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The upwelling radiance attenuation coefficient KLu in the upper 10 m of the water column can be significantly influenced by inelastic scattering processes and thus will vary even with homogeneous water properties. The Marine Optical Buoy (MOBY), the primary vicarious calibration site for many ocean color sensors, makes measurements of the upwelling radiance Lu at 1, 5, and 9 m, and uses these values to determine KLu and to propagate the upwelling radiance directed toward the zenith, Lu, at 1 m to and through the surface. Inelastic scattering causes the KLu derived from the measurements to be an underestimate of the true KLu from 1 m to the surface at wavelengths greater than 575 nm; thus, the derived water-leaving radiance is underestimated at wavelengths longer than 575 nm. A method to correct this KLu, based on a model of the upwelling radiance including Raman scattering and chlorophyll fluorescence, has been developed that corrects this bias. The model has been experimentally validated, and this technique can be applied to the MOBY dataset to provide new, more accurate products at these wavelengths. When applied to a 4-month MOBY deployment, the corrected water-leaving radiance Lw can increase by 5% (600 nm), 10% (650 nm), and 50% (700 nm). This method will be used to provide additional and more accurate products in the MOBY dataset.

We demonstrate a method for estimating absorption and backscattering coefficients by inverting glider-measured profiles of the downwelling irradiance and upwelling radiance. The inversion method was validated against approximately 1,300 profiles of data from 22 glider missions within the Gulf of Maine over a 10 year period. The backscattering coefficient at 532 nm was estimated with a mean absolute error of 21% and bias of 0.01% compared to measured values. We could only quantitatively evaluate the absorption coefficient against the fluorometry data, but found that profiles of fluorescence and absorption were in quantitative agreement. With absorption and backscattering coefficients acting as a basis for studying the biogeochemical parameters of the constituents in the water column, these results show the potential of bio-optical gliders for studying marine ecosystems under varying sky conditions.

We challenge a recent paper in this journal suggesting that the well-established formula governing the transmittance of radiance across a refracting interface needs revision [Optics Express, 25(22) 27086 (2017)]. We provide a simple example of radiative transfer across an interface showing that the accepted formula is correct.

We analyze the effects of the sensor signal-to-noise ratio (SNR) requirements for atmospheric correction of satellite ocean color remote sensing using the near-infrared (NIR) and shortwave infrared (SWIR) bands. Using the Gaussian noise model for the sensor noise distribution in the NIR and SWIR bands, some extensive simulations have been carried out to evaluate and assess the effects of sensor NIR and SWIR SNR values on the retrieved normalized water-leaving reflectance spectra ρwN(λ), which are used to derive all ocean or inland water biological and biogeochemical property data. The standard atmospheric correction algorithm for global oceans and inland waters using the two NIR bands, i.e., Gordon and Wang (1994) [Appl. Opt.33, 443 (1994)Appl. Opt.46, 1535 (2007)], is assumed in the evaluation. Specifically, the minimum and goal SNR requirements for the NIR and SWIR bands for atmospheric correction are estimated. The minimum SNR values are those with which sufficiently accurate ρwN(λ) can be derived, while the goal SNR requirements are those with which the atmospheric correction algorithms reach to their corresponding inherent limitations (or inherent errors), i.e., no gains can be achieved with further increase of SNR values in the NIR and SWIR bands. Evaluation results show that the minimum SNR requirement for the two NIR bands is ~200-300, while the minimum SNR requirement for the three SWIR bands is ~100. For the goal SNR requirements, the recommendations are SNR's of ~600 and ~200 for the two NIR bands and three SWIR bands, respectively.

We describe in detail the implementation of the spectral optimization algorithm (SOA) for Case 2 waters for processing of ocean color data. This algorithm uses aerosol models and a bio-optical reflectance model to provide the top-of atmosphere (TOA) reflectance. The parameters of both models are then determined by fitting the modeled TOA reflectance to that observed from space, using non-linear optimization. The algorithm will be incorporated into the SeaDAS software package as an optional processing switch of the Multi-Sensor Level-1 to Level-2 code. To provide potential users with an understanding of the accuracy and limitations of the algorithm, we generated a synthetic data set and tested the performance of the SOA with both correct and incorrect bio-optical model parameters. Application of the SOA to actual SeaWiFS data in the Lower Chesapeake Bay (for which surface measurements were available) showed that 20% errors in the bio-optical model parameters still enabled retrieval of chlorophyll a and the total absorption coefficient of dissolved plus particulate detrital material at 443 nm with an error of less than 30% and 20%, respectively. In a companion paper we present a validation study of the application of the algorithm in the Chesapeake Bay.

