Science Focus:Sea Surface Temperature Measurements of the MODIS and AIRS Instruments Onboard of Aqua Satellite
Dongliang Yuan,NASA Goddard Earth Sciences Data and Information Services Center,
On May 24, 2002, the Aqua satellite was launched into a Sun synchronous orbit as part of the NASA-centered
international Earth Observing System (EOS) to provide observations of the Earth´s oceans, atmosphere,
land, ice, snow covers, and vegetation.Onboard Aqua carries several of the most important instruments of
the EOS system, the Moderate Resolution Imaging Spectroradiometer (MODIS), the Atmospheric InfraRed Sounder
(AIRS), and the Advanced Microwave Sounder Unit (AMSU).Both of MODIS and AIRS measure radiances in the
infrared bands so that the surface temperature of the ocean can be derived.The infrared measurements of
AIRS and the microwave measurements of AMSU have been combined to generate a sea surface skin temperature
product.This article explains the sea surface temperature (SST) products of MODIS and AIRS and gives an
introduction to the synchronized use of the two products in scientific researches.
Over the surface of the ocean, there frequently exists a very thin layer called the surface skin layer in
remote sensing sciences (Schluessel et al., 1990) (Figure 2).The existence of the surface skin layer can
be demonstrated both in theory (Hinzpeter, 1967, 1968) and in observations (Ewing and McAlister, 1960;
Saunders, 1967; Clauss et al., 1970; Schluessel et al., 1990) by the need to regulate the long wave radiation
and the sensible and latent turbulent heat fluxes across the sea surface.Above and below the thin skin layer,
turbulent eddy fluxes enhance heat flux in the ocean and/or atmosphere across the interface.However, the
eddy cannot transport heat across the ocean surface by itself.The heat balance in the skin layer must be
accomplished by molecular processes, hence the thin skin layer.The actual thickness of the skin layer
depends on the local energy flux of the molecular transports, which is usually less than 1 mm thick and can
persist at wind speed up to 10 m/s.For stronger winds, the skin layer is destroyed by breaking waves.
Observations indicate that the skin layer can re-establish itself within 10 to 12 seconds after the dissipation
of the breaking waves (Ewing and McAlister, 1960; Clauss et al., 1970).
The infrared wave can only penetrate water no deeper than ~500 µm.The MODIS instrument measures
infrared radiances at around two wavelengths: 3.9 µm and 11 µm.The penetration depths for
these two bands are ~100 µm and ~10 µm.The AIRS uses wavelengths from 3.75 µm to 13 µm
for surface properties retrievals, which have penetration depths ranging from ~100 µm to ~10 µm.
Thus, the radiances measured by the MODIS and AIRS satellite instruments are emitted from within the surface
skin layer of the ocean.Figure 1 shows the infrared optical constants of clear water.
Figure 1. Absorption coefficient and penetration depths of infrared waves.Adopted from Wieliczka et al., 1989).
The SST can also be measured by the microwave radiometer.The penetration depth of the microwaves can be an
order of magnitude larger than that of the infrared waves.For low-frequency (6-10 GHz) microwaves, the
penetration depth can exceed 1 mm.The penetration depth of the microwaves is sensitive to the surface
salinity and roughness conditions.For the frequencies used in the AMSU instrument, which span from 23 GHz
to 90 GHz, the penetration depth is generally smaller than 1 mm and the surface radiation is heavily influenced
by the skin temperature of the surface ocean.
The vertical structure of the skin layer SST can be generally described as in Figure 2.The interface SST,
SSTint, is the temperature at the infinitely thin layer at the exact air-sea interface.This
temperature cannot be measured using current technology.The skin SST, SSTskin, is the temperature
measured by an infrared radiometer at a depth of order of 500 µm depending on the wavelength of the
measurement.This temperature is depth (wavelength) dependent, but the differences measured by the infrared
radiometers are very small (less than 0.01 K due to the small penetration depth differences).Therefore, the
wavelength dependence of SSTskin is usually ignored.The subskin SST, SSTsubskin, is representative
of the SST at the bottom of the skin temperature layer and is usually the value measured by a low-frequency
(6-10 GHz) microwave radiometer.The SST at depth, SSTdepth, (traditionally referred to as the
bulk SST) represents the temperature of the upper mixed layer produced by the turbulence associated with wind
stirring and convective overturning, etc.
