Nares StraitAug.-3, 2009

Level-1BBand-1 @ 645nm

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]

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 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]

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 forRemote SensingCloud Detection

Standard NASACloud Detection

[from Luo et al., 2008]

This really isanotherfull lecture

[from Robinson, 2004]

[from Robinson, 2004]

2008

[from Luo et al., 2008]

Modis BandsBi 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)

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?

Standard SST algorithm

Arctic SSTalgorithm

[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