Post on 15-Dec-2015
transcript
Infrared measurements of sea surface temperature (SST)
• 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.
Andreas Muenchow, University of Delaware, May-11, 2010
My first MODISSST image(made 4 days ago):
Pretty, but wrongon so many levels…
Nares StraitAug.-3, 2009
What is SST?
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)
MODIS Ocean Standard Products (Level-2)
[from Gumley, 2006]
sst4 sst
sst4 usable only at night (solar contributions)sst usable day and night (negligible solar contributions)
2 bands usedto estimatesst and sst4
Planck’s Law:
[from Robinson, 2004]
Sensor CalibrationBand-integrated radiance as a function of temperature(Planck’s Law) at detector:
L(Tb) = ∫ C1 () / [5 exp(C2/ Tb)-1] d
whereTb blackbody temperature() detector response function (determined pre-launch)C1, C2 constants
L = gain*S + offset or Tb = A + B ln(L)
Calibration finds gain and offsetto relate the digital output signal S to radiance at detector L:
Need 2 known points to find gain and offset for each detector
Striping dueto imperfectinter-detectorcalibrations
SST
MODIS has 10 detectors scanned by 2 mirror-sides --> 20 calibrations
[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]
Canadian Center forRemote SensingCloud Detection
Standard NASACloud Detection
[from Luo et al., 2008]
SST = a + b*T4 + c*(T4-T5) + d* (T4-T5)*(sec-1)
Tbi brightness temperature channel “i”, e.g, T4 (Band-31 in Modis)Tbj brightness temperature channel “j”, e.g, T5 (Band-32 in Modis)
Atmosphere-A: Atmosphere-B
[from Robinson, 2004]
SST = a + b*T4 + c*(T4-T5) + d* (T4-T5)*(sec-1)
a=-263.006b=0.963563c=2.579211d=0.242598 sensor zenith
Daytime Coefficients for NOAA-12 AVHRRSST algorithm(McClain et al., 1985)
SST
SST4
[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 - BT12BT11 = brightness temperature at 11 um, in deg-CBT12 = brightness temperature at 12 um, in deg-Cbsst = 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 - BT40BT39 = 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 match-ups 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-seaskin temperature forSST validation andalgorithm development
[from Minnett et al., 2001]
My first MODISSST image(made 4 days ago):
Pretty, but wrongon so many levels…
Nares StraitAug.-3, 2009
What is SST?
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