Sherwin D. LadnerSherwin D. Ladner11, Robert A. Arnone, Robert A. Arnone22, Richard W. Gould, Jr., Richard W. Gould, Jr.22, Alan Weidemann, Alan Weidemann22, Vladimir I. Haltrin, Vladimir I. Haltrin22, Zhongping Lee, Zhongping Lee22, Paul M. Martinolich, Paul M. Martinolich3,3, and and Trisha BergmannTrisha Bergmann44

Planning Systems Incorporated Naval Research Laboratory Neptune Sciences Incorporated Planning Systems Incorporated Naval Research Laboratory Neptune Sciences Incorporated Rutgers UniversityRutgers University

MSAAP Building 9121 Ocean Optics Section Code 7333 40201 Highway 190 East Institute of MSAAP Building 9121 Ocean Optics Section Code 7333 40201 Highway 190 East Institute of Marine and Coastal SciencesMarine and Coastal Sciences

Stennis Space Center, MS 39529 Stennis Space Center, MS 39529 Slidell, LA 70461 New Stennis Space Center, MS 39529 Stennis Space Center, MS 39529 Slidell, LA 70461 New Brunswick, NJ 08901Brunswick, NJ 08901

voice: (228)688-5754; email: voice: (228)688-5754; email: ladner@nrlssc.navy.mil

PSIPSI

VARIABILITY IN THE BACKSCATTERING TO SCATTERING AND F/Q RATIOS IN NATURAL WATERSVARIABILITY IN THE BACKSCATTERING TO SCATTERING AND F/Q RATIOS IN NATURAL WATERSOcean Optics XVI November 18-22, 2002 Santa Fe, New MexicoOcean Optics XVI November 18-22, 2002 Santa Fe, New Mexico

(1)(1) (3)(3) (4)(4)

Variability of the backscattering to scattering (bVariability of the backscattering to scattering (bbb/b) and the upwelling irradiance/upwelling radiance (f/Q) /b) and the upwelling irradiance/upwelling radiance (f/Q)

ratios affect the accurate retrieval of inherent optical properties from ocean color satellite algorithms (SeaWiFS ratios affect the accurate retrieval of inherent optical properties from ocean color satellite algorithms (SeaWiFS and MODIS). We investigate the variability of band MODIS). We investigate the variability of bbb/b and the f/Q in coastal and open-ocean surface waters of the /b and the f/Q in coastal and open-ocean surface waters of the

Northern Gulf of Mexico and U.S. East Coast off New Jersey. In situ measurements of scattering (b) from an ac9 Northern Gulf of Mexico and U.S. East Coast off New Jersey. In situ measurements of scattering (b) from an ac9 probe were collected with concurrent measurements of bprobe were collected with concurrent measurements of bbb from a six channel Hydroscat sensor. We compare the from a six channel Hydroscat sensor. We compare the

measured bmeasured bbb/b values with values derived from the Petzold Volume Scattering Functions (Petzold 1972), examine /b values with values derived from the Petzold Volume Scattering Functions (Petzold 1972), examine

the spectral variability of the ratio and examine the variability in different water types related to the changes in the spectral variability of the ratio and examine the variability in different water types related to the changes in the Volume Scattering Function (VSF). In addition, we estimate the T*f/Q (Mobley 1999) term from above-water the Volume Scattering Function (VSF). In addition, we estimate the T*f/Q (Mobley 1999) term from above-water measurements of remote sensing reflectance (Rrs) coupled with direct measurements of absorption (a) and measurements of remote sensing reflectance (Rrs) coupled with direct measurements of absorption (a) and backscattering (bbackscattering (bbb) coefficients. We will examine the spectral dependence of the T*f/Q term and its relationship ) coefficients. We will examine the spectral dependence of the T*f/Q term and its relationship

to the bto the bbb/b ratio, which we use as a substitute for the changing VSF. Finally, we will show how the estimated /b ratio, which we use as a substitute for the changing VSF. Finally, we will show how the estimated

T*f/Q values vary from the commonly used value of T*f/Q values vary from the commonly used value of 0.0510.051 used for satellite processing. used for satellite processing.

IntroductionIntroduction

Remote sensing algorithms for ocean color visible imagery are based on relationships between remote Remote sensing algorithms for ocean color visible imagery are based on relationships between remote sensing reflectance and such inherent optical properties as absorption and scattering. Previous studies have sensing reflectance and such inherent optical properties as absorption and scattering. Previous studies have simplified these relationships and are currently used in present ocean color algorithms such as with SeaWiFS. simplified these relationships and are currently used in present ocean color algorithms such as with SeaWiFS. The limitations of the relationships in different optical water types are under investigation. This effort is The limitations of the relationships in different optical water types are under investigation. This effort is directed at examining different water types defined by the backscattering to scattering ratio and the changes in directed at examining different water types defined by the backscattering to scattering ratio and the changes in the T*f/Q term (where Q is the ratio of upwelling irradiance to upwelling radiance). Both the ratios affect the the T*f/Q term (where Q is the ratio of upwelling irradiance to upwelling radiance). Both the ratios affect the accurate retrieval of the inherent optical properties.accurate retrieval of the inherent optical properties.

The remote sensing reflectance (Rrs) is dependent on the angular scattering described by the volume The remote sensing reflectance (Rrs) is dependent on the angular scattering described by the volume scattering function (VSF). The VSF describes the angular distribution of scattered light in the water column. scattering function (VSF). The VSF describes the angular distribution of scattered light in the water column. The integral of the VSF over full solid angle is the scattering coefficient (b); the integral over angles in the The integral of the VSF over full solid angle is the scattering coefficient (b); the integral over angles in the backward direction only (90-180 degrees) is the backscattering coefficient (bbackward direction only (90-180 degrees) is the backscattering coefficient (bbb). The VSF is influenced by ). The VSF is influenced by

particle characteristics such as shape, index of refraction, and size distribution. Because the VSF is difficult to particle characteristics such as shape, index of refraction, and size distribution. Because the VSF is difficult to measure (new instruments are just now becoming available), we use the bmeasure (new instruments are just now becoming available), we use the bbb/b ratio (or probability of /b ratio (or probability of

backscattering) as a substitute for the VSF to describe water mass differences. This ratio provides a method to backscattering) as a substitute for the VSF to describe water mass differences. This ratio provides a method to describe the differences in the VSF for different water masses and how the angular differences influence the describe the differences in the VSF for different water masses and how the angular differences influence the Rrs. Understanding the differences in bRrs. Understanding the differences in bbb/b for different water masses will help characterize the changes in the /b for different water masses will help characterize the changes in the

VSF, which affect Rrs estimates, and provide better estimates of inherent optical properties in coastal and VSF, which affect Rrs estimates, and provide better estimates of inherent optical properties in coastal and open-ocean waters (Northern Gulf of Mexico and New Jersey Coast) where different bopen-ocean waters (Northern Gulf of Mexico and New Jersey Coast) where different bbb/b water types are /b water types are

observed. This is important for researchers to model optical remote sensing algorithms, visibility and laser observed. This is important for researchers to model optical remote sensing algorithms, visibility and laser propagation in seawater. propagation in seawater.

Compare the measured bCompare the measured bbb/b ratios and linear Petzold relationship./b ratios and linear Petzold relationship.

Determine how the bDetermine how the bbb/b ratio (VSF surrogate to characterize water mass differences) changes both /b ratio (VSF surrogate to characterize water mass differences) changes both spectrally and in a wide variety of coastal and open-ocean water using spectrally and in a wide variety of coastal and open-ocean water using in situin situ measurements. measurements.

Use the relationship between measured remote sensing reflectance, backscattering and absorption to Use the relationship between measured remote sensing reflectance, backscattering and absorption to evaluateevaluatethe T*(f/Q) term. the T*(f/Q) term.

Examine the spectral nature of the estimated T*(f/Q) term using Examine the spectral nature of the estimated T*(f/Q) term using in situin situ measurements, variation in measurements, variation in relation to the commonly used satellite processing value of 0.05 and compare to past studies.relation to the commonly used satellite processing value of 0.05 and compare to past studies.

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33

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GULF OF MEXICO CRUISESGULF OF MEXICO CRUISES

20012001 - 65 Stations- 65 Stations20022002 - 18 Stations- 18 Stations

High scattering waters with elevated suspendedHigh scattering waters with elevated suspendedsediment loads and CDOM.sediment loads and CDOM.

NEW JERSEY CRUISESNEW JERSEY CRUISES

2000 - 18 Stations2000 - 18 Stations2001 - 73 Stations2001 - 73 Stations

Clearer waters containing less sediment.Clearer waters containing less sediment.

Above Water RrsAbove Water Rrs – Analytical Spectral Device (ASD) field spectroradiometer processed using Near-Infrared surface glint – Analytical Spectral Device (ASD) field spectroradiometer processed using Near-Infrared surface glintremoval which accounts for the Rrs at NIR wavelengths in high scattering waters (Gould et al 2001). This instrument removal which accounts for the Rrs at NIR wavelengths in high scattering waters (Gould et al 2001). This instrument measures the spectrum at 1.3 nm resolution from 400-800nm and a 12% spectralon (gray) card was used for calibration.measures the spectrum at 1.3 nm resolution from 400-800nm and a 12% spectralon (gray) card was used for calibration.

Absorption [a] and Beam Attenuation [c]Absorption [a] and Beam Attenuation [c] were measured using a WetLabs ac9 at 9 wavelengths (412, were measured using a WetLabs ac9 at 9 wavelengths (412, 440440, , 488488, 510, , 510, 532532,,555, 650, 555, 650, 676676, 715nm). The instrument was calibrated using milli-Q water and the (Zaneveld et al. 1994) scatter correction , 715nm). The instrument was calibrated using milli-Q water and the (Zaneveld et al. 1994) scatter correction was applied. We derived scattering by difference (c-a). was applied. We derived scattering by difference (c-a). Note: Red wavelengths are coincident between the ac9 and Hydroscat Note: Red wavelengths are coincident between the ac9 and Hydroscat instruments.instruments.

The spectral backscattering coefficient [bThe spectral backscattering coefficient [bbb]] was measured with the Hydroscat instrument, which measures the scattering at was measured with the Hydroscat instrument, which measures the scattering at 140 degrees and extrapolates the b140 degrees and extrapolates the bb b at 6 wavelengths (at 6 wavelengths (442442, , 488488, , 532532, 589, 620, , 589, 620, 676676nm). The instrument was calibrated at nm). The instrument was calibrated at the factory prior to the deployments. the factory prior to the deployments.

Pure Water CorrectionPure Water Correction : Backscattering and scattering due to pure water were removed using (Smith and Baker 1981) prior to : Backscattering and scattering due to pure water were removed using (Smith and Baker 1981) prior to investigating the differences in the spectral binvestigating the differences in the spectral bbb/b ratio./b ratio.

In Situ MeasurementsIn Situ Measurements

2. Spectral b2. Spectral bbb/b/b

Measured bMeasured bbb (532nm) vs. b (532nm) for (532nm) vs. b (532nm) for the two Gulf of Mexico Cruises. the two Gulf of Mexico Cruises.

Measured bMeasured bbb (532nm) vs. b (532nm) for (532nm) vs. b (532nm) for the two New Jersey Cruises (LEO15). the two New Jersey Cruises (LEO15).

Measured bMeasured bbb/b (532nm) ratio (y-axis) as a function of /b (532nm) ratio (y-axis) as a function of Wavelength (x-axis) for the two Gulf of Mexico Cruises. Wavelength (x-axis) for the two Gulf of Mexico Cruises.

Measured bMeasured bbb/b (532nm) ratio (y-axis) as a function of /b (532nm) ratio (y-axis) as a function of Wavelength (x-axis) for the two New Jersey Cruises. Wavelength (x-axis) for the two New Jersey Cruises.

Two distinct water types for both Gomex and NJTwo distinct water types for both Gomex and NJregions. regions. bbbb/b ratios of the /b ratios of the Northern Gulf of Mexico Northern Gulf of Mexico waters waters during during 20022002 (left) and the (left) and the New Jersey 2000New Jersey 2000 (right) (right) are similar to the (Petzold 1972) ratio.are similar to the (Petzold 1972) ratio. bbbb/b ratio can vary widely from the (Petzold 1972) /b ratio can vary widely from the (Petzold 1972) ratio. ratio.

