23].These methods are simple and computationally efficient and have been developed for fuels ranging from grasslands to shrublands and forests in different regions [16,[24][25][26][27].However, empirical approaches tend to be site-specific and data-dependent, therefore requiring frequent re-calibrations ensuring these are applicable to new circumstances (e.g., more severe fire as a result of climate change or different fuel arrangements).To that end, empirical approaches lack generality [28].Physical models, such as the FIRETEC [29], AIOLOS-F [30], FIRESTAR [31] and WFDS [32] are more robust since they take into account the mechanism of fire propagation by combining thermodynamics, air-dynamics and botany [33].However, calculating the balanced equations of the conservation of energy and momentum is usually time-consuming.Wildfires tend to burn in more extreme conditions and uncertain locations [34] which makes near real-time FSR estimation using traditional methods more difficult.Remote sensing technology can directly obtain information from the ground and provide a unique, cost-effective source of information for a spatial and temporal understanding of wildfire behavior.Satellite products widely used for extracting the FSR are the Moderate Resolution Imaging Spectroradiometer (MODIS) global scale active fire (MCD14ML) and burned area (MCD64A1 and MCD45A1) products.For example, Loboda et al. [35] used MODIS Terra (MOD14) and Aqua (MYD14) satellite products to reconstruct the fires that occurred in Siberia between 2001 and 2004 and then obtained historical annual maps of FSR.Based on the morphological spread structure of each fire event, Benali et al. [19] used the spatial-temporal information of the MODIS active fire product (MCD14ML) and the official burned area database for Portugal between 2001 and 2009 to get the major fire routes and correspondent mean spread directions.However, the temporal resolution of the solar orbit MODIS products is one-day or longer [36] which makes the retrieval of near real-time FSR unfeasible.Some studies have used sequential airborne data to extract the FSR [34].However, the flight of an airplane is generally limited in time and space.
Compared with solar orbit satellites, geostationary satellites provide higher temporal resolution, usually ranging from 10 to 30 min [37,38].Therefore, these satellites have the potential to provide near real-time monitoring of wildfire behavior.Zhang et al. [39] proposed an algorithm to simulate the diurnal burned area using the Geostationary Operational Environmental Satellites (GOES) active fire data.The results demonstrated its potential value in near real-time burned area estimation.Calle and Laneve et al. [40,41] successfully used data from the Meteosat Second Generation (MSG) Spinning Enhanced Visible and Infrared Imager (SEVIRI) with a temporal resolution of 15 min, to detect forest fires in near real-time in Greece and Portugal.Kim et al. [42] used the Korean geostationary meteorological satellite to identify wildfires in Korea in 2013 with an overall accuracy of over 85% of the fire activity detected.Zhang et al. [38] used multi-geostationary satellites (GEOS-West and East, MSG SEVIRI and Multifunctional Transport Satellite (MTSAT)) to obtain hourly data on global biomass burning emissions products.However, the spatial resolution of the geostationary data used in these studies was generally above 4 km, making them suitable only for extremely large-scale wildfire detection and limiting the near real-time FSR extraction.
With the launch of the Himawari-8 stationary meteorology satellite on October 17th 2014, near-infrared (NIR) and infrared band images with a temporal resolution of ten minutes (data distribution with 15-20-min latency) and higher spatial resolution of 2 km were available (500 m for the red band and 1 km for other visible bands) [43], making the near real-time monitoring and detection of fire behaviors more feasible.Fatkhuroyan et al. [44] used Sataid software [45] to combine the Red-Green-Blue (RGB) image of Himawari-8 data from channel 3, 4 and 6, to detect forest fire smoke in Indonesia from August to October 2015.Xu et al. [46] also presented a straightforward algorithm based on the Himawari-8 data to detect and track near real-time wildfires in Australia.Xu et al. [47] applied the geostationary FRP (Fire Radiative Power) algorithm designed for the MSG SEVIRI to Himawari-8 and other geostationary sensors, such as Feng Yun-2 (FY-2) and MTSAT.The latter study showed that data from the Himawari-8 achieved the best results.Wickramasinghe et al. [48] added NIR and red reflectance to a traditional active fire detection algorithm and developed a multi-spatial resolution approach for Himawari-8, which could map fire lines with a 500 m spatial resolution.However, up to now there have been no studies using the Himawari-8 data for near real-time FSR extraction.
This paper presents the first study to assess the capability of Himawari-8 data in estimating near real-time FSR.To that end, a novel method is presented based on the movement rate of the fire center that is extracted from Himawari-8 (H8-FSR).The method was applied to the Esperance bushfire which ignited on 17 November 2015, in Western Australia.The results were compared with the FSR obtained from the CSIRO GFS model broadly used by Australian rural fire authorities.

