Nares Strait
Aug.-3, 2009
Level-1B
Band-1 @ 645nm
Infrared measurements of sea surface
temperature (SST)
Andreas Muenchow, University of Delaware, May-11, 2010
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Nares Strait motivation
Vertical and temporal temperature changes
Sensor calibration
Cloud detection
Atmospheric correction
Validation
References:
1. Robinson, I.S., 2004: Measuring the Oceans from Space, chapt.-7
2. Gumley, L., 2006: Modis Ocean Products, http://www.ssec.wisc.edu/library/coursefiles/SouthAfrica/Gumley_MODIS_Ocean.ppt
3. Vincent et al., 2008: Arctic waters and marginal ice zones, a composite Arctic sea surface temperature algorithm, J. Geophys. Res., 113,
C04021, doi:10.1029/2007JC004353.
4. Luo, et al., 2008: Developing clear sky, cloud, and cloud shadow mask for producing clear-sky composites at 250-meter spatial
resolution, Rem. Sens. Env., 112, 4167-4185.
5. Minnett, P.J., 2001: The marine-atmosphere emitted radiance interferometer, a high accuracy, seagoing infrared spectrometer, J, Atmos.
Ocean. Tech., 18, 994-1013.
Nares Strait
Aug.-3, 2009
My first MODIS
SST image
(made 4 days ago):
Pretty, but wrong
on so many levels
…
What is SST?
MODIS Ocean Standard Products (Level-2)
Geophysical
Parameter Name
Description
nLw_412
Normalized water-leaving radiance at 412 nm
nLw 443
Normalized water-leaving radiance at 443 nm
nLw_488
Normalized water-leaving radiance at 488 nm
nLw_531
Normalized water-leaving radiance at 531 nm
nLw_551
Normalized water-leaving radiance at 551 nm
nLw_667
Normalized water-leaving radiance at 667 nm
Tau_869
Aerosol optical thickness at 869 nm
Eps_78
Epsilon of aerosol correction at 748 and 869 nm
Chlor_a
OC3 Chlorophyll a concentration
K490
Diffuse attenuation coefficient at 490nm
Angstrom_531
Angstrom coefficient, 531-869 nm
SST
Sea Surface Temperature: 11 micron
SST4
Sea Surface Temperature: 4 micron (night only)
[from Gumley, 2006]
Planck’s Law:
sst4
sst
2 bands used
to estimate
sst and sst4
sst4 usable only at night (solar contributions)
sst usable day and night (negligible solar contributions)
[from Robinson, 2004]
sst4
sst
[from Robinson, 2004]
[from Robinson, 2004]
[from Robinson, 2004]
[from Robinson, 2004]
[from Minnett et al., 2001]
[from Robinson, 2004]
Night
Day
[from Robinson, 2004]
[from Robinson, 2004]
[from Robinson, 2004]
Sensor Calibration
Band-integrated radiance as a function of temperature
(Planck’s Law) at detector:
L(Tb) = ∫ C1 () / [5  exp(C2/ Tb)-1] d 
where
Tb blackbody temperature
() detector response function (determined pre-launch)
C1, C2 constants
Calibration finds gain and offset
to relate the digital output signal S to radiance at detector L:
L = gain*S + offset
or
Tb = A + B ln(L)
Need 2 known points to find gain and offset for each detector
MODIS has 10 detectors scanned by 2 mirror-sides --> 20 calibrations
SST
Striping due
to imperfect
inter-detector
calibrations
[from Gumley, 2006]
Chlor_a
[from Gumley, 2006]
MODIS Chlorophyll Algorithm
Semi-analytical algorithm(1)
Chl_a = 10**(0.283 - 2.753*R + 1.457*R2 + 0.659*R3 - 1.403*R4)
where:
R = log10((Rrs443 > Rrs488) / Rrs551)
Rrs = nLw / F0; remote sensing reflectance
F0 = extraterrestrial solar irradiance
nLw = water leaving radiance at 443, 488, 551
(1)
