2.4 Product Evaluation Results: March 2021 to Feb 2022 AF & FRP Night-time
2.4.1 Fire Pattern & FRP Magnitude Analysis
Figure 2‑11 shows a visual comparison between the spatial patterns of active fire pixel count and FRP contained within the Level 3 Monthly Summary AF & FRP Night-time Products retrieved from one year’s Sentinel-3A and -3B data and from Terra MODIS. Both products covering March 2021 to February 2022 show very similar spatial patterns, indicating a broad degree of agreement, even if the MODIS data includes all night-time observations and not only those collected near-simultaneously with SLSTR.
The AF pixel count data show that the SLSTR products include many more AF pixel detections than MODIS, but the grid-cell FRP totals shown in Figure 2‑11 are similar between the two records, because the additional AF pixels that SLSTR detects in many of the grid cells are dominated by low FRP values. These findings mirror those reported in Xu et al. (2020) and results from Section 2.3.1 – in that the recorded FRP is similar between SLSTR and MODIS, but that SLSTR can detect smaller fires due to the AF detection algorithm sensitivity and the smaller mean SLSTR pixel size across the swath compared to MODIS.
Figure 2‑11: Total active fire (AF) pixel count and total FRP of fires detected within 0.25° grid cells from March 2021 to February 2022 using all Sentinel-3A, -3B SLSTR and Terra MODIS data. Note that the Terra MODIS active fire pixel count has a scale that is five times smaller than that of S3A and S3B due to the lower number of AF pixels the former sensor typically detects.
2.4.2 Level-2 Monthly Global Fire Location and FRP Summary Night-time Product
The AF detection process is inherently a trade-off between attempting to detect the lowest FRP fires (which are typically the most common type in most geographic areas), whilst also minimising false alarms (i.e., the triggering of the AF detection algorithm by a non-fire phenomena). Raising the minimum FRP detection limit in the contextual AF detection algorithm of Xu et al. (2020) very likely reduces the number of false alarms, but will also prevent some lower FRP fires from being identified. Comparison to global data from Terra MODIS taken within a scan angle limit of 30° and within ±6 minutes of a Sentinel-3 acquisition (see section 2.3.1 and Xu et al., 2020) shows that >90% of MODIS-identified active fire pixels (MOD14 Collection 6 product) had a matching Sentinel-3 AF pixel detection, representing an apparent SLSTR product omission error of <10% compared to MODIS. Conversely, of the Sentinel-3 AF pixel detections present in the same dataset, only ~60% had a matching MODIS AF pixel detection, and ~80% of the additional detections made by SLSTR had an FRP of less than 5 MW. The additional SLSTR AF detections are mostly below the minimum MODIS FRP detection limit, but SLSTR has a slightly lower minimum FRP detection limit than MODIS due to the smaller mean SLSTR pixel area and the intricacies of the AF pixel detection algorithm used to generate the Setinel-3 AF products (Xu et al., 2020).
Figure 2‑12 shows a visual comparison between the spatial patterns of active fire pixel count and FRP contained within the Level-3 monthly Night-time FRP products retrieved from Sentinel-3A and -3B data, and from Terra MODIS in September 2021. Very similar spatial patterns are seen, indicating a broad degree of agreement despite the MODIS data including all night-time observations The errors of omission and commission are expected similar to what reported in section 2.3.1 and indeed in Xu et al. (2020) as the spatial pattern of AF is similar between SLSTR and MODIS. For the same reason, as the spatial pattern of FRP is similar between SLSTR and MODIS in Figure 2‑11 and Figure 2‑12, the FRP between SLSTR and MODIS is expected to be similar to what reported in section 2.3.1 and Xu et al. (2020).
Figure 2‑12: Total active fire (AF) pixel count and total FRP of fires detected within 0.25° grid cells for the month of September 2021 using all Sentinel-3A, -3B SLSTR and Terra MODIS data. Note the Terra MODIS active fire pixel count has a scale that is five times smaller than that of S3A and S3B due to the lower number of AF pixels the former sensor typically detects.
2.4.3 Level 3a Daily Gridded AF & FRP Night-time Product
Similar to section 2.3.3, the fire season metrics derived from the C3S Level 3a Daily Gridded AF & FRP Night-time Product files were compared to those derived from MOD14 data. The comparisons are made globally as well as within the GFED regions (GFED) (e.g., see Figure 1‑6). Comparisons were made in terms of AF detections as well as with FRP.