Coastal waters (Case 2) are generally more optically complex than oceanic waters and contain much higher quantities of colored detrital matter (CDM, a combination of dissolved organic matter and detrital particulates) as well as suspended sediment. Exclusion of CDM in the retrieval can lead to an overestimation of chlorophyll a concentration (C). We present a validation of a Case 2 version of the coupled spectral optimization algorithm (SOA) for simultaneous atmospheric correction and water parameter retrieval using Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite ocean color data. Modeling of water constituents uses the Garver, Siegel and Maritorena (GSM) semi-analytic bio-optical model locally tuned for Chesapeake Bay. This includes a parameterization for CDM through its absorption spectrum. SOA-retrieved C and CDM are compared with in situ measurements in Chesapeake Bay. Results are also compared with output from two alternate models 1) the standard algorithm (Std) and 2) the standard atmospheric correction combined with the locally tuned GSM model (StdGSM). The comparisons indicate that the SOA is a viable alternative to both given models in Chesapeake Bay. In contrast, StdGSM appears to require improvement before it can be considered for operational use in these waters. Perhaps the most important result is the high-quality of CDM retrievals with the SOA. They suggest that there is value added using the SOA method in Chesapeake waters, as the Std method does not retrieve CDM. In a companion paper we describe in detail the model implementation, and its accuracy and limitations when applied to the Chesapeake Bay.

The portion of the radiance exiting the ocean and transmitted to the top of the atmosphere (TOA) in a particular direction depends on the angular distribution of the exiting radiance, not just the radiance exiting in the direction of interest. The diffuse transmittance t relates the water component of the TOA radiance to that exiting the water in the same direction. As such t is a property of the ocean–atmosphere system and not just the atmosphere. Its computation requires not only the properties of the atmosphere but the angular distribution of the exiting radiance as well. The latter is not known until a determination of the water properties can be made (which is the point of measuring the radiance in the first place). Because of this, it has been customary to assume an angular distribution (uniform upward radiance beneath the water surface) in the computation of t, which is referred to as t⁎. However, it is known that replacing t with t⁎ can result in an error of several percent in the retrieved water-leaving radiance. Since the error depends on sun-viewing direction, this error could be particularly important when water-leaving radiance from two or more sensors in different orbits are compared. Even given an estimate of the angular distribution of the water-leaving radiance, computation of t using full radiative transfer theory in an image processing environment is not practical. Thus, we developed a first-order correction to t for bidirectional effects in the water-leaving radiance that captures much of the variability of t with viewing direction. The correction computed across a SeaWiFS scan line shows that a t⁎-induced error of as much as 5–6% could occur near the edges of the scan; however, limiting the scan to polar viewing angles (θ)

During the southwest monsoon (SWM) in the northern Arabian Sea, the phytoplankton bloom associated with upwelling has remained incompletely characterized by satellite ocean-color observations. This is due to the prevalence of dust storms in the region from May to September that prevents good satellite-derived chlorophyll retrievals. To ameliorate this problem, SeaWiFS data from May to September 2000 were processed with the spectral-matching algorithm (SMA) using a suite of Saharan dust models. The use of SMA significantly increased chlorophyll data recovery in areas otherwise completely masked by standard SeaWiFS processing, and permitted a description of the bloom over the SWM period. Maps of the frequency and spatial distribution of TOMS-absorbing aerosol index indicate that Africa and Arabia are the most probable dust sources, and that the nearshore area parallel to the Arabian coast was the most affected by dust events during the SWM, which is also where chlorophyll retrievals are also poorest. Weekly averages of the SeaWiFS images produced using the SMA show that at the onset of upwelling in June, the bloom along the Omani coast has limited offshore extent except for a few small thin filaments. From July to September, the high-chlorophyll features evolve into more complex structures that extend offshore into the Arabian Sea interior. In October, when the SWM has ended, the filaments appear to merge, forming a high-chlorophyll band off the Omani coast. Quikscat wind data suggest that the widespread filament formation occurs only when the SWM winds begin to weaken, which results in greater spatial variability in the zone of maximum wind stress (the Findlater Jet). Although the bloom is generally restricted to the cold-water upwelling area (as indicated by TMI sea-surface temperatures) that lies northwest of the Findlater Jet axis, the high-chlorophyll structures do not occur solely in association with coldest upwelling features. This suggests that thermal satellite imagery cannot be used to infer chlorophyll distributions for periods in which no ocean-color data are available, e.g., the earlier JGOFS cruises.