Slope 2002 = 0.0138Slope 2002 = 0.0138

Slope 2001 = 0.01Slope 2001 = 0.01

Slope 2000 = 0.018Slope 2000 = 0.018

Slope 2001 = 0.004Slope 2001 = 0.004

Note: Each point represents the averageNote: Each point represents the averagebb/b value for each wavelength (slope of thebb/b value for each wavelength (slope of theline from the plot in section 1).line from the plot in section 1).

Data seems to depict two distinct water types although Data seems to depict two distinct water types although there is spread and overlap in the data.there is spread and overlap in the data. First water type (First water type (Gomex 2002Gomex 2002) is similar to) is similar tothe (Petzold 1972) relationship (the (Petzold 1972) relationship (white circleswhite circles) which) whichconsisted of 15 VSF measurements (515nm) made in consisted of 15 VSF measurements (515nm) made in San Diego Harbor over 30 years ago. San Diego Harbor over 30 years ago. Second water type consist of Second water type consist of Gomex 2001Gomex 2001 stations stations collected in waters containing less scattering, sediment collected in waters containing less scattering, sediment loads, and CDOM. loads, and CDOM. Both Gulf of Mexico cruises were conducted in the sameBoth Gulf of Mexico cruises were conducted in the samevicinity one year apart. vicinity one year apart.

Data seems to depict two distinct water types more Data seems to depict two distinct water types more clearly.clearly.

First water type (First water type (NJ 2000NJ 2000) is similar to Petzold ) is similar to Petzold relationship (relationship (white circleswhite circles) and Gulf of Mexico waters.) and Gulf of Mexico waters.

Second water type (Second water type (NJ 2001NJ 2001) consist of mostly open ) consist of mostly open ocean waters dominated by biological rather than ocean waters dominated by biological rather than inorganic particles and produced a bb/b ratio lower inorganic particles and produced a bb/b ratio lower than other cruises.than other cruises.

Both New Jersey cruises were conducted in the Both New Jersey cruises were conducted in the vicinity one year apart.vicinity one year apart.

Spectral shapes for both Gomex and NJ regions are Spectral shapes for both Gomex and NJ regions are nearly flat.nearly flat.

New Jersey waters show more distinct differenceNew Jersey waters show more distinct differencein magnitude.in magnitude.

Changes in bChanges in bbb/b ratio can possibly be used to /b ratio can possibly be used to characterize the VSF in different water types and characterize the VSF in different water types and agrees with the behavior seen in actual VSFagrees with the behavior seen in actual VSFmeasurements (Haltrin et al., OOXVI 2002)measurements (Haltrin et al., OOXVI 2002)

3.3. Measured Rrs vs. [bMeasured Rrs vs. [bbb/(a+b/(a+bbb)] : Evaluating the T*(f/Q) Term)] : Evaluating the T*(f/Q) TermRrs = (T*f/Q) * [bRrs = (T*f/Q) * [bbb/(a+b/(a+bbb)] ; (Mobley 1999) )] ; (Mobley 1999)

Theoretically, the T*(f/Q) should range between approximately 0.0366 – 0.0612. (0.051 is commonly used in most satellite Theoretically, the T*(f/Q) should range between approximately 0.0366 – 0.0612. (0.051 is commonly used in most satellite processing techniques).processing techniques).

Used measured properties of a, bUsed measured properties of a, bbb, and computed the ratio [b, and computed the ratio [bbb/(a+b/(a+bbb)] for Gulf of Mexico and New Jersey stations.)] for Gulf of Mexico and New Jersey stations.

Regions were plotted separately but did not show a regional dependence.Regions were plotted separately but did not show a regional dependence.

Relationships show a spectral variance in the T*(f/Q) term.Relationships show a spectral variance in the T*(f/Q) term.

Using in situ measurements of a, bUsing in situ measurements of a, bbb, and rrs, we observed the T*(f/Q) term to range from 0.04 (532nm) – 0.07 (440nm)., and rrs, we observed the T*(f/Q) term to range from 0.04 (532nm) – 0.07 (440nm).