Study Area
The Esperance bushfire happened in Esperance (121 Esperance is one of the harbor cities located near the South Indian Ocean Coast of Western Australia (Figure 1).The study area is situated in a typical Mediterranean climate consisting of high temperatures from October to March.The average monthly precipitation is approximately between 18.9-97.0mm, with an average afternoon relative humidity of around 58%. Esperance is flat with an average elevation of 10.8 m.The Esperance fire burned more than 128,000 hectares of crops, shrubs, and grasslands and resulted in the deaths of four people as well as serious damage to facilities.

Himawari-8 Data
Himawari-8 is a next-generation geostationary meteorological satellite from the Himawari series satellites designed and manufactured by Japan Aerospace Exploration Agency (JAXA) [49].It was launched on 17 October, 2014 and began generating data on 7 July, 2015.Compared with previous generations of satellites e.g., MTSAT, the Himawari-8 offers improved spatial (500 m-2 km) and temporal (10 min) resolutions.Himawari-8 is also the world first still-weather satellite with the capability of capturing color images [49].It has 16 observation bands distributed over the range of visible (VIS), NIR and thermal infrared (TIR), in which bands 7 (3.9 µm) and 14 (11.2 µm) are most useful in detecting active fire and burned area.In addition, the Himawari-8 satellite has a sound positioning accuracy, with 0.5 pixels for the VIS and NIR bands [49].The Advanced Himawari Imager (AHI) carried by Himawari-8 satellites scan the Pacific Ocean hemispheric region every 10 min [49], which can result in the production of 142 images per day.This further enhances the capability for continued ground observation.In this study, a total of 37 scenes (Level 1 Gridded full disk) of Himawari-8 data taken by AHI on 17 November 2015 were used.The Coordinated Universal Time (UTC) time of the data is from 03:00 to 09:00, corresponding to Australia Western Standard Time (AWST) from 11:00 to 17:00.Himawari-8 data were downloaded from JAXA's website [50].

Meteorological Data
Meteorological data are also essential inputs to the CSIRO GFS model.In this study, the ground-based meteorological observation data (wind speed and direction, temperature and relative humidity) from the three stations nearest (Figure 1) to the Esperance wildfire (ESPERANCE AERO (No.956380), SALMON GUMS RES.STN (No. 956390), MUNGLINUP WEST (No.956440)) were downloaded from the Australian Bureau of Meteorology [51].The data has a temporal resolution of 10-min, which is in accordance with the Himawari-8 data.

Methodology
The methodology in this study can be divided into two stages (see Figure 2): (i) The extraction of near real-time FSR (H8-FSR), consisting of data preprocessing, near real-time burned area detection, near real-time fire center extraction, and near real-time FSR calculation; and (ii) The comparison of H8-FSR results with those obtained using the CSIRO GFS Model.

Data Preprocessing
The Himawari-8 data downloaded from the JAXA were in the 'NetCDF' format, therefore, data preprocessing was needed prior to operational use.This included converting the format, writing geographic information, clipping and re-projecting onto the Geocentric Datum of Australia (GDA) 1994 South Australia Lambert coordinates.

Near Real-Time Burned Area Detection
In this study, the burned area data were identified based on each Himawari-8 image using Equation (1).From this equation, the burned area data at timestamp i equates to the sum of the active fire data in the past two hours.The two-hour interval was found to be suitable for the area chosen in this study.However, the suitability of this time interval should be explored further, if applied to other wildfires.The active fire detection algorithm developed by Xu et al. [46] was used to detect the Esperance wildfire.The algorithm was modified from the active fire detection algorithm used in the MOD64 collection 6 product [52].This modification is based on a certain threshold required to detect a possible active fire and then eliminating any false positives (e.g., water and cloud masking).The pixels that match the Equation (3) are classified as active fire.The empirical thresholds used in Equation (2) were specifically adjusted for the case study.