We report application and validation of a spectral optimization algorithm for processing SeaWiFS data in Case 1 waters. The algorithm couples a simplified aerosol model with a sophisticated water-reflectance model to simultaneously retrieve both atmospheric and ocean parameters. Two of the retrieved ocean properties—the absorption coefficient of colored detrital material and the chlorophyll a concentration—are validated by comparison with ‘‘surface’’ truth obtained with airborne and space-borne sensors. We show that employing a more complete water reflectance model significantly improves the decoupling between the oceanic and atmospheric optical signals. Methodologies for applying the algorithm to Case 2 waters and for delineating terrestrial vs. marine chromophoric dissolved organic matter (CDOM) are suggested. D 2002 Elsevier Science Inc. All rights reserved.

We simulate vicarious calibration (VC) of a Sea-viewing Wide Field-of-view Sensor (SeaWiFS)-like ocean color sensor relative to its longer near infrared (NIR) spectral band (865 nm) to understand the influence of calibration error at 865 nm, which is difficult to assess in orbit. We show that as long as the calibration error at 865 nm less than ∼10% in magnitude, the post-vicarious-calibration-corrected radiances are sufficiently accurate to retrieve useful water-leaving reflectances at moderate aerosol optical depths. This is completely independent of the initial calibration error in the shorter-wave bands, but assumes an atmospheric correction approach similar to that currently used with SeaWiFS. Retrievals are only slightly improved by reducing the magnitude of the error at 865 nm below ∼5%. The simulations immediately suggest that pre-launch calibration is necessary only to the extent required to set the sensitivity of the instrument in the desired range. Rather than trying to achieve a highly accurate pre-launch calibration, e.g., uncertainty

The Aerosols99 cruise (January 14 to February 8, 1999) went between Norfolk, Virginia, and Cape Town, South Africa. A Micropulse lidar system was used almost continually during this cruise to profile the aerosol vertical structure. Inversions of this data illustrated a varying vertical structure depending on the dominant air mass. In clean maritime aerosols in the Northern and Southern Hemispheres the aerosols were capped at 1 km. When a dust event from Africa was encountered, the aerosol extinction increased its maximum height to above 2 km. During a period in which the air mass was dominated by biomass burning from southern Africa, the aerosol layer extended to 4 km. Comparisons of the aerosol optical depth (AOD) derived from lidar inversion and surface Sun photometers showed an agreement within 0.05 RMS. Similar comparisons between the extinction measured with a nephelometer and particle soot absorption photometer (at 19 m altitude) and the lowest lidar measurement (75 m) showed good agreement (0.014 km 1 ). The lidar underestimated surface extinction during periods when an elevated aerosol layer (total AOD 0.10) was present over a relatively clean (aerosol extinction 0.05 km 1 ) surface layer, but otherwise gave accurate results.

Computation of the light scattering properties of marine particles has typically been effected using Mie theory (i.e., modeling the particles as homogeneous or layered spheres). Because scattering by irregularly shaped particles is significantly different from that of spheres, particularly in backscattering directions, it is of interest to examine the efficacy of using more complex formulations of light scattering that are not limited to spherically symmetric particles. We applied the discrete dipole approximation (DDA) to the computation of the scattering properties of detached calcium carbonate coccoliths from the coccolithophorid Emiliania huxleyi. Three distinct models of E. huxleyi coccoliths were studied: thin disks with a diameter of approximately 2.75 µm, washers with a 1.38- µm hole in the disk, and two parallel disks joined by a hollow tube (a "fishing reel"). The model coccoliths all had the same volume (mass ~ 0.19 pg carbon) and disk diameter and a refractive index of 1.20 relative to water. DDA computations for randomly oriented model coccoliths showed that the total scattering cross section and its spectral variation are similar for each of the three particle shapes and agree well with measurements made in natural E. huxleyi blooms both on a per-coccolith and per-calcite concentration basis. The backscattering cross section and its spectral variation was found to be strongly dependent on particle morphology. This dependence was shown to be due to multiple reflections within the particle. Scattering and backscattering coefficients for volume-equivalent spheres were within a factor of two of those for the disklike models.

We examined blooms of the coccolithophorid E. huxleyi, observed in SeaWiFS imagery, with a new algorithm for the retrieval of detached coccolith concentration. The algorithm uses only SeaWiFS bands in the red and near infrared (NIR) to minimize the influence of the absorption by chlorophyll and dissolved organic material. We used published experimental determinations of the calcite specific backscattering and its spectral dependence, and assumed that the absorption coefficient of the medium was that of pure water, to estimate the marine contribution to the SeaWiFS radiance. The aerosol (and Rayleigh-aerosol interaction) contribution to the radiance was modeled as an exponential function of wavelength. These allow derivation of the coccolith concentration on a pixel-by-pixel basis from SeaWiFS imagery. Application to a July 30, 1999 SeaWiFS image of a bloom south of Plymouth, UK indicates that the SeaWiFS estimates are in good agreement with surface measurements of coccolith concentration.