Figure 2.Schematic plot of open ocean surface thermal structures: a) nighttime; b) daytime.
SSTint is the temperature at the air-sea interface, SSTskin at about 500 µm,
SSTsub-skin at about 1 mm and SSTdepth the bulk SST.Figure adapted from Donlon et al. (2004).
Within the surface mixed layer usually of a thickness on the order of 1 m to 100 m, the bulk SST has very
small vertical gradient.In fact, the depth of the surface mixed layer is frequently defined as the depth
where the temperature drops from the surface bulk SST by a small amount (say 0.5 K or 1 K).Occasionally,
vertical gradient of SST is present in the upper mixed layer due to interleaving and overturning processes of
high-frequency heating/cooling.But the gradient is quickly eliminated by the turbulence over periods of a
few hours to a day.The mean skin temperature is generally several tenths of a degree colder than the mean
bulk temperature (Schluessel et al., 1990).The instantaneous bulk-skin temperature differences can be as
large as 1.0 K to -1.0 K (Robinson, 1985), depending on the wind and surface flux conditions.For instance,
when the long wave radiation from the upper few micrometers of the ocean is upward, the skin temperature is
usually cooler than the bulk SST.Latent and sensible heat fluxes can cool the sea surface further if the
air is dryer or colder.
Because of the small penetration depths of the infrared and microwave radiation, the satellite instruments
from space can only measure the upwelling long wave or microwave radiations from the surface skin layer.
However, most oceanographers are interested in the bulk SST in the surface mixed layer because traditionally
this is the temperature measured by ships, by drifter buoys, and by moored thermometers, etc., and because
the variations of the bulk SST involve large heat exchange, which can impact the earth´s climate.The
errors of satellite SST measurements due to the bulk-skin temperature difference can cause significant
inaccuracies in global climate studies.
Very few in situ measurements of the surface skin temperature are made on a regular basis, so the
MODIS/Aqua SST data have been calibrated primarily by the bulk SST of in situ and ship-board measurements
(Smith et al., 1996; Minnett, 1999; Barton, et al., 2003).The calibration is necessary because the
atmospheric corrections, which the infrared measurement is sensitive to, involve large uncertainties.
Thus the MODIS SST can be regarded as the best representation of the bulk SST based on the space-borne
instruments, particularly during high-wind conditions (wind speed at 10 m above the sea level > 6 m/s)
(Donlon et al., 2002).Over the tropical oceans, the MODIS algorithms have been calibrated by the Tropical
Atmosphere-Ocean (TAO) mooring measurements at a nominal depth of 1 m.However, the use of ship-borne
measurements may introduce errors of depth variations and may impact the accuracy of the SST retrievals (Donlon et al, 2002).
The microwave can penetrate clouds and thus the AMSU instrument is advantageous in measuring SST under
cloud cover.However, the microwave SST measurements are strongly frequency and surface dependent and can be
highly erroneous in the areas of strong precipitation, which make the microwave-only measurements less
attractive in many areas of the ocean.The design of the AIRS/AMSU instrument is to achieve effective
de-clouding using the high spectral resolution of the AIRS instrument combined with the AMSU field of
view (FOV)(Aumann, et al., 2003).The de-clouding makes use of the multiple FOVs of the AIRS within an
AMSU footprint and retrieves the surface skin temperature and the vertical temperature profiles of the
atmosphere simultaneously in an iterative procedure (Susskind et al, 2003).Because of the intrinsic
structure of the vertical temperature profile retrieved, the AIRS/AMSU SST is believed to be close to the
skin SST in theory.Thus, the difference between the MODIS SST and the AIRS/AMSU SST contains the bulk-skin
SST difference in addition to algorithm errors and instrument inaccuracies and others.Indeed, the AIRS/AMSU
SST has been found to be colder than the MODIS SST on global scales (Aumann and Strow, 2003).