Near Real-Time Fire Center Extraction
In this study, the centroid of the burned area was defined as the fire center.The centroid of a target is one of the most useful measurements for describing the spatial distribution of a target [53][54][55][56].Therefore, the centroid of the burned area can represent the average location of a fire at any time stamp.After obtaining the burned area data (see Section 3.2), Equation (4) was applied together with R software [57] to extract the geographical coordinates of each fire center.
where X c and Y c are the geographical coordinates of the fire center, n is the number of pixels flagged burned, x i and y i are the geographical coordinates of the i-th pixel, and m i is the area of the i-th pixel.

Near Real-Time FSR Calculation
Himawari-8 provides an observation every 10 minutes, which makes it possible to retrieve FSR at near real-time by analyzing the spread progression of the fire center.Wildfire spread models that are based on Huygens' wavelet principle presume that there is an elliptical spread at each point of the fire-front, and that a fire spreads outwards elliptically and at a constant speed in windless and flat area conditions [58][59][60][61][62][63].Therefore, it can be assumed that the FSR can be represented by the rate of burned area centroid movement in a windy environment.Additionally, fire spread models define the fire spread direction (FSD) as the reverse of wind direction [23].The wind direction is generally expressed in 16 directions on the land with an average of 22.5 degrees [64].Following these principles, the azimuth of centroid movement (α i , also called FSD) from time stamp i − 1 to i is first calculated using Equation ( 5), and then the difference ( ) between the azimuth of centroid movement and reverse wind direction (RWD) is calculated (Equation ( 6)).
where X c t i , Y c t i , X c t i+1 , and Y c t i+1 are the geographical coordinates of the burned area centroid in time stamp i and i + 1, DR Wind is the reverse wind direction.Finally, the near real-time FSR, corresponding to lower than 22.5 degrees, is calculated at different timesteps using Equation (7).
where T r is the time resolution of Himawari-8, V (i−1,i) is the FSR during the timestep i − 1 to i, D (i−1,i) is the distance of the centroid movement during timestep i − 1 to i. D (i−1,i) can be obtained with Equation (8).

Comparison of Results with the CSIRO GFS Model
The FSR computed from the CSIRO GFS model [27] was used as a benchmark to assess the performance of H8-FSR.The CSIRO GFS model was calibrated using 121 experimental fires and 20 major wildfires in Australia [65].This model is broadly used by the Australian fire authorities [65].The inputs of the model include meteorological information (10 m open wind speed, temperature, relative humidity) as well as a Grassland curing index (GCI).In this study, the meteorological data was obtained from three stations as described in Section 2.3.GCI, defined as the percentage of dead materials over the grass sward, has a large influence on grassland fire propagation [66].To date, there are two GCI satellite-based products for Australia, with a time and spatial resolution of eight days and 500 m respectively.The GCI product developed by Martin et al. [67] is more accurate than the curing product developed by Newnham et al. [68].The algorithm developed by Martin et al. [67] retrieves GCI using the Normalized Difference Vegetation Index (NDVI) and the Global Vegetation Monitoring Index (GVMI) computed from three window bands of MODIS (MOD09A1) satellite data, centered at 0.64 µm (band 1), 0.86 µm (band 2) and 1.64 µm (band 6).The algorithm was trained on curing visual estimates obtained by following the Australia Country Fire Authority (CFA) guide [67].Therefore, GCI was eventually obtained by applying this algorithm to the Himawari-8 data acquired from the Esperance bushfire that broke out at AWST time 08:30.
where V H−8 is the FSR extracted from THE Himawari-8 data, V M is the FSR estimated from the CSIRO GFS model and n is the number of observations.