International audience; Airborne plumes of desert dust from North Africa are observable all year on satellite images over the Tropical Atlantic. In addition to its radiative impact, it has been suggested that this mineral dust has a substantial influence on the marine productivity. This effect is however difficult to gauge because present atmospheric correction algorithms for ocean color sensors are not capable of handling absorbing mineral dust. We apply a new approach to atmospheric correction in which the atmosphere is removed and the case 1 water properties are derived simultaneously. Analysis of SeaWiFS images acquired off Western Africa during a dust storm demonstrates the efficacy of this approach in terms of increased coverage and more reliable pigment retrievals.

A radiometric system, deployed from a ship, is used to measure directly the influence of the presence of breaking waves (whitecaps) on the upwelling radiance above the sea surface. Estimates of their remote sensing augmented spectral reflectance, i.e., the temporally averaged or spatially averaged increase in the ocean's reflectance over and above the reflectance in the absence of breaking waves, are provided from measurements in the tropical Pacific. The accuracy of these estimates is dependent on their ability to determine radiometrically the background reflectance of the water. In the visible the remote sensing augmented spectral reflectance of whitecaps measured in the open ocean was found to be essentially independent of wavelength and in the range 0.001- 0.002 for wind speeds of 9 -12 m s 21 . This is in reasonably good agreement (within a factor of 2) with earlier predictions based on the statistical relationship between fractional coverage and wind speed and the estimated average reflectance of individual whitecaps. In the near infrared (860 nm) the remote sensing augmented spectral reflectance falls to ;80% of its value in the visible.

A measurement system for determining the spectral reflectance of whitecaps in the open ocean is described. The upwelling radiance is obtained from a ship by observing a small region of the water surface over time using a six-channel radiometer (410, 440, 510, 550, 670, and 860 nm) extended from the bow of the ship. Downwelling irradiance is simultaneously measured and used to provide surface reflectance. The system includes a TV camera mounted beside the radiometer that provides a visual reference of surface events. Air/water temperature and wind speed/direction are also measured along with global positioning system data. Calibration procedures and radiometric characterization of the system for operation under different sky conditions and solar zenith angles are emphasized so that full advantage is taken of ship time whenever whitecap events occur. The radiometer was operated at sea and examples of the spectral reflectance of different foam types (thick foam layers to thin residual patches) generated by the ship’s bow in coastal waters are presented and found to vary spectrally. The presence of submerged bubbles in the foam measurement results in a lower reflectance at the longer wavelengths. For wavebands in the visible region, the spectral reflectance values tend to equalize with higher reflecting foam from thicker foam layers.

To recover the ocean water-leaving radiance and derive biophysical parameters from observations of space-borne ocean color sensors, the required uncertainty in the measured top-of-atmosphere radiance is at present impossible to achieve prior to launch. A methodology and strategy for achieving the required uncertainty in the post-launch era is presented here. The method consists of combining direct measurements of the water-leaving radiance, whitecap radiance, and aerosol optical thickness made simultaneously with satellite overpasses, with radiative transfer theory to reduce the calibration uncertainty of the visible bands with respect to the near-infrared (NIR). This procedure is possible over the open ocean, where, in the absence of aerosol transported from land over long distances by the wind, the atmosphere can be very clear with most of the aerosol generated by local processes such as breaking waves, for example, the aerosol optical thickness in the visible ∼0.05–0.10. In this case, the radiative transfer process is considerably simplified and molecular scattering is the dominant atmospheric component in the visible. It is shown that such a procedure alone is sufficient to reduce the calibration uncertainty to required levels. Further reduction is possible by reducing the uncertainty in the NIR calibration by measuring sky radiance from island locations (or a ship), and using these to predict the at-sensor radiance. For the most part, this NIR calibration is limited by the uncertainty in the calibration of the radiometer used to measure the sky radiance. Finally, the sensor calibration is maintained by monitoring the actual water-leaving radiance continuously at a single location, where the atmosphere is sufficiently clear that atmospheric correction introduces only a small error, and directly comparing the true and the sensor-derived water-leaving radiances.