Figure 2‑13a shows daily global total FRP as derived from Sentinel-3A, -3B SLSTR and Terra MODIS daily gridded global products. All three products show a very similar temporal development. At the global scale, the fire season peak occurs in August, starts in May and ends in November for each of S3A, S3B and MODIS. The cumulative percentage in Figure 2‑13b shows a similar trend, although using the 10% and 90% of the total FRP as the definitions of the start and end of the fire season respectively we identify May 2021 as the start for two sentinel-3 satellites but June 2021 as the start of Terra MODIS. For the end of the fire season, November 2021 is the end according to MODIS but December according to SLSTR. This peak, as well as the duration of the global “fire season”, agrees with findings reported in Giglio et al. (2006).
Figure 2‑13: Daily active fire count and FRP comparison between SLSTR and MODIS. (a) Total global daily FRP; (b) Cumulative percentage of total global daily FRP; (c) Total global daily active fire count; (d) Cumulative percentage of total global daily active fire count. Notice that there are few days in March and May 2021 when SLSTR has no input Level-2 FRP data.
Figure 2‑14 shows the monthly peak of the fire season both globally and in the GFED regions defined in Figure 1‑6, as derived from SLSTR and from Terra MODIS. Globally, the three satellite datasets (S3A, S3B and Terra MODIS) agree very well: they all peak at the beginning of August 2021, for example. For the GFED regions, 10 out of the 14 regions show a very good agreement, with the three satellites showing a fire season peak that is identical to within a few days. Note that landscape fire is dynamic phenomena having a high daily variance, and further analysis is needed to study the difference for regions showing greater disparity between each of the datasets.
Figure 2‑15 and Figure 2‑16 (image has been divided into two distinct parts to enhance user accessibility and clarity) show the seasonal pattern of daily cumulative FRP for both the globe and for the 14 GFED regions, again as derived from each of the three satellite datasets. The data from SLSTR shows a very strong agreement with that from MODIS in almost all regions, despite these figures being based on all Terra MODIS data and not just that collected near-simultaneously with SLSTR. Globally the coefficient of variation (r2) between the data from S3A and from Terra MODIS is 1.00, and the slope of the linear best fit is 1.08. For S3B and Terra MODIS the values are 1.00 and 1.08. For the GFED regions, the coefficient of variation (r2) between S3A and MODIS ranges from 0.77 to 1.00 and the slope from 0.66 to 1.12. Between S3B and MODIS r2 ranges from 0.80 to 1.00, and the slope from 0.73 to 1.10.
Figure 2‑14: Peak timing of the fire season as determined from Level 3a Daily Gridded FRP Product of S3A and S3B and from daily Terra MODIS data. Temporal resolution of the data is one day. The abbreviation of the names is as follows: Boreal North America (BONA); Temperate North America (TENA); Central America (CEAM); NH South America (NHSA); SH South America (SHSA); Europe (EURO); Middle East (MIDE); NH Africa (NHAF); SH Africa (SHAF); Boreal Asia (BOAS); Central Asia (CEAS); SE Asia (SEAS); Equatorial Asia (EQAS); Australia and New Zealand (AUST).
Figure 2‑15: Seasonal pattern of cumulative monthly FRP (%) at the global and GFED region scale, as derived from the C3S Level 3 Daily FRP Product and Terra MODIS – Part a. The abbreviation of the names is as follows: Boreal North America (BONA); Temperate North America (TENA); SH South America (SHSA); Europe (EURO); Middle East (MIDE); Boreal Asia (BOAS); Central Asia (CEAS); SE Asia (SEAS).
Figure 2‑16: Seasonal pattern of cumulative monthly FRP (%) at the global and GFED region scale, as derived from the C3S Level 3 Daily FRP Product and Terra MODIS – Part b. The abbreviation of the names is as follows: Central America (CEAM); NH South America (NHSA); NH Africa (NHAF); SH Africa (SHAF); Equatorial Asia (EQAS); Australia and New Zealand (AUST).
2.4.4 Level 3a 27-Day Gridded AF & FRP Night-time Product
The C3S Level 3a 27-Day Gridded FRP Product is simply the accumulation of 27 C3S Level 3a Daily Gridded AF & FRP Night-time Products, so its evaluation will simply focus on verifying the correctness of the lower temporal resolution statistical summary derived from the former data. The 27-Day products were verified thoroughly in this way, with the active fire pixel count, FRP and all other parameters being found to agree with the accumulation of the daily product data. This indicates the correctness of the C3S 27-Day products. An example is shown in Figure 2‑17 where the active fire pixel count from 4 August to 30 August 2021 derived from the 27-Day product perfectly agrees with that derived from the accumulation of the 27 individual daily products.
