### SEVIRI Errors of Omission and Commission
#### 5.1.4 SEVIRI's Low Spatial Resolution Detection Bias  
The proportion of MODIS active fire pixels that SEVIRI failed to detect and were completely surrounded by SEVIRI non-potential fire pixels (_i.e._, cloud-free land pixels) continually increased from 4% in ROI 1% to 30% in ROI 4 (Table 2). Thus in the absence of confounding artefacts, SEVIRI unconditionally missed more MODIS active fire pixels in the south and west of the CAR compared to the north and east. Without an alternative explanation for the failed detections, these statistics (more than any other presented here) demonstrate (i) the isolated impact of SEVIRI's low spatial resolution detection bias; and (ii) a geographic variability in SEVIRI's active fire detection performance. Depending on the location in the CAR, between 16% and 40% of SEVIRI's omission rate is attributable to a low spatial resolution detection bias.  
In the absence of confounding artefacts, the probability that SEVIRI will detect a fire depends on a combination of the reaction intensity and the instantaneous area of the subpixel combustion components. Although it is possible to retrieve the effective temperature and area of a subpixel fire using either a \"dual-band\" algorithm [61] or a multi-spectral analysis (e.g., [62]), these approaches were not attempted here due to the potentially large uncertainties in the retrieval of these parameters [63]. Instead two basic remote sensing parameters were used to infer differences in fire behaviour across the CAR:  
Figure 6: Comparisons between the internal cloud mask used during the processing of the MODIS active fire products and the SANWC cloud mask used during processing of the SEVIRI FRP-PIXEL product. In (**a**) are 1139 concurrent observations of the instantaneous cloud cover fraction in each ROI shown in Figure 1a, calculated for MODIS, \(f_{MODIS}\), and SEVIRI, \(f_{SEVIRI}\) (see Equation (2)). Observations in (a) are aggregated over all ROIs and sorted into 10% bins of \(f_{MODIS}\), as shown in (**b**).  
* Coherent with an increase in SEVIRI's omission rate, the median, mean, and maximum FRP measured by both MODIS and SEVIRI continually decreased from ROI 1 to ROI 4 (Table 1). The distributions of FRP used to derive the MODIS statistics are shown in Figure 8a.
* An independent clustering of MODIS active fire pixels demonstrates that coherent with an increase in SEVIRI's omission rate, the cluster size of contiguous active fire pixels decreases from ROI 1 to ROI 4 (Figure 8b). Whilst the maximum cluster size progressively decreased from 29 active fire pixels in ROI 1 to six in ROI 4, solitary MODIS active fire pixels accounted for 29% of the clusters in ROI 1 and nearly 60% in ROI 4.  
Hence we find that regional variations in the performance of the SEVIRI FTA detection algorithm coincide with a geographical gradient in observed active fire characteristics.  
Figure 7: Simultaneous comparisons between the cloud masks used in the MODIS and SEVIRI active fire products at 12:00 UTC on 11 January 2009 within ROI 1 (Figure 1a). The 500 m MODIS Aqua true colour image is presented in (**a**); the MODIS fire mask retrieved from the coincident MYD14 fire product is presented in (**b**); the status flags retrieved from the coincident SEVIRI FRP-PIXEL quality file are shown in (**c**); and the MODIS cloud mask retrieved from the coincident MYD35 cloud product is shown in (**d**). Aside from the true colour image (which has been reprojected into geographic coordinates), the masks are shown in their native image coordinate system along with the boundary of ROI 1.