The scatter could be due to higher in-water transparency at 532 nm than the other channels and is the wavelength of minimum The scatter could be due to higher in-water transparency at 532 nm than the other channels and is the wavelength of minimum absorption. Any small variation in absorption, whether it is due to the error in measurement or stability of the instrument, absorption. Any small variation in absorption, whether it is due to the error in measurement or stability of the instrument, could possibly cause such scatter.could possibly cause such scatter.

Measured Rrs vs. bMeasured Rrs vs. bbb/(a+b/(a+bbb) ratio for all stations collected in the Gulf of Mexico and New Jersey regions by ) ratio for all stations collected in the Gulf of Mexico and New Jersey regions by wavelength. wavelength.

4. Spectral T*(f/Q) Comparison4. Spectral T*(f/Q) Comparison

Mean T*(f/Q) vs. wavelength. Error Mean T*(f/Q) vs. wavelength. Error barsbars

indicate + or – one standard indicate + or – one standard deviation.deviation.

ConclusionsConclusions

(Morel and Gentili 1993) results (Case I waters) are (Morel and Gentili 1993) results (Case I waters) are stable and linear as a function of wavelength stable and linear as a function of wavelength whereas results inwhereas results inthis study (non-Case I waters) show a larger this study (non-Case I waters) show a larger variation. variation.

The bThe bbb/b ratio derived from in situ measurements varies /b ratio derived from in situ measurements varies spectrally over different water types and could be related spectrally over different water types and could be related to regional differences in the VSF. This ratio could act as to regional differences in the VSF. This ratio could act as a surrogate for differences in the VSF for different watera surrogate for differences in the VSF for different watermasses. masses.

Understanding the bUnderstanding the bbb/b relationship will help characterize /b relationship will help characterize the changes in the VSF, which affect Rrs estimates.the changes in the VSF, which affect Rrs estimates.

The estimated T*(f/Q) in this study derived using above-The estimated T*(f/Q) in this study derived using above-water measurements of Rrs coupled with direct water measurements of Rrs coupled with direct

measurements measurements of absorption (a) and backscattering (bof absorption (a) and backscattering (bbb) has more spectral ) has more spectral variability in variability in nonnon-Case I waters compared to values -Case I waters compared to values

presented presented by Morel and Gentili in case I waters.by Morel and Gentili in case I waters.

We found the spectral range of the T*(f/Q) term to range We found the spectral range of the T*(f/Q) term to range fromfrom

0.04 – 0.07 (different from the constant 0.051 commonly 0.04 – 0.07 (different from the constant 0.051 commonly used used

in most satellite processing techniques). From theoreticalin most satellite processing techniques). From theoreticalconsiderations, the term should range between 0.0366 and considerations, the term should range between 0.0366 and 0.0612. A regional dependence due to water type was not0.0612. A regional dependence due to water type was notobserved in this study. observed in this study.

The parameterization of the T*(f/Q) term for different water The parameterization of the T*(f/Q) term for different water types certainly can improve the accuracy of algorithms types certainly can improve the accuracy of algorithms applied to remotely sensed ocean color data. applied to remotely sensed ocean color data.

AbstractAbstract

ObjectivesObjectives

MethodsMethods

1. Spectral b1. Spectral bbb/b vs. Petzold’s Relationship/b vs. Petzold’s Relationship

(2)(2)

Petzold SlopePetzold Slope0.018290.01829

Constant 0.051 :Constant 0.051 :Common SatelliteCommon Satellite

ValueValue

Petzold SlopePetzold Slope0.018290.01829

Using bUsing bbb/b as a surrogate to the VSF to characterize water masses./b as a surrogate to the VSF to characterize water masses.

Slope = 0.04Slope = 0.04

Slope = 0.051Slope = 0.051

Slope = T*(f/Q) = 0.07Slope = T*(f/Q) = 0.07

Slope = 0.058Slope = 0.058

All Stations & Regions – All Stations & Regions – Black linesBlack lines indicate least squares linear regression. indicate least squares linear regression.

440 nm440 nm

676 nm676 nm

488 nm488 nm

532 nm532 nm