Estimating Fire Radiative Energy From MODIS

An alternative approach to biomass burning emission estimates has emerged from remote sensing science in which the fire intensity measured during combustion serves as a proxy for emissions released. The rate of energy emitted is referred to as the fire radiative power, or FRP. Integrating FRP over the lifespan of the fire event provides the total fire radiative energy (FRE) released, which in turn is directly proportional to the total fire emissions. We present an approach to estimate FRE from the Moderate Resolution Imaging Spectrometer (MODIS) 8-day, 0.5° climate modeling grid (CMG) FRP observations. An important characteristic of the radiative energy emitted from fires is the temporal trajectory which describes how the fire intensity behaves over the span of seconds, minutes, hours, days, and even seasons. To characterize the fire diurnal cycle we first examined FRP retrievals from Africa using the European Space Agency's geostationary Meteosat SEVIRI sensor. SEVIRI's temporal sampling resolution (15-minute) provides the capability to integrate FRP observations and calculate FRE. We also included diurnal cycle probability density functions (PDFs) from the Tropical Rainfall Measuring Mission (TRMM) sensor to expand our investigation area beyond Africa. A modified Gaussian function was shown to provide a simple and accurate representation of the observed diurnal cycles. Aqua and Terra MODIS 30-day gridded FRP observations were then used to parameterize the Gaussian function based on the ratio between the morning (Terra) and afternoon (Aqua) MODIS FRP values. Our contention is that the variation in the Terra/Aqua ratio can serve as a proxy for the diurnal cycle trajectory. The relationship between the Terra/Aqua ratio and Gaussian function parameters was established for multiple regions within the tropics. In addition, we supplemented the diurnal cycle parameterization with MODIS FRP retrievals at high latitudes. High latitude FRP retrievals were chosen because of Terra and Aqua's polar orbit and therefore greater frequency of observations made by each sensor. The greater frequency of FRP retrievals allowed us to characterize the temporal trajectory for higher latitude fires, which tend to have a distinctly different diurnal burning cycle than tropical fires. A comparison of estimated FRE with observed FRE from SEVIRI demonstrates the accuracy of our approach. For example, within a southern African region (1900 km2), which emitted a total FRE of 4.84e+06 MJ in July 2004, results from 49 2.5°x2.5° test cells showed that our calculation of FRE was only slightly overestimated (bias = 7%) with little error (RMSE = 3.05e+04 MJ). Comparable results were found in northern African and within other tropical regions when compared with TRMM PDFs.