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Abstract:
Characterising the dynamics of landscape-scale wildfires at very high
temporal resolutions is best achieved using observations from Earth
Observation (EO) sensors mounted onboard geostationary satellites. As a
result, a number of operational active fire products have been developed
from the data of such sensors. An example of which are the Fire
Radiative Power (FRP) products, the FRP-PIXEL and FRP-GRID products,
generated by the Land Surface Analysis Satellite Applications Facility
(LSA SAF) from imagery collected by the Spinning Enhanced Visible and
Infrared Imager (SEVIRI) onboard the Meteosat Second Generation (MSG)
series of geostationary EO satellites. The processing chain developed to
deliver these FRP products detects SEVIRI pixels containing actively
burning fires and characterises their FRP output across four geographic
regions covering Europe, part of South America and Northern and Southern
Africa. The FRP-PIXEL product contains the highest spatial and temporal
resolution FRP data set, whilst the FRP-GRID product contains a
spatio-temporal summary that includes bias adjustments for cloud cover
and the non-detection of low FRP fire pixels. Here we evaluate these two
products against active fire data collected by the Moderate Resolution
Imaging Spectroradiometer (MODIS) and compare the results to those for
three alternative active fire products derived from SEVIRI imagery. The
FRP-PIXEL product is shown to detect a substantially greater number of
ac- tive fire pixels than do alternative SEVIRI-based products, and
comparison to MODIS on a per-fire basis indicates a strong agreement and
low bias in terms of FRP values. However, low FRP fire pixels remain
undetected by SEVIRI, with errors of active fire pixel detection
commission and omission compared to MODIS ranging between 9-13% and
65-77% respectively in Africa. Higher errors of omission result in
greater underestimation of regional FRP totals relative to those derived
from simultaneously collected MODIS data, ranging from 35% over the
Northern Africa region to 89% over the European region. High errors of
active fire omission and FRP underestimation are found over Europe and
South America and result from SEVIRI's larger pixel area over these
regions. An advantage of using FRP for characterising wildfire emissions
is the ability to do so very frequently and in near-real time (NRT). To
illustrate the potential of this approach, wildfire fuel consumption
rates derived from the SEVIRI FRP-PIXEL product are used to characterise
smoke emissions of the 2007 "mega-fire" event focused on Peloponnese
(Greece) and used within the European Centre for Medium-Range Weather
Forecasting (ECMWF) Integrated Forecasting System (IFS) as a
demonstration of what can be achieved when using geostationary active
fire data within the Copernicus Atmosphere Monitoring Service (CAMS).
Qualitative comparison of the modelled smoke plumes with MODIS optical
imagery illustrates that the model captures the temporal and spatial
dynamics of the plume very well, and that high temporal resolution
emissions estimates such as those available from a geostationary orbit
are important for capturing the sub-daily variability in smoke plume
parameters such as aerosol optical depth (AOD), which are increasingly
less well resolved using daily or coarser temporal resolution emissions
data sets.
Quantitative comparison of modelled AOD with coincident MODIS and
AERONET (Aerosol Robotic Network) AOD indicates that the former is
overestimated by similar to 20-30 %, but captures the observed AOD
dynamics with a high degree of fidelity. The case study highlights the
potential of using geostationary FRP data to drive fire emissions
estimates for use within atmospheric transport models such as those
implemented in the Monitoring Atmospheric Composition and Climate (MACC)
series of projects for the CAMS.