We investigate the roles of climate forcings and chaos (unforced variability) in climate change via ensembles of climate simulations in which we add forcings one by one. The experiments suggest that most interannual climate variability in the period 1979–1996 at middle and high latitudes is chaotic. But observed SST anomalies, which themselves are partly forced and partly chaotic, account for much of the climate variability at low latitudes and a small portion of the variability at high latitudes. Both a natural radiative forcing (volcanic aerosols) and an anthropogenic forcing (ozone depletion) leave clear signatures in the simulated climate change that are identified in observations. Pinatubo aerosols warm the stratosphere and cool the surface globally, causing a tendency for regional surface cooling. Ozone depletion cools the lower stratosphere, troposphere and surface, steepening the temperature lapse rate in the troposphere. Solar irradiance effects are small, but our model is inadequate to fully explore this forcing. Well-mixed anthropogenic greenhouse gases cause a large surface wanning that, over the 17 years, approximately offsets cooling by the other three mechanisms. Thus the net calculated effect of all measured radiative forcings is approximately zero surface temperature trend and zero heat storage in the ocean for the period 1979–1996. Finally, in addition to the four measured radiative forcings, we add an initial (1979) disequilibrium forcing of +0.65 W/m2. This forcing yields a global surface warming of about 0.2°C over 1979–1996, close to observations, and measurable heat storage in the ocean. We argue that the results represent evidence of a planetary radiative imbalance of at least 0.5° W/m2; this disequilibrium presumably represents unrealized wanning due to changes of atmospheric composition prior to 1979. One implication of the disequilibrium forcing is an expectation of new record global temperatures in the next few years. The best opportunity for observational confirmation of the disequilibrium is measurement of ocean temperatures adequate to define heat storage.

By validation of atmospheric correction, we mean quantification of the uncertainty expected to be associated with the retrieval of the water-leaving radiance from the measurement of the total radiance exiting the ocean-atmosphere system. This uncertainty includes that associated with the measurement or estimation of auxiliary data required for the retrieval process, for example, surface wind speed, surface atmospheric pressure, and total ozone concentration. For a definitive validation this quantification should be carried out over the full range of atmospheric types expected to be encountered. However, funding constraints require that the individual validation campaigns must be planned to address the individual components of the atmospheric correction algorithm believed to represent the greatest potential sources of error. In this paper we develop a strategy for validation of atmospheric correction over the oceans that is focused on EOS/MODIS. We also provide a description of the instrumentation and methods to be used in the implementation of the plan.

Sensors that can be used for the observation of ocean color in NASA's Earth Observing System era (SeaWiFS, MODIS, and MISR) have been designed with 2–4 times the radiometric sensitivity of the proof-of-concept ocean color instrument CZCS (coastal zone color scanner). To realize an improvement in the retrieval of biologically important ocean parameters, e.g., the concentration of the photosynthetic pigment chlorophyll a, from this increased sensitivity, significantly better atmospheric correction than was applied to CZCS is required. Atmospheric correction improvement necessitates the inclusion of the effects of multiple scattering, which are strongly dependent on the aerosol size distribution, concentration, and absorption properties. We review the basic concepts of atmospheric correction over the oceans and provide the details of the algorithms currently being developed for SeaWiFS, MODIS, and MISR. An alternate correction algorithm that could be of significant value in the coastal zone is described for MISR. Related issues such as the influence of aerosol vertical structure in the troposphere, polarization of the light field, sea surface roughness, and oceanic whitecaps on the sea surface are evaluated and plans for their inclusion in the algorithm are described. Unresolved issues, such as the presence of stratospheric aerosol, the appropriateness of the aerosol models used in the assessment of multiple scattering, and the identification of, and difficulties associated with the correction for, the presence of absorbing aerosols, e.g., urban pollution or mineral dust, are identified, and suggestions are provided for their resolution.

The launch of ADEOS in August 1996 with POLDER, TOMS, and OCTS instruments on board and the future launch of EOS-AM 1 in mid-1998 with MODIS and MISR instruments on board start a new era in remote sensing of aerosol as part of a new remote sensing of the whole Earth system (see a list of the acronyms in the Notation section of the paper). These platforms will be followed by other international platforms with unique aerosol sensing capability, some still in this century (e.g., ENVISAT in 1999). These international spaceborne multispectral, multiangular, and polarization measurements, combined for the first time with international automatic, routine monitoring of aerosol from the ground, are expected to form a quantum leap in our ability to observe the highly variable global aerosol. This new capability is contrasted with present single-channel techniques for AVHRR, Meteosat, and GOES that although poorly calibrated and poorly characterized already generