Addressing the spatiotemporal sampling design of MODIS to provide estimates of the fire radiative energy emitted from Africa

摘要

Satellite-based estimates of the fire radiative power (FRP) and energy (FRE) emitted from open biomass burning are affected by the spatiotemporal resolution of polar-orbiting and geostationary sensors. Here the impacts of the MODIS sampling design on estimates of FRE are characterized by superimposing the timing and extents of the Terra and Aqua granules onto the SEVIRI active fire product. Results for different land-cover types across Africa indicate that the FRE measured by SEVIRI during eight days is linearly related to the sum of FRP measured by SEVIRI within the MODIS granules. These relationships are least variable during the height of the fire season when diurnal cycles of FRP measured by SEVIRI are most consistent. Relationships between FRE and the sum of FRP developed using the SEVIRI active fire product are directly applied to the sum of FRP retrieved from the MODIS Terra and Aqua Climate Modeling Grid (CMG) fire products. Estimates of FRE from MODIS herein agree within 5% of those obtained from previously published methods, but remain a factor of 0.72 times those obtained by adjusting SEVIRI measurements of FRE to account for low spatial resolution detection biases. An examination of the MODIS scan geometry suggests that the latter underestimation is attributed to the coupling between a MODIS imaging artefact referred to as the "bow-tie" effect and the typical calculation used to retrieve the sum of FRP from the MODIS CMG fire products. Depending on the availability of MODIS scan angle information, we offer rigorous and simplified calculations to account for the bow-tie effect. Applying the simplified adjustment to the MODIS CMG fire products yields national estimates of monthly FRE that are 1.44 times greater than originally predicted.