Figure 2‑17: Example of verification of the Level-3a 27-Day gridded FRP product. (a) Total active fire pixel count from the C3S 27-Day gridded FRP product covering 4 August to 30 August 2021; (b) Total active fire pixel from the 27 daily C3S daily products covering the same period. The active fire pixel count is identical in all the grid cells between (a) and (b).
2.4.5 Level 3 Monthly Grided AF & FRP Night-time Product
The C3S Level 3 Monthly Summary AF & FRP Night-time Product was compared to the MODIS MOD14 product. As the spatial patterns of absolute AF pixel counts and FRP have been analysed already in section 2.6.1, we focus here on the analysis of fire season and metrics defined at the global and GFED region scale. Degrees of agreement between the C3S and MODIS products are quantified using the coefficient of determination (r2) and slope of the linear best fit.
Figure 2‑18 and Figure 2‑19 (image has been divided into two distinct parts to enhance user accessibility and clarity) show the seasonal pattern of cumulative monthly FRP at both the global and GFED region scale, from both the C3S FRP products and Terra MODIS. As with the C3S daily product (Figure 2‑15 to Figure 2‑16), the seasonal pattern from SLSTR appears very close to that derived from MODIS. The r2 value of cumulative FRP formed by the S3A and Terra data is 1.00 and the slope of the OLS linear best fit, 1.08, whilst that from S3B and Terra is 1.00 and 1.08 respectively. For the GFED regions, the r2 between the S3A and MODIS values range from 0.75 to 1.00 and slopes 0.50 to 1.11, whilst S3B and MODIS are 0.80 to 1.00 and 0.55 to 1.09.
Figure 2‑18: Seasonal pattern of cumulative monthly FRP at the global and GFED region scales, as derived from the C3S Level 3 Monthly Summary FRP Product and Terra MODIS. The abbreviation of the names is as follows: Boreal North America (BONA); Temperate North America (TENA); SH South America (SHSA); Europe (EURO); Middle East (MIDE); Boreal Asia (BOAS); Central Asia (CEAS); SE Asia (SEAS).
Figure 2‑19: Seasonal pattern of cumulative monthly FRP at the global and GFED region scales, as derived from the C3S Level 3 Monthly Summary FRP Product and Terra MODIS. The abbreviation of the names is as follows: Central America (CEAM); NH South America (NHSA); NH Africa (NHAF); SH Africa (SHAF); Equatorial Asia (EQAS); Australia and New Zealand (AUST).
Figure 2‑20 shows the fire season start, peak, end and duration as derived from the C3S Level 3 Monthly Summary FRP Product and from Terra MODIS, both globally and from Terra MODIS. We used 25% of the total FRP to define the beginning of the fire season, and 75% to define the end of the fire season. Our analysis shows that the fire season duration and intensity are more accurately captured by the 25% and 75% percentiles of the annual cumulative FRP curve than by the 10% and 90% percentiles for the GRED regions. This is because the lower and upper percentiles reflect the onset and end of the fire activity more closely, while the intermediate percentiles may include periods of low or no fire occurrence. Overall, the start, peak, end and duration of the fire season agree very well between S3A, S3B and MODIS. At the global scale, according to the satellite data, the fire season begins and reaches its peak at the same time globally. However, there are some discrepancies between SLSTR and MODIS in terms of the end and duration of the fire season. SLSTR shows that the fire season ends one month later and lasts one month longer than MODIS. For all the GFED regions, ~20% of the 14 regions have 100% agreement for all the four fire season metrics analysed, ~80% have a difference of one month, and almost all the regions have a difference of less than two months.
Figure 2‑20: Fire season metrics as determined from Level 3 Monthly Gridded FRP Product of S3A and S3B and from daily Terra MODIS data. The temporal resolution of the data is one month. Analysis is conducted globally and for the GFED regions. (a) Start of the fire season; (b) Peak of the fire season; (c) End of the fire season; (d) Duration of the fire season. The abbreviation of the names is as follows: Boreal North America (BONA); Temperate North America (TENA); Central America (CEAM); NH South America (NHSA); SH South America (SHSA); Europe (EURO); Middle East (MIDE); NH Africa (NHAF); SH Africa (SHAF); Boreal Asia (BOAS); Central Asia (CEAS); SE Asia (SEAS); Equatorial Asia (EQAS); Australia and New Zealand (AUST).
