Water Leaving Radiance and the Validation of NASA Satellites with Biospherical Instruments Inc.

Alexandra Olivier, Biospherical Instruments, San Diego, CA http://sos.noaa.gov/images/atmosphere/biosphere.png

Inherent Optical Properties (IOPs)

Independent of ambient lightDepend on composition, morphology, and concentration of particles IOPs:

Beam Attenuation Coefficient (c) c = a + b

Scattering Phase Function (β)Absorption Coefficient (a)

depends on the material and also on the wavelength of light which is being absorbed.

Volume Scattering Function (b)Petzold’s measurements

Apparent Optical Properties(AOPs)

Depend on medium & directional structure of ambient light fieldShow regular features & stabilityRadiance/various irradiances are not AOPsAOPs:

Various reflectances / etc.)Average cosines (µ) Diffuse attenuation coefficients (i.e., ...)

“K-Functions”-logarithmic derivatives units: 1/m or “per meter”

Water Leaving Radiance () • Determined by using above

or in water measurements

• Carries information about the water column and bottom conditions

• Usually measured, but can be calculated taking atmospheric corrections into consideration

http://remotesensing.spiedigitallibrary.org/data/Journals/APPRES/926148/JARS_7_1_073558_f003.png

Atmospheric Correction Process of removing contributions by surface glint and atmospheric scattering from the measured total radiance to get

NIR & SWIR bands are used for determination

) or

Self-Shading Causes upwelling radiance values and remote sensing reflectance to be too low

Affected by marine and atmospheric conditions; dependent on solar zenith angle

Simulated responses with or without the presence of a shading object are computed using the Monte Carlo Simulation

The magnitude of error depends on wavelength, sensor size, water turbidity, and illumination conditions; greatest at small solar zenith angles.

Self-Shading Error

Shading error for in general sky conditions

Shading error for in general sky conditions:

Fractional shading error (no atmospheric scattering):

NomenclatureK = constant that depends on measurement type & illumination conditions corrected radiance, irradiance measured radiance, irradiancea = water beam absorption coefficientr = radius of housing shading error for direct sunlight from appropriate sun angleshading error for indirect sunlight increase with increased solar zenith angle and increased chlorophyll concentration refracted solar zenith angle empirical factor for which they give values determined by fitting their model results (

Self-Shading CorrectionShading corrections should be routinely applied to upwelling light measurements from in-water instruments

Corrected radiance:

Corrected irradiance:

Monte Carlo SimulationStatistical simulation technique that provides approximate solution to mathematically expressed problems Method that utilizes the sequence of random numbers to perform simulations Does not give an exact answer, instead a statistical estimate with errorMost common use is evaluation of integral and calculation of mathematical constant variable

Essential feature is that the known probability of occurrence of each separate event in sequence of events is used to establish probability of occurrence of entire sequence

Monte Carlo Simulation a reasonable approach to developing a Monte Carlo algorithm:

figure out how to numerically simulate a process as it occurs in naturethen figure out how to simulate another, perhaps artificial, process that will give the same answer as the "natural" process, but with less computational time.

Process:Consider how photons propagate through a mediumUsing a ratio of scattering efficiency to total extinction efficiency (“attenuance”) to determine the probability of photon survival in any particular interaction:

1. Draw a random number, t, from a U[0.1] distribution. 2. Compare t with .

• If t, then the photon is scattered. • If t, then the photon is absorbed.

Oceanic Remote Sensing Use of optical measurements made from satellites or aircraft to obtain information about the constituents of natural waters, corresponding IOPs, or bottom depth and type

Two types:Active: signal of known characteristics sent from sensor platform, to the ocean & the return signal is detected after time of delayPassive: observe light that’s naturally emitted or reflected by water body

Ocean Color Remote Sensing: The term “Ocean Color” is loosely used to mean radiometric data at two or more visible wavelengths, from which useful information about water bodies can be extracted

http://eijournal.sensorsandsystems.com/newsite/wp-content/uploads/2013/12/astrium_figure-11.jpg

Ocean Color Remote SensingSome of the current applications of ocean color data:

Mapping of chlorophyll concentrations Measurement of IOPs Determination of phytoplankton physiology, phenology, and functional groups Studies of ocean carbon fixation & cycling Monitoring of ecosystem changes resulting from climate changeFisheries management Monitoring of water quality for recreationDetection of harmful algal blooms & pollution events

