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Appendix C Comprehensive Oceanic and Atmospheric Optical Datasets S ome past data collection campaigns have been designed Consequently, verification of a given model against indepen- only for validation of specific products without regard dently developed numerical models becomes an expedient for subsequent possible uses and long-term value of substitute for rigorous and complete model validation (e.g., the data. This results in a partial dataset, which, when later Mobley et al., 1993). examined for other purposes, lacks one or more crucial The lack of comprehensive datasets is understandable “missing pieces” that preclude its use. given agency funding constraints for personnel, instrumenta- A comprehensive dataset has all the information neces- tion, and ship time. Unfortunately, data collection for its own sary for a complete radiative transfer (RT) calculation to sake is almost never viewed as fundable science, even though propagate sunlight from the top of the atmosphere (TOA), model and algorithm development and validation always through the atmosphere to the sea surface, through the sea need comprehensive datasets. Finally, there are instrument surface into the water, and then from the water back to the limitations for measurement of some needed parameters. atmosphere, and finally through the atmosphere to the sen- Nevertheless, the collection of even a few comprehensive sor. This RT process is the physical basis for all ocean color datasets for selected water and atmospheric conditions would remote sensing and must be fully understood when evaluat- greatly advance ocean color remote sensing and environmen- ing the performance of any particular sensor and the products tal optics in general. it generates. In addition to collecting the data needed for model and Another way to summarize the necessary information is algorithm validation, data collection programs should be to keep in mind that to validate an environmental parameter viewed as opportunities to compare various instruments and or ocean color product (such as the chlorophyll or Colored methodologies for making the same kind of measurement. Dissolved Organic Matter [CDOM] concentration, or depth Measurement redundancy is absolutely necessary in a field and bottom type in shallow water), it first is necessary to vali- experiment. date the atmospheric correction algorithm, which requires The necessary measurements are dictated by the inputs knowing the absorbing and scattering properties of the atmo- needed to solve the radiative transfer equation (RTE) and to sphere. To validate the bio-optical inversion algorithm that validate its output. Conceptually, atmospheric and oceanic retrieves an ocean color product from the sea-level remote absorbing and scattering properties + boundary conditions → RTE → radiance → other optical quantities of interest. sensing reflectance, it is necessary to know both the value of the product and the water-leaving radiance. To understand To validate a model or algorithm at one point and one how the product influences the water-leaving radiance, it is time, simultaneous and co-located measurements are needed necessary to know the water absorbing and scattering proper- for the following quantities: ties (the inherent optical properties [IOPs]) and the in-water radiance distribution. Oceanic Measurements RT models are presently validated to the extent possible with incomplete datasets. In such exercises, the available In principle, the two fundamental IOPs should be mea- IOP measurements plus reasonable assumptions about the sured. See Light and Water (Mobley, 1994) or similar texts missing pieces are used as inputs. The model predictions for a complete discussion of the quantities discussed here. are then compared with the available radiometric measure- These are the: ments. However, there are always too many missing inputs and outputs to claim rigorous and complete model validation. 90