Near Real-Time FSR
Using Equation ( 3), the difference between the FSD and the RWD for Esperance wildfire (36 time periods) were calculated.The number of time periods falling in different difference intervals were counted (Table 1).The FSR of 30 time periods with differences ( ) lower than 22.5 degrees were calculated in three directions.The extracted direction was FSD and the south and east directions were calculated by decomposing wind speed.Compared with the CSIRO GFS model, the H8-FSR under-estimated the FSR in the south, east and extracted directions (MBE = −0.52 m/s, −0.52 m/s and −0.75 m/s, respectively) (Table 2) with a MAPE of 43.33%, 35.53% and 33.20% (Table 2).The statistical metrics demonstrated a better agreement between the model estimates in the east and extracted directions compared to the south direction (Table 2 and Figure 4).The south direction demonstrated a larger MAPE (43.33%), a smaller R 2 (0.35) and a regression line that deviated from the 1:1 ideal line.However, the RMSE (0.80 m/s) was found to be smaller than the RMSE of the other two directions.Figure 4c demonstrates a less scattered distribution of dots, while the maximum FSR extracted from the Himawari-8 data (2.2 m/s) in the south direction was smaller than the FSR extracted from Himawari-8 data in the east and extracted directions.These may account for the low RMSE value.Figure 5 indicates that the FSR extracted from the Himawari-8 data is very close to the FSR estimated from the CSIRO GFS model for the 10-min time period from 11:00 to 12:40, especially in the east direction (Figure 5b).A large deviation occurs from 12:50 to 15:10 when CSIRO GFS model generally obtains higher FSR.However, the FSR derived from the Himawari-8 data is higher than the FSR estimated from the CSIRO GFS model at 15:30−15:40 and 15:50−16:00 periods.

Discussion
This is the first study to evaluate the performance of near real-time FSR extraction from the Himawari-8 data.The FSR estimated from the CSIRO GFS model was used as a benchmark to compare and assess the performance of H8-FSR.The H8-FSR compared favorably to the CSIRO GFS model when it came to the extraction of the near real-time FSR in the Esperance wildfire and produced errors that were acceptable within the literature (~30% MAPE [28]).However, H8-FSR underestimated FSR in some instances.These underestimations may have been the result of several reasons.Firstly, the CSIRO GFS model may have overestimated the FSR because this model assumes a fire will not be constrained during its growth [70].Additionally, the CSIRO GFS model does not take into account topography.However, weather, fuel and topography together are the key factors affecting fire behavior [71].Nevertheless, Esperance, the site that was studied is very flat and hence, the effect of topography may be insignificant.Another potential reason for the underestimations may have arisen from the meteorological information that was used.The meteorological information was calculated from the average values obtained from three different meteorological stations.For example, the wind speed and relative humidity values reported by the three stations highly differed at certain time periods (13:20-13:30, 14:10-14:20, 14:40-14:50, and 15:00-15:10), coinciding with the time periods where the FSR estimated with H8-FSR and CSRO GFS model differed the most (Figure 5).Using values for the average wind speed and relative humidity for the data recorded at the three meteorological stations as inputs to the CSIRO GFS model may have led to the observed deviations.On the other hand, fuel moisture content (FMC), including live and deal fuels moisture content, has proven to be a key indicator for fire risk assessment and FSR prediction [72][73][74][75].However, the CSIRO GFS model only considers the dead FMC, which is derived from meteorological factors [27].In recent years, several studies have propagated for the use of remote sensing data to retrieve FMC and indeed, this has achieved reasonable results [75][76][77][78].Therefore, further work will focus on applying the method to the Himawari-8 data to retrieve near real-time live and dead FMC and using those estimates as input parameters for fire behavior and risk assessments.Moreover, different vegetation types and growth conditions may also affect the FSR.The NDVI of the vegetation near the Esperance bushfire that ignited (at AWST time 08:30) was also computed from the Himawari-8 data, and then the NDVI of the burned area which increased at different timesteps were eventually extracted.The relationship between the NDVI and the FSR is shown in Figure 6.The FSR is faster in the area with lower NDVI.In contrast, when the NDVI is higher, which means the vegetation is very lush and difficult to ignite [79], the FSR becomes smaller.In this study, the region that was selected for this research was dominated by grassland but also comprised of little crops and shrubs.Consequently, this may also affect the evaluation of H8-FSR since the benchmark model was constructed within the context of grassland and without accounting for other vegetation types.The H8-FSR also has certain limitations.Firstly, it was assumed that the centroid of the burned area represented the average location of the fire, thus, the burned area data formed the base of FSR extraction.The FSR extracted from the Himawari-8 data was small at the time periods 12:50-13:00, 13:20-13:30 and 15:00-15:10 (Figure 5).As a result, the rate of burned area growth was also small.Therefore, the inaccuracy of burned area data affects the precision of the H8-FSR.The spatial resolution (2 km) of the Himawari-8 data is still not adequate enough, and this may introduce challenges to the detection of mixed pixels with different degrees of combustion.Unfortunately, at present, there is no officially reliable Himawari-8 burned area product.In this study, the sum of the active fire data was used to get the burned area data.The active fire detection algorithm is based on certain thresholds, which may not be robust for different regions.Therefore, in order to better apply the Himawari-8 data to fire research, it is necessary to develop a reliable product of burned extend for Himawari-8 fire.The second important limitation of H8-FSR is that it estimates an overall distribution of FSR by computing the rate of burned area centroid movement in a windy climate.The rate of fire center movement can well represent overall FSR when all the pixels have a similar direction of movement at every time step.However, small-scale wind has a substantial direction variation [80,81], which means the burned pixels in varying locations have different directions when the wind speed is very small.As a result, the movement of a fire center cannot actually reflect the fire spread, and thus affects the effectiveness of the H8-FSR.

