Quantifying the potential for high-altitude smoke injection in the North American boreal forest using the standard MODIS fire products and subpixel-based methods

All chemical transport models require an estimation of the vertical distribution of smoke particles near the source. This study quantitatively examines the strengths and weaknesses of several fire products for characterizing plume buoyancy and injection heights in the North American boreal forest during 2004–2005. Observations from the Multiangle Imaging Spectroradiometer show that 21% of smoke plumes are injected more than 500 m above the boundary layer (BL500) and 8% exceed 2.5 km above ground level. Corresponding observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) show that probability of injection above the BL500 exceeds 60% for pixel-based fire radiative power (FRPp) values above ~2500 MW. Increasing values of subpixel-retrieved fire area and temperature also correspond to higher injections but only after removing fire pixels with a weak 11 µm fire signal and clustering. The probability of injection above the BL500 reaches 50% when the subpixel radiant flux (FRPf flux) exceeds 20 kW/m2, highlighting its potential for estimating plume buoyancy. However, these data have limitations similar to FRPp, where the highest probability of injection corresponds to a small percentage of the data set (5–18%), and many high-altitude injections occur with lower values. Examinations of individual smoke plumes highlight the importance of combining pixel-level and subpixel outputs and show that plume injection is also sensitive to the fire pixel spatial distribution and meteorology. Therefore, an optimal method for predicting high-altitude injections will require some combination of injection climatology, FRPp, FRPf flux, and meteorology, but each variable's importance will depend on fire event characteristics.