Performance of the MODIS Semi-analytical Ocean Color Algorithm for Chlorophyll-a Carder, K.L.; Chen, F.R.; Cannizzaro,
J.P.; Campbell, J.W.; Mitchell, B.G. Advances in Space Research. Vol. 33, no. 7, pp. 1152-1159. 2004
[from Gumley, 2006]
[from Robinson, 2004]
Canadian Center for
Remote Sensing
Cloud Detection
Standard NASA
Cloud Detection
[from Luo et al., 2008]
This really is
another
full lecture
[from Robinson, 2004]
[from Robinson, 2004]
2008
[from Luo et al., 2008]
Modis Bands
Bi with i=1,2,3,6
[from Luo et al., 2008]
[from Luo et al., 2008]
[from Luo et al., 2008]
[from Robinson, 2004]
SST = a + b*T4 + c*(T4-T5) + d* (T4-T5)*(sec-1)
Atmosphere-A:
Atmosphere-B
Tbi brightness temperature channel “i”, e.g, T4 (Band-31 in Modis)
Tbj brightness temperature channel “j”, e.g, T5 (Band-32 in Modis)
[from Robinson, 2004]
Daytime Coefficients for NOAA-12 AVHRR
SST algorithm(McClain et al., 1985)
SST = a + b*T4 + c*(T4-T5) + d* (T4-T5)*(sec-1)
a=-263.006
b=0.963563
c=2.579211
d=0.242598
 sensor zenith
SST4
SST
[from Robinson, 2004]
MODIS Longwave Infrared Sea Surface Temperature ---> SST
dBT <= 0.5
sst = a00 + a01*BT11 + a02*dBT*bsst + a03*dBT*(sec() - 1.0)
dBT >= 0.9
sst = a10 + a11*BT11 + a12*dBT*bsst + a13*dBT*(sec() - 1.0)
0.5 < dBt < 0.9
sstlo = a00 + a01*BT11 + a02*dBT*bsst + a03*dBT*(sec() - 1.0)
ssthi = a10 + a11*BT11 + a12*dBT*bsst + a13*dBT*(sec() - 1.0)
sst = sstlo + (dBT - 0.5)/(0.9 - 0.5)*(ssthi - sstlo)
where:
dBT = BT11 - BT12
BT11 = brightness temperature at 11 um, in deg-C
BT12 = brightness temperature at 12 um, in deg-C
bsst = Either sst4 (if valid) or sstref (from Reynolds OISST)
sec() = 1/(cosine of sensor zenith angle)
a00, a01, a02, a03, a10, a11, a12, a13 derived from match-ups
[from Gumley, 2006]
MODIS Shortwave Infrared Sea Surface Temperature --> SST4
sst4 = a0 + a1 * BT39 + a2 * dBT + a3 * (sec() - 1.0 )
where:
dBT = BT39 - BT40
BT39 = brightness temperature at 3.959 um, in deg-C
BT40 = brightness temperature at 4.050 um, in deg-C
sec() = 1/(cosine of sensor zenith angle)
a0, a1, a2, and a3 are time dependent coefficients derived from matchups between observed MODIS brightness temperature and field
measurements of SST.
Note: sst4 is not valid during daytime because of solar reflection.
[from Gumley, 2006]
Measuring at-sea
skin temperature for
SST validation and
algorithm development
[from Minnett et al., 2001]
Nares Strait
Aug.-3, 2009
My first MODIS
SST image
(made 4 days ago):
Pretty, but wrong
on so many levels
…
What is SST?
Standard SST algorithm
Arctic SST
algorithm
[from Vincent et al., 2008]
[from Vincent et al., 2008]
[from Vincent et al., 2008]
Is atmospheric correction always appropriate?
SST = a + b*T4 + c*(T4-T5) + d* (T4-T5)*(sec-1)
Is anything lost by applying atmospheric corrections?
Is atmospheric correction always appropriate?
SST = a + b*T4 + c*(T4-T5) + d* (T4-T5)*(sec-1)
Is anything lost by applying atmospheric corrections?
•Image noise may be enhanced
•Includes noise from 2channels
•Thermal gradients are modified
Is atmospheric correction always appropriate?
SST = a + b*T4 + c*(T4-T5) + d* (T4-T5)*(sec-1)
Is anything lost by applying atmospheric corrections?
•Image noise may be enhanced
•Includes noise from 2channels
•Thermal gradients are modified
If spatial structures, patterns, fronts, eddies, plumes are studied
Use brightness temperatures Ti, not SST