A sub-pixel-based calculation of fire radiative power from MODIS observations: 2. Sensitivity analysis and potential fire weather application

In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: a b s t r a c t a r t i c l e i n f o Keywords: Fire Wildfire Biomass burning MODIS Atmospheric correction Water vapor Emissivity Background temperature Airborne Fire radiative power Sub-pixel Retrieval Fire weather Radiative transfer Fire area Fire radiative power (FRP) over a pixel area has been highlighted as a valuable parameter for quantitatively deriving smoke emissions. However, smoke plume rise forecasts and characterizations of fire intensity require additional information, including the FRP over the fire area (FRP f) and per fire area (or fire radiative flux), both of which can be calculated through a bi-spectral retrieval of sub-pixel fire area and temperature. This study, the second in a two-part series, examines the sources of error and the corresponding uncertainties in a sub-pixel algorithm that calculates FRP f for fire pixels identified at 1 km 2 nominal spatial resolution by the MODerate Resolution Imaging Spectroradiometer (MODIS) fire detection algorithm (collection 5). Radiative transfer simulations are incorporated to account for atmospheric effects as a function of Earth-satellite geometry at 3.96 and 11 μm (MODIS fire detection channels), and show that the 11 μm channel is highly sensitive to variations in column water vapor amount. By investigating several fire events in California, considerable variations in retrieved fire area, occasionally by more than 50%, are observed when comparing the mid-latitude summer climatology and observation-based atmospheric profiles that are used in the sub-pixel retrieval algorithm. While regions of dry, brown vegetation may also increase the potential for error via the surface emissivity assumption, the algorithm is much more sensitive to errors in 11 μm background brightness temperature, where an error of only 1.0 K may alter the retrieved fire area by an order of magnitude or more. An independent application, using the Texas wildfire event of September 2011, reveals that the sub-pixel retrieval can even become irrelevant for 17.6% of the available MODIS fire pixels as a result of noise in the background region causing the 11 μm background brightness temperature to become warmer than the fire pixel, especially during daytime scenes. The various sources of uncertainty in the estimates of FRP f and fire area can …