PMC

Purpose is to demonstrate how spot fire merging may increase combined rate spread (ROS), even with identical main fire ROS. Black arrow is spread direction. Dotted lines are time 1 and solid lines are time 2. At time 1, the fires are the same, but Fire A has two separate spot fires (small circles) and Fire B has none. At time 2, “combined” ROS for Fire A is greater than for Fire B because Fire A includes the two merged spot fires, thus is farther downwind. T1 and T2 indicates head fire location at time 1 and time 2 respectively. Note that the spot fires are not considered when identifying the head fire location for Fire A time 1.

The fire occurrence numbers are from the Wildfire Atlas of China (WFAC) and the Moderate Resolution Imaging Spectroradiometer (MODIS) data, including MOD14A1 provided by Terra and MYD14A1 provided by Aqua. a Total fire occurrences from MODIS, b crop fire occurrences from MODIS, c the ratio between total fire occurrences from MODIS and WFAC, and d the ratio between non-crop fire occurrences from MODIS and the WFAC. The fire occurrences were aggregated via combining the spatiotemporal adjacent fire points from MODIS Terra and Aqua from 2005 to 2018 into one fire event. Crop fire was picked out based on the land cover data of China in 2018. The The 2 × 2 degree fire data is a sum of the fire occurrences.

Percentage of fire days with a given fire spread rate (a) and percentage of burned area with a given fire spread rate (b), as a function of fire type. A fire day is a day on which active fire detections were present for a given fire. Fire spread rate is defined as the 95th percentile of the spread rate over all active fire detections on the fire line. The mean daily rate-of-spread is 1.83 km day⁻¹ for the human-caused fires and 0.84 km day⁻¹ for lightning-caused fires shown in panel a. Similarly, mean rate-of-spread normalized by burned area (shown in panel b) is 5.6 km day⁻¹ for the human-caused fires and 3.5 km day^–1 for lightning-caused fires. The frequency distributions of daily rate-of-spread between human and lightning-caused fires as a function of fire day or burned area are significantly different.

Head fire was measured between y1 and y2, off-centre fire between y3 and y4. x1 = Line 1, x2 = Line 2, x3 = Line 3. Images extracted to show examples of (a) before head fire has reached any measurement line or spot fire has merged, (b) head fire has just crossed Line 1 (x1) and spot fire 1 has merged, (c) head fire has just crossed Line 2 (x2). Seconds after ignition are (a) 25, (b) 37 and (c) 48.