Because of the use of the surrounding FOVs, the de-clouding of AIRS/AMSU SST has been achieved at the
sacrifice of the spatial resolution.Currently, the MODIS swath resolution at nadir is 1 km in comparison
with the AIRS/AMSU resolution at swath nadir of 40.5 km.Figures 3 and 4 show examples of the MODIS
(product MYD28L2) and AIRS SST (product AIRX2RET) nighttime swaths over the Japan area.The black areas
in Figure 3 over the ocean indicate cloud coverage in the MODIS SST data.The missing data in Figure 3
indicate bad retrievals of the AIRS/AMSU SST data.It is noticed that the MODIS data has separate SST retrievals
at 11 µm and 4 µm while the AIRS/AMSU use the radiances from 13 µm to 3.75 µm for
surface property retrievals.We have only compared the MODIS 11 µm SST with the AIRS/AMSU SST in this article.
Figure 3. MODIS SST image on April 11, 2004.
In Figures 3 and 4, the warm SST fronts south of Japan indicate the Kuroshio fronts.North of Japan, in
the Sea of Japan, the SST fronts of the Tsushima Current are visible in both images.Direct comparison of
the two images shows that the MODIS SST image contains much more cloud coverage but much higher spatial
resolution than the AIRS/AMSU image.Along the confluence zone of the Kuroshio and the Oyashio at the 36°
N east of Japan, the MODIS image indicates clear sky with detailed structures of the SST variations revealed
while the AIRS/AMSU image indicates bad retrievals of the SST pixels along the front.The AIRS/AMSU data
are only retrieved within the latitudinal range of 40° S to 40° N at present while the MODIS SST
covers areas at much higher latitudes.Because of the coarse spatial resolution, the AIRS/AMSU SST data
in coastal waters are not retrieved well.Despite the differences, the basic features of the Kuroshio and
the Tsushima Current are observed well by both instruments.For example, the meander of the Kuroshio just
east of Japan is captured by both images.
Figure 4.AIRS SST images on April 11, 2004.
MODIS algorithms for SST retrieval sometimes fail at the front where large SST gradient is indicated by the
observations.The black points along the eastern front of the Oyashio are an example of it.However, these
SST pixels are kept in the MODIS SST swath data.If a user wants to use them, they can simply relax the
quality filtering criterion.
Because of the bulk-skin SST difference and of the different retrieval algorithms, there are discrepancies
between the MODIS and the AIRS/AMSU measurements of the SST at the same location.Note that both of the
MODIS and AIRS/AMSU instruments are onboard of Aqua satellite so that they scan the same point of the
earth's surface at exactly the same time.A few examples of the comparisons are made in Table 1.
The comparisons are made at AIRS/AMSU SST pixels of good quality by averaging the MODIS SST pixels of
good quality within the AIRS/AMSU pixel.
Table 1.Comparison of AIRS and MODIS SST
| || AIRS || MODIS || NOBS
|142°E,34°N ||18.59° C ||19.37° C ||1241
|142°E,34.5°N ||18.79° C ||19.52° C ||1215
|142°E,35°N ||19.47° C ||19.20° C ||902
|142°E,35.5°N ||19.21° C ||19.65° C ||1174
|132°E,28°N ||21.04° C ||21.90° C ||758
|133°E,28°N ||20.66° C ||20.80° C ||880
The values in Table 1 indicate that the AIRS/AMSU SST is generally colder than the MODIS SST, but the
difference is within ±1.0 K, consistent with the bulk-skin SST difference and with the uncertainty
of the satellite SST retrievals.
Although the AIRS/AMSU SST is generally colder than the MODIS SST on global scales (Aumann and Strow, 2003),
the difference between the two SSTs can vary significantly over space.Figure 5 shows a comparison of the
two SST measurements along a section of 30° N south of Japan.According to the MODIS image, the area
is clear of clouds.The comparison shows that the MODIS SST is colder than the AIRS/AMSU SST along this section.
Figure 5. AIRS/AMSU and MODIS SST along 30° N south of Japan.The unit of SST is °C.
In summary, the MODIS SST and the AIRS/AMSU SST are equivalent measurements to the accuracies of the bulk-skin
SST difference and of the satellite retrieval.The MODIS SST data have batter spatial resolution and high-latitude
retrievals but more cloud coverage.In comparison, the AIRS/AMSU SST data have less cloud coverage but much
coarser spatial resolution and fewer coastal SST retrievals.AIRS/AMSU SST data indicate bad retrieval near the
Kuroshio and Oyashio confluence zone under clear sky.Both SST datasets have captured the major oceanography
phenomena, like the Kuroshio meander and the Tsushima Current front, etc.
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