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91 APPENDIX C • absorption coefficient a(z,λ), measured as a function • the above-water, downwelling plane irradiance Ed(in of depth z and wavelength λ. air,λ) incident onto the sea surface, which can and should • volume scattering function VSF(z,λ,y); y is the scat- be partitioned into direct and diffuse contributions (Gordon, tering angle, 0-180 degree. 1989). • sun zenith angle (or compute from latitude, longi- tude, date, and time). What can be measured today: • sky and cloud conditions. Commercial instruments are available for in situ absorp- • wind speed. tion measurements, and promising new instruments are under development. Bottom irradiance reflectance Rb(l) = Eu(l)/Ed(l) can be No commercially produced instruments are currently used along with the assumption that the BRDF is Lambertian available for in situ measurement of the VSF over the full to obtain satisfactory predictions of water-leaving radiance range of scattering angles, but several unique instruments for most remote sensing purposes (Mobley et al., 2003). exist. Others are under development. Bench-top commercial instruments exist for measurements made on water samples. In-Water Outputs Given the current lack of readily available in situ VSF instru- ments, a reasonable proxy is to measure: Ideally, the following should be measured for compari- son with RT model predictions: • the beam attenuation coefficient c(z,λ), measured as • the full radiance underwater distribution L(z,q,f,λ) a function of depth and wavelength. • the backscatter coefficient bb(z,λ), measured as a as a function of depth, direction, and wavelength. • the irradiances, Ed(z,λ), Eu(z,λ), and Eo(z,λ), which function of depth and wavelength. give a consistency check by integrating the radiance to com- Commercial instruments are available for in situ mea- pare with the irradiances. • the in-air upwelling radiance Lu(in air,q,f,λ). surement of beam attenuation, although (as with many mea- surements) there are instrument design issues that require standardization (Boss et al., 2009). The same is true for What can be measured today: measurement of the backscatter coefficient. Measurement of c allows the scattering coefficient b There are no commercial instruments for in situ mea- to be obtained from b(z,λ) = c(z,λ) – a(z,λ). Knowing the surement of the full radiance distribution, although a few scattering and backscatter coefficients allows the scattering unique instruments do exist (Voss and Chapin, 2005). Com- phase function to be estimated from bb(z,λ)/b(z,λ), which mercial instruments are available for Ed and Eu, which are can give acceptable inputs to the RTE (Mobley et al., 2002). routinely measured. Commercial instruments are available for radiance measurements in a given direction, so radiance is usually measured only for selected directions (most com- Boundary Conditions Needed to Solve the RTE monly the upwelling direction, which can be used in estimat- In principle the needed measurements are: ing the remote sensing reflectance Rrs). An acceptable set of radiometric measurements is then: • the in-air, sea-level downwelling (sun and sky) radi- ance Ld (in air,q,f,λ) as a function of direction (polar angle • the upwelling (nadir-viewing), in-water radiance q and azimuthal angle f) and wavelength. Lu(z,λ). • the sea surface wave spectrum. • the upwelling and downwelling in-water plane irradi- ances, Ed(z,λ) and Eu(z,λ). • the bidirectional reflectance distribution function, BRDF(q′,f′,q,f,λ), of the bottom, if not infinitely deep water, • the above-water upwelling radiance in one direction, as a function of all incident (q′,f′) and reflected (q,f) direc- e.g., Lu(in air,q=40,f=135,λ). The recommended direction tions and wavelength. is at 40 deg off-nadir and at 135-degree relative to the sun, which minimizes the sun glint (Mobley, 1999). • the in-air downwelling (sky) radiance in the reflection What can be measured today: direction, e.g., Ld(in air,q=40,f=135,λ), plus a gray-card mea- Although these boundary conditions can be measured, surement for estimation of Ed and Rrs (the so-called Carder they are almost never measured in the field because of instru- method of estimating Rrs; see Mobley, 1999). • the downwelling in-air plane irradiance, Ed(in air, λ) ment limitations. Therefore, it is reasonable to measure the following: for both direct and diffuse lighting.
