Test (3) T, or T5 > 275°K to exclude
clouds and sensor noise (damaged
pixels)
Test (4) pj, p2 < 5-6 % to prevent
false detections by highly reflective
surfaces
Where T3, T, and T; represent the top of atmosphere
(ToA) brightness temperatures (K) for channel 3, 4
and 5 respectively, pi, p2 the % ToA reflectance in
channel 1 and 2 respectively while a; is the
brightness temperature threshold for test i = 1, 2, 3,
4 and 5.
However, the GAC images are spatial sub-sample
produced from the original Local Area Coverage
(LAC) images. The spatial resolution of GAC data
is effectively 4.4 km x 1.1 km with a 2.2 km gap
between each scan line. This spatial degradation of
GAC data may result in a bias of the derived fire
information for Borneo (Belward and Lambin,
1990). Therefore, the capability of GAC data in
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fire count in LAC
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
active fire detection was examined in relation to that
of LAC data. Thirteen pairs of LAC, and the
corresponding GAC data from the same orbit, were
collected for the 1997-98 El Niño related year.
Since the data volume of the original LAC images is
15 times larger than the contemporaneous GAC
product, in order to create an analogous GAC fire
count comparable to the LAC fire product, the GAC
derived fire counts were multiplied by a factor of
15.
Results showed that the adjusted GAC fire count
numbers were very well related to the LAC fire
counts of the coincident imagery (r^ = 0.99, n = 13 p
< 0.001) (Figure la). The mean percentage
difference between the two fire count datasets was —
1.6% with a standard deviation of 13.9%. Taking
into account the substantial degradation of LAC
data during the GAC production, these variations
are minor, indicating the efficacy of GAC data for
providing quantitative fire information in Borneo
during El Nifio periods when the fire occurrence is
high.
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Figure 1 (a) Regression analysis between LAC and adjusted GAC fire counts during El Niño period. (b)
Temporal evolution of Niño 3 anomaly during the five El Niño events studied herein. It is clearly indicated
the two strongest El Niño events of 1982-83 and 1997-98, these having the highest absolute anomaly values.
These data were obtained from the Climate Prediction Centre of the NOAA National weather Service in
USA.
The fire counts detected in each GAC image were
further adjusted for different cloud cover and
observation time of each GAC image (Kaufman ef
al., 1990; Giglio et al., 2000). The GAC derived fire
activity in Borneo for the five El Nifio events from
1982 to 1998 is showed in Figure 2. Obviously,
there is a distinct interannual constant pattern of fire
activity. The major fire activity tends to occur
between August-October (1* fire sub-season) of the
first year (Year 0), and between February-April (2
fire sub-season) of the following year (Year 1). The
only pronounced exception was the 1993-94 fire
event when the major fire activity appeared in the
3d fire sub-season (August-October of Year 1),
most likely associated with prolonged ENSO
anomalies (i.e. 1991-92). On the contrary, in
November-January (NDJ) and March-July (MJJ) of
the El Nifio period, the fire activity appeared
significantly weakened, revealing the domination of
local climate conditions driven by the monsoon
597
circulation. As it is depicted in Figure 1b, during
NDJ of Year 0 and MJJ of Year 1, the ENSO index,
in most of the studied El Niño events, remained
high enough to trigger fire occurrence. However, the
Asian monsoon system is active over Borneo during
these time periods, particularly during the NDJ of
Year 0 when the winter (west) monsoon is
substantially stronger than the summer (east)
monsoon, resulting strong convection over that
region. This is also supported by the fact that
although El Niño is being in its mature phase during
NDJ of Year 0 (MJJ of Year 1) and the ENSO index
is very high, moving (or just start to decrease)
towards to (away from) the peak, negligible fire
activity occurred. On the contrary, during the MJJ of
Year 0, El Niño was still not fully developed, while
in NDJ of Year 1, El Niño was already demised,
contributing (resulting) in this way, together with
the strong winter monsoon influence, to the almost
complete absence of major fire events.
