## 5 Discussion
At a fine grid resolution (e.g., 0.25\({}^{\circ}\)), there is a considerable number of grids with only VIIRS fire detections but without MODIS fire detections. In these grids, the missing MODIS FRP cannot be adjusted using the corresponding models that require FRP values available from both MODIS and VIIRS fire detections. For the grid size being larger than 2.5\({}^{\circ}\), almost all grids contain both VIIRS and MODIS fire detections. In this case, the fitted models can be applied to improve MODIS FRP underestimation at almost all coarse grids, but spatial details are missed relative to fine-resolution ones. Therefore, choosing an appropriate grid resolution to adjust the MODIS FRP underestimation depends on the application purposes. The FRP comparisons in different grid resolutions provide direct evidence to evaluate the published methods for adjusting the bow-tie effect caused MODIS FRP underestimation. First, our results show that MODIS FRP in 0.5\({}^{\circ}\) grids is relatively underestimated by 40-50% compared to VIIRS (Figure 8d and Table 1). This is very similar to the previous finding that MODIS FRP at a 0.5\({}^{\circ}\) grid could be improved by 44% across Africa by correcting the MODIS bow-tie effect [30]. However, this study indicates that the MODIS FRP underestimation should be attributed to two factors: (1) the MODIS bow-tie effect and (2) the MODIS coarser-resolution effect relative to VIIRS I-band in nadir. In other words, VIIRS I-band, theoretically, is able to sense smaller and/or cooler fires than MODIS even at nadir, although both sensors observe fires in the same grid. Thus, it is likely the algorithm of bow-tie correction [30] could overestimate MODIS FRP. Second, [PERSON] et al. [33] correct the MODIS bow-tie effect on FRP by mapping the CDF of grid FRP at large VZA to nadir and find that MODIS FRP could be improved by up to 44% for 1\({}^{\circ}\) grids with a VZA \(>\) 55\({}^{\circ}\). Our results show that MODIS FRP is underestimated by 47-85% relative to VIIRS in 1\({}^{\circ}\) grids as VZA varies from nadir to the scan edge. Because VIIRS fire detection capability is almost consistent across VZA (Figures 1 and 4b), it can be derived that the underestimation of MODIS FRP attributed to VZA effect could be as large as 38% (38% = 85% - 47%), which is slightly smaller than the 44% found by [PERSON] et al. [33]. However, MODIS FRP in nadir is still underestimated by 47% compared to VIIRS FRP. Third, [PERSON] et al. [35] corrects MODIS FRP at large view angles (VZA\(>\)35\({}^{\circ}\)) in 27 km \(\times\) 27 km grids by using FRP at smallest VZA observed within two days and finds that emissions estimates could be increased by a factor of two across Northern Hemisphere Africa, including the FRP improvements by correction of clouds and swath gaps. However, our results show that MODIS grid FRP on average is smaller than VIIRS grid FRP by up to 31% at a grid resolution of 0.25\({}^{\circ}\) (\(\sim\)25 km at the equator, Figure 7c). Even when all VIIRS I-band FRP inside and outside MODIS swath gaps across Africa are considered, the continental-scale MODIS FRP is relatively underestimated by \(\sim\)62.5% (49% + bias interpreted as 13.5% of daily mean MODIS FRP in Figure 5a), much smaller than the difference of a factor of two. It is likely that the assumption proposed by [PERSON] et al. [35] for correcting MODIS FRP at large VZA and swath gaps may not work well and could overestimate MODIS FRP. This is demonstrated in Figure 11, which shows the variations of mean VZA and daily FRP of MODIS and VIIRS I-band in a 1\({}^{\circ}\) grid centering at 30.232\({}^{\circ}\)E and 4.139\({}^{\circ}\)N in January 2017. The grid FRP estimates from the two sensors show similar temporal patterns, although MODIS swath gap occurred on January 4 and 20. According to the assumption in [PERSON] et al. [35], MODIS FRP requires correction for a total of 23 days in January 2017.