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92 APPENDIX C Ancillary Measurements • aerosol optical depth t(λ) as a function of wavelength. • aerosol scattering phase function. These measurements are not needed to solve the RTE, • aerosol albedo of single scattering wo(λ) (= b/c, so but they are necessary for validation of bio-optical algo- related to the absorption and scattering coefficients). rithms for retrieval of Chl, CDOM, TSM, etc. They also are needed to understand the fundamental connections between If highly accurate atmospheric RT calculations are to be water column constituents and inherent optical properties. In performed, vertical profiles of temperature, water vapor, and principle, the following should be measured: cloud type should be measured (typically with balloon-borne instruments or ground-based LIDAR). Ozone concentration • Phytoplankton pigments (at the minimum, measure should be determined from ancillary data such as sea-level Chl-a). measurements or satellite observation (e.g., the TOMS sen- • Absorption coefficient a(z,λ) partitioned into the sor) if RT calculations are to be done below 350 nm. Ozone contributions by water, phytoplankton, CDOM, organic, and can be optically important also in the visible part of the inorganic particles. At the minimum use filtered and unfil- spectrum (the wide Chappuis band in the green wavelengths). tered instruments to partition the absorption into dissolved In particular, the variations in the O3 column content have and particulate contributions. to be accounted for when processing ocean color data, and • The VSF(z,λ,y) (or total scattering and backscat- the green signal (550-560 nm) corrected accordingly. It has tering) partitioned into contributions by water, phytoplank- also been demonstrated that nitrogen oxide may affect the ton, and minerals (assuming CDOM is non-scattering). blue channels. Chlorophyll-a is often the only pigment measured. Filtered and unfiltered instruments can be used to partition absorption Polarization into particulate and dissolved fractions. Partitioning the VSF into component concentrations is almost never done. Polarization is an inherent feature of all electromag- netic radiation, including ocean color radiance. However, Atmospheric Measurements the ocean color community has usually ignored polarization with a few notable exceptions such as the POLDER satellite. To solve the RTE in the atmosphere, the same inputs are This is both because of measurement difficulties and because needed as for the ocean, viz. the atmospheric absorption and unpolarized measurements can yield acceptably accurate scattering properties and sea surface boundary conditions. answers for many (but not all) problems of interest. How- However, these IOPs are usually cast in a different form, ever, polarization carries information that can be exploited to using a different vocabulary. The vastly greater path lengths improve ocean color product retrievals. For example, surface needed for atmospheric attenuation measurements precludes reflection is strongly dependent on polarization, so that sun the development of instruments that can directly measure glint is partially polarized, depending on the relative sun the needed IOPs. The measurements that can and should be and viewing directions. In addition, biological and mineral made are the following: particles have different indices of refraction and different size distributions, and thus scatter light differently, includ- • Sea-level pressure, temperature, humidity, and wind ing polarization changes during the scattering. The French speed. These are standard meteorological measurements that POLDER satellite exploited these effects to improve retriev- allow the Rayleigh scattering contribution to the atmospheric als of biological vs. mineral particulate loads in the water. path radiance to be calculated. Some atmospheric RT codes now include polarization • Atmospheric gas contributions to absorption are (e.g., 6SV; Vermote et al., 2006), and a few researchers known for gases whose mixing ratios are constant. Ozone have developed proprietary coupled ocean-atmosphere RT and water vapor concentrations are highly variable and need codes. Polarization likely will become more important in to be determined for detailed atmospheric RT calculations. future OCR applications. Therefore, the above measure - ments should be made with polarization in mind. Inclusion Aerosol concentration and optical properties are highly of polarization effects in RT computations requires the fol- variable and remain the source of the greatest uncertainty lowing measurements: in atmospheric RT calculations and atmospheric correction. The standard measurement used to deduce aerosol proper- • Instead of the VSF (z,λ,y), measure the full Mueller ties is sun photometry using, for example, the CIMEL sun matrix. The Mueller matrix has 16 elements, although not all photometer (the Aeronet, from which it is possible to extract are independent. The (1,1) element is the VSF. the needed aerosol properties [Wang and Gordon, 1993; • Instead of the radiance L, measure the Stokes Vector Dubovik and King, 2000]), which are: (4 elements).
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93 APPENDIX C There are currently no instruments for in situ measure- and the atmosphere and using scalar (unpolarized) RT codes ment of the Muller matrix, although one measurement of results in errors in the order of 10 percent in predictions of the full matrix has been made in the laboratory on a sea TOA radiances as needed for development and validation of water sample (Voss and Fry, 1984). Instruments are under satellite sensors and atmospheric corrections algorithms. The development for measurement of selected elements of the magnitude and wavelength dependence of the errors depend Muller matrix, which if available would enable underwater on the atmospheric and oceanic properties, sun angle, and polarized RT calculations to be made for three elements of viewing direction. The errors therefore cannot be quantified the Stokes Vector. without detailed vector (polarized) RT calculations for the Unique instruments do exist for underwater measure- particular environmental and viewing conditions of interest. ment of the Stokes Vector (Tonizzo et al., 2009). Although Therefore, the development of a user-friendly and publicly such measurements are not yet common, they likely will available coupled ocean-atmosphere vector RT code would become more so in the future. greatly benefit future ocean color sensor and algorithm Doing unpolarized radiative transfer in both the ocean development.