14 The Validation of Sea Surface Temperature Retrievals
235
In all of the ship-based radiometers, the data are recorded internally or on a
local computer and transmitted to shore via satellite telemetry link. The ISAR and
CIRIMS use commercial radiometers with bandpass filters covering the 9.6–11.5
μm wavelength range. The SISTeR has a filter wheel with three filters corresponding to the mid- and thermal infrared bands of the AVHRRs and (A)ATSRs. All
three types of ship-board radiometers have two small internal blackbody calibration
targets, for real-time calibration in the field.
The hyperspectral validation data are derived from the Marine-Atmospheric
Emitted Radiance Interferometer (M-AERI; Minnett et al., 2001). This is a Fourier
Transform InfraRed (FTIR) interferometric spectroradiometer that operates in the
range of infrared wavelengths from ∼3 to ∼18 μm and measures spectra with a
resolution of ∼0.5/cm. It was developed specifically for the validation of MODIS
skin SST retrievals and includes two very accurate internal blackbody cavities for
calibration in the field. Two infrared detectors, needed to achieve this wide spectral
range, are cooled to close to the boiling point of liquid nitrogen (∼78 K) by a Stirling
cycle mechanical cooler to reduce the noise equivalent temperature difference to
levels well below 0.1 K. A gold-plated scan mirror, which is programmed to step
through a pre-selected range of angles, directs the field of view from the interferometer to either of the internal blackbody calibration targets or to the environment from
nadir to zenith. The interferometer integrates measurements over a pre-selected time
interval, typically 60 s–100 s, to obtain a good signal-to-noise ratio. A typical cycle
of measurements, including two view angles to the atmosphere, one to the ocean,
and calibration measurements, takes about 5–10 min.
The absolute accuracy of the infrared spectra produced by the M-AERI is determined by the effectiveness of the blackbody cavities as calibration targets. The
absolute accuracy of the spectral measurements of the M-AERI is better than
0.03 K (Minnett et al., 2001). The absolute uncertainties of the retrieved skin SST,
determined by operating two M-AERI’s side-by-side and by comparing M-AERI
measurements with those from other well-calibrated radiometers, are less than
0.05 K (Barton et al., 2004; Minnett et al., 2001) which are sufficiently small to
give confidence in the use of such data in the validation of satellite SST retrievals.
Since the launch of the NASA Earth Observing System satellite Terra in
December 1999, over 40 M-AERI cruises have been undertaken on research vessels
(Fig. 14.4). These and other radiometer cruises (e.g. Jessup and Branch, 2008) have
spanned a wide range of climatological regimes, from polar regions to the tropics.
In addition to sampling the full range of SST, from freezing to the high temperatures in the Red Sea and Tropical Western Pacific, validation data have been taken
in a wide range of marine atmospheres. In addition, an M-AERI and a suite of atmospheric sensors have been deployed on the cruise ship Explorer of the Seas, operated
by Royal Caribbean International. This ship was equipped at construction with scientific laboratories for oceanographic and atmospheric research (Williams et al.,
2002). This vessel has been a valuable source of infrared radiometric measurements
for the validation of satellite SST retrievals.
By taking validation data from moving ships, the integration time of the radiometric measurements along the shiptrack, or the averaging of rapidly sampled data,
235
In all of the ship-based radiometers, the data are recorded internally or on a
local computer and transmitted to shore via satellite telemetry link. The ISAR and
CIRIMS use commercial radiometers with bandpass filters covering the 9.6–11.5
μm wavelength range. The SISTeR has a filter wheel with three filters corresponding to the mid- and thermal infrared bands of the AVHRRs and (A)ATSRs. All
three types of ship-board radiometers have two small internal blackbody calibration
targets, for real-time calibration in the field.
The hyperspectral validation data are derived from the Marine-Atmospheric
Emitted Radiance Interferometer (M-AERI; Minnett et al., 2001). This is a Fourier
Transform InfraRed (FTIR) interferometric spectroradiometer that operates in the
range of infrared wavelengths from ∼3 to ∼18 μm and measures spectra with a
resolution of ∼0.5/cm. It was developed specifically for the validation of MODIS
skin SST retrievals and includes two very accurate internal blackbody cavities for
calibration in the field. Two infrared detectors, needed to achieve this wide spectral
range, are cooled to close to the boiling point of liquid nitrogen (∼78 K) by a Stirling
cycle mechanical cooler to reduce the noise equivalent temperature difference to
levels well below 0.1 K. A gold-plated scan mirror, which is programmed to step
through a pre-selected range of angles, directs the field of view from the interferometer to either of the internal blackbody calibration targets or to the environment from
nadir to zenith. The interferometer integrates measurements over a pre-selected time
interval, typically 60 s–100 s, to obtain a good signal-to-noise ratio. A typical cycle
of measurements, including two view angles to the atmosphere, one to the ocean,
and calibration measurements, takes about 5–10 min.
The absolute accuracy of the infrared spectra produced by the M-AERI is determined by the effectiveness of the blackbody cavities as calibration targets. The
absolute accuracy of the spectral measurements of the M-AERI is better than
0.03 K (Minnett et al., 2001). The absolute uncertainties of the retrieved skin SST,
determined by operating two M-AERI’s side-by-side and by comparing M-AERI
measurements with those from other well-calibrated radiometers, are less than
0.05 K (Barton et al., 2004; Minnett et al., 2001) which are sufficiently small to
give confidence in the use of such data in the validation of satellite SST retrievals.
Since the launch of the NASA Earth Observing System satellite Terra in
December 1999, over 40 M-AERI cruises have been undertaken on research vessels
(Fig. 14.4). These and other radiometer cruises (e.g. Jessup and Branch, 2008) have
spanned a wide range of climatological regimes, from polar regions to the tropics.
In addition to sampling the full range of SST, from freezing to the high temperatures in the Red Sea and Tropical Western Pacific, validation data have been taken
in a wide range of marine atmospheres. In addition, an M-AERI and a suite of atmospheric sensors have been deployed on the cruise ship Explorer of the Seas, operated
by Royal Caribbean International. This ship was equipped at construction with scientific laboratories for oceanographic and atmospheric research (Williams et al.,
2002). This vessel has been a valuable source of infrared radiometric measurements
for the validation of satellite SST retrievals.
By taking validation data from moving ships, the integration time of the radiometric measurements along the shiptrack, or the averaging of rapidly sampled data,