Normalized Water Leaving Radiance

From Field:[ = = µW

From Satellite: [ = (ϴ’) µW

Exact Normalized Water Leaving Radiance

From Field:[ µW

From Satellite: [ µW

Transferring [ to [: [

Summary of Measuring From the Field:

Step 1- Water Leaving Radiance (if not measured): OR µW

Step 2- Normalized Water Leaving Radiance:[ = = µW

Step 3- Exact Normalized Water Leaving Radiance:[ µW

From Satellite Summary:

Step 1- Normalized Water Leaving Radiance: [ = (ϴ’) µW

Step 2- Exact Normalized Water Leaving Radiance:[ µW

Nomenclature extraterrestrial solar spectral irradiance at mean earth-sun distance (extraterrestrial solar flux atRemote-sensing reflectance reflection x refractionratio of the upwelling irradiance just beneath the ocean surface to the upwelling radiance just beneath the ocean surfacebackscatter to absorption ratio for nadir viewing instrument’s unknown radiance response calibration factor shipboard radiometer measured radiance from the sea surface at Zenith total skylight actually reflected from wave-roughened sea surface into direction divided by sky radiance sky radiance measured with radiometer looking upward light incident upon medium of index the air–water Fresnel reflection at the interface for the whole (Sun + Sky) downwelling

irradiance the water–air Fresnel reflection for the whole diffuse upwelling irradiance

Why?Radiance and irradiance are measured together because separately, they do not make good candidates for AOPs.

Fail to meet the requirement of being stable enough to be a useful descriptor of water

Exact normalized water leaving radiance is the only measurement that can be successfully compared for validation

Ocean Color Remote Sensing:reveals large and small scale structures that are very difficult to observe from the surface.Locates and enables monitoring of regions of high and low bio-activity

Food (phytoplankton associated with chlorophyll) Climate (phytoplankton possible CO2 sink)

Reveals ocean current structure and behavior:Seasonal influences River and Estuary influences Boundary currents

ReferencesGordon, Howard R., and Kenneth J. Voss. "MODIS Normalized Water-leaving Radiance Algorithm Theoretical Basis Document." 30 Apr. 1999. Web. 15 May 2015. <http://oceancolor.gsfc.nasa.gov/DOCS/atbd_mod18.pdf>.

Hooker, Stanford, Curtis Mobley, et al. "Ocean Optics Protocols For Satellite Ocean Color Sensor Validation, Revision 4." III (2003). Web. 20 May 2015. <http://oceancolor.gsfc.nasa.gov/DOCS/Protocols_Ver4_VolIII.pdf>.

Leathers, Robert A., T. Valerie Downes, Curtiss O. Davis, and Curtis D. Mobley. “Self-Shading Corrections for Oceanographic Upwelling Radiometers." 6 July 2004. Web. 18 May 2015.

Leathers, Robert A., and T. Valerie Downes. "Self-shading Correction for Upwelling Sea-surface Radiance Measurements Made with Buoyed Instruments." Optical Society of America, 1 May 2001. Web. 17 May 2015.

Mobley, Curtis. "Ocean Optics." Ocean Optics Web Book. 21 Apr. 2010. Web. 15 May 2015. <http://www.oceanopticsbook.info/view/introduction/overview>.

Mobley, Curtis D. Light and Water: Radiative Transfer in Natural Waters. San Diego: Academic, 1994. Print.

Morel, Andre. "Normalized Water-Leaving Radiance." 24 Jan. 2001. Web. 27 Apr. 2015. <http://www.ioccg.org/reports/morel_nLw.pdf>.

Morel, Andre', and Bernard Gentili. "Diffuse Reflectance of Oceanic Waters. III. Implication of Bidirectionality for the Remote-sensing Problem." Applied Optics 35.24 (1996): 4850-862. Web. 20 May 2015.

Siegel, David A., Menghua Wang, Stéphane Maritorena, and Wayne Robinson. "Atmospheric Correction of Satellite Ocean Color Imagery: The Black Pixel Assumption." Applied Optics Appl. Opt. (1999): 3582. Web. 20 May 2015.