MODIS Ocean Science Team Contributors to the Ocean Presentations

1. Just the sunglint slides… MODIS Ocean Science TeamContributors to the Ocean Presentations L’Aquila Summer Course - August 26, 2002 Mark Abbott Oregon State University Barney Balch Bigelow Otis Brown University of Miami Dennis.K.Clark NOAA Janet Campbell University of New Hampshire Ken Carder University of South Florida Wayne Esaias NASA Robert Evans University of Miami Howard Gordon University of Miami Frank Hoge NASA Eddie Kearns University of Miami Ricardo Letelier Oregon State University Peter Minnett University of Miami Ken Voss University of Miami

2. Where to get data and more information Information locations: MODIS Oceans home page • http://modis-ocean.gsfc.nasa.gov MODIS Oceans QA Browse 36km Imagery (MQABI) • http://jeager.gsfc.nasa.gov/browsetool/ Select Terra collection 4 Useful links to documentation and related web pages • http://modis-ocean.gsfc.nasa.gov/doclinks.html Data Ordering locations: NASA GES DAAC WHOM (NASA - Goddard DAAC) http://daac.gsfc.nasa.gov/ • Select Ocean color ->MODIS->ocean EOS DATA GATEWAY EDG - http://modis.gsfc.nasa.gov/data/ordering.html

3. http://modis-ocean.gsfc.nasa.gov/qa MQABI Quality Assurance Web Page … provides access to the available MODIS Ocean data collections, the ability to browse selected fields and links to information concerning the status of the Ocean products.

4. MODIS Compared to AVHRR, SeaWiFS • 12 bit digitization vs 10 bit --> improved precision • Lower noise detectors --> subtle features better resolved • Global 1 km data stream vs 4km --> larger data sets • Additional spectral channels --> improved and additional product algorithms, better quality determination • Equatorial crossing time --> significant impact on atmospheric correction • Shared calibration sources: • optical:- MOBY buoy, • infrared: MAERI interferometer, buoys

5. Radiance sources and sinks affecting visible and IR wavebands

6. Time-series of M-AERI measurements on Explorer of the Seas M-AERI The Explorer of the Seas is a Royal Caribbean Cruise Liner, operating a bi-weekly schedule out of Miami. It is outfitted as an oceanographic and atmospheric research vessel, very suitable for satellite validation. For more details see http://www.rsmas.miami.edu/rccl/

7. Atmospheric CorrectionEffects of the Atmosphere • We want to measure the "color" of the ocean, but the satellite actually measures “ocean + atmosphere”. The atmosphere is 90% of the signal in the ‘blue’ segment of the spectrum, and it must be accurately modeled and removed. • Some of the atmospheric effects that are included in visible “atmospheric correction” for retrieval of ocean water leaving radiance or reflectance include: • Gaseous absorption (ozone, water vapor, oxygen). • Molecular scattering (air molecules), also referred to as Rayleigh scattering. Reason for blue skies and red sunsets. • Aerosol scattering and absorption (haze, dust, pollution). Whitens or yellows the sky. • Adapted from http://seawifs.gsfc.nasa.gov/SEAWIFS/TEACHERS/CORRECTIONS/Bryan Franz, SeaWiFS Project

8. Ocean-Atmosphere SpectrumWater-Leaving Radiance Retrieval Challenge MODIS - At satellite radiance Rayleigh removed radiance Water leaving radiance: Lw

9. Atmospheric Correction Equation t = r + (a + ra) + twc + tg + t w t : total reflectance measured at satellite w : water-leaving reflectance. r: contribution due to molecular (Rayleigh) scattering, which can be accurately modeled. MODIS requires accurate measurement of change in its mirror reflectivity with angle of incidence a + ra : contribution due to aerosol and Rayleigh-aerosol scattering, estimated in NIR from measured radiances and extrapolated to visible using aerosol models. wc : contribution due to whitecaps, estimated from statistical relationship with wind speed. g : Sunglint reflectance from sea surface; SeaWiFS avoids by tilting sensor; MODIS does not tilt so sunglint must be removed.

10. Atmospheric Correction Differences: MODIS vs SeaWiFS • Glint : Spectral diffuse glint term, modified Cox-Munk distribution with spectral weighting, must be removed for MODIS (non-tilting), SeaWiFS minimizes glint by tilting • Rayleigh : Polarization varies with satellite and solar zenith angles, MODIS mirror angle of incidence (AOI) affects reflectivity, SeaWiFS has constant AOI mirror • Multiple (10) detectors per spectral band : Affects Rayleigh, especially near nadir, SeaWiFS has 1 detector/band • Equatorial crossing time : SeaWiFS noon, MODIS Terra 1030, MODIS Aqua 1330. Affects sun glint, bidirectional reflectance.

11. Atmospheric correction enhancements for MODIS • Instrument effects : detector and mirror side banding (banded appearance) • Polarization (effect on MODIS optical train) • Sunglint • Sun-satellite-observation point viewing geometry, overpass time, BRDF (bidirectional reflectance)

12. Sunglint Correction Sun glint influences large portions of the image. Several approaches to correcting glint problem were investigated. a) Assumed sunglint was direct, i.e. no scattering component. Result: Lw’s (water-leaving radiances) decreased as increased sunglint was removed. b) Removed diffuse Rayleigh scattering component of the sun glint. Result: Lw retrievals showed a spectral behavior. Lw’s were correct in green region but under-corrected in blue. c) Added a diffuse aerosol component to the sun glint. Result: improved Lw retrievals in regions of sun glint contamination with reasonable spectral behavior.

13. Glint Corrected La 865nm Note wind-wave-current interaction. Glint suppressed in Gulf Stream region Uncorrected La 865nm Yellow and red region is glint contaminated (Lg > 5*La). > 70% of swath affected. Corrected La 865nm Sun glint removed

14. Future Sunglint Enhancement • Present sunglint correction relies on assimilation-modeled wind fields and Cox-Munk surface reflectance model (no wind direction) • Does not reflect local, kilometer-scale winds or wind-current interaction • Alternative : ‘measure’ surface reflectance by utilizing 4 and 11 micron infrared bands

15. 865-nm Aerosol Op Depth: May 8, 2000 Note aerosol fronts off U.S. East Coast and mid-ocean Goal : remove sunglint but retain aerosol signal

16. 412nm Water-Leaving Radiance: Cox-Munk glint corrected Note residual glint along Gulf Stream Core

17. Alternate approach: replace Cox-Munk with MODIS IR 869-nm Reflectance, Total minus Rayleigh MODIS IR regression fit F(4/11) where 4 includes reflected sunlight, 11 does not

18. Cross-scan glint behaviour Cox-Munk sun glint correction, slope after pixel 800 is residual sun glint IR sun glint correction

19. Glint Reflectance: Modeled vs Empirical Cox-Munk Glint Field MODIS IR Channel Glint Field (‘Measured’) Note IR glint field not aliased by presence of aerosol front

20. Cox-Munk minus IR-Empirical reflectance Use of IR bands enables detection of sunglint missed with Cox-Munk approach

21. Conclusions • MODIS, a nadir pointing sensor, is significantly affected by sunglint • Successful removal of sun-glint extends useful coverage ~20% of total scan or 30% of non-saturated scan • Glint modeled using Cox-Munk and diffuse Rayleigh+Aerosol spectral propagation provides excellent correction if a proper wind field is available and ocean velocity is negligible • MODIS IR bands are being investigated as an alternative providing a ‘measured’ surface reflectance to better accommodate wind-wave interaction • Initial results are encouraging

22. Strategies for sun-glint identification and correction in water-leaving radiances measured from MODIS University of Miami Rosenstiel School of Marine and Atmospheric Science February 2002 Robert H. Evans Edward Kearns Katherine Kilpatrick Modis Chl A3, May 8, 2000 K. Carder Semi-analytic

23. MODIS Polarization and cross-scan correction with reduced sunglint mask Daily images of Chlorophyll_a2 (SeaWiFS equivalent) Dec 4, 2000 (solstice) Apr 8, 2001 (equinox) June 10, 2001 (solstice)

24. Sun Glint Reflectance Equation glint reflectance g = tg * t tg = glintsc() * zglint * zbst() * t_star() • glintsc() = 2.4-2.5 (spectral scale factor) • zglint = sun glitter coefficient using Cox and Munk (1954a,b; 1956) ; assumes vector wind, modified wind speed input to adjust shape • zbst = two-way (Sun --> surface --> satellite) Rayleigh and ozone diffuse transmittance; • t_star = one-way (surface --> satellite) aerosol diffuse transmittance using chosen models;