Conclusions
The spatial-temporal distribution of the FSR is vital for fire management; for example, the establishment of firebreaks and the evacuation of personnel.The near real-time FSR extracted from the Himawari-8 data can provide more information for fire agencies in predicting the location of fire propagation.This in turn could lead to improvements in the efficiency of operational firefighting.This paper first defined the fire center as the burned area centroid and then proposed a novel method for near real-time FSR extraction based on the movement rate of the fire center which is extracted from the high-temporal resolution Himawari-8 data.The methodology was applied to a wildfire case that occurred in Esperance, Western Australia on 17 November, 2015.The results demonstrated that the H8-FSR could extract the FSR from Himawari-8 data well, compared with the CSIRO GFS model.The accuracy of burned area extraction and the scale of wind may also affect the efficacy of H8-FSR, which was discussed in Section 5 and remains to be investigated further.Further improvement in terms of the H8-FSR and its application to other vegetation types such as forest and shrubland, and testing the H8-FSR with other geostationary satellites needs to be implemented.Such satellites could include GaoFen-4 (GF-4), Feng Yun-4 (FY-4), GEO-R and Next Geostationary Korea multipurpose Satellite-2A (GK-2A).This would serve to improve the development of a global geostationary fire observation and prevention system.

Figure 1 .
Figure 1.Location of the study area.The background is a pseudo-color composited Himawari-8 image (band 6, 4 and 3 corresponding to RGB) acquired at 8:30 a.m.Australia Western Standard Time (AWST) on 17 November 2015.

Figure 2 .
Figure 2. A flowchart that demonstrates the methodology used in this study.

Figure 3 .
Figure 3. Variation in the burned area at each period computed as the difference in burned area between two consecutive 10 min time periods.

Figure 4 .
Figure 4. Scatterplots between the FSR extracted from Himawari-8 data (V H-8 ) and the CSIRO GFS model (V M ) in three different directions.The short dash lines are 1:1 lines and the solid lines are linear regression lines.

Figure 5 .
Figure 5.Time series of FSR extracted from the Himawari-8 data (V H-8 ) and the CSIRO GFS model (V M ).

Figure 6 .
Figure 6.Scatterplot showing the FSR extracted from the Himawari-8 data (V H-8 ) and the NDVI of different timesteps.The solid line indicates a linear regression.

Table 1 .
Statistical summary of the differences between the FSD and RWD.

Table 2 .
Quantitative comparison of the FSR extracted from Himawari-8 data and CSIRO GFS model.MBE, Mean Bias Error, MAPE, Mean Absolute Percent Error; RMSE, Root Mean Square Error and R 2 , coefficient of determination (*** p-Value = 0.00).