Environmental Engineering Reference
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have been observed in situ, using a satellite method similar to that employed by
another study (Smyth et al.
2004
). We examine these results in relation to physical
processes and climatic indices.
18.2 Methods
Blooms of
E
.
huxleyi
were identified in global AVHRR and SeaWiFS imagery as
described in the following sections.
18.2.1 AVHRR Imagery
Daily global, 1/4
o
resolution remote sensing reflectances (
R
rs
), derived from
Rayleigh-corrected radiances of the AVHRR visible and near-infrared channels,
were acquired from the Clouds from AVHRR Extended (CLAVR-X) Project
(Heidinger et al.
2002
). Input radiances from the different AVHRR platforms had
been intercalibrated to ensure a reliable and comparable time series of
R
rs
.CLAVR-X
was developed specifically to separate the cloud and sea-surface signals. To separate
the effect of atmospheric contamination,
R
rs
was estimated using a method for
detecting
E
.
huxleyi
blooms developed by Smyth et al. (
2004
):
R
rs
¼ðR
1
R
2
Þ=ð
exp
ðð
0
:
057
=
2
Þp
1
Þ
where
R1
and
R2
are Rayleigh-corrected channel 1 (0.580-0.680
μ
m) and channel 2
μ
(0.725-1.1
m) reflectances and pl is the optical path length, a function of the
satellite and solar zenith angle. The factor of
0.057 is the calculated value of the
Rayleigh optical depth in channel 1 for a
p
l of unity and a standard atmosphere.
18.2.2 SeaWiFS Imagery
SeaWiFS Global Area Coverage Level 1a data were acquired from the SeaWiFS
Project and Distributed Active Archive Center, processed to Level 2 radiances
while retaining the coccolith-flagged pixels normally masked during standard
processing, and spatially binned to 9-km resolution. SeaWiFS classification of
pixels into
E
.
huxleyi
bloom and non-bloom was based on 8-day mean normalized
water-leaving radiances using the supervised, multispectral classification scheme of
Iglesias-Rodriguez et al. (
2002
). The spectral signature of
E
.
huxleyi
blooms was
empirically ascertained to distinguish them from other oceanic conditions and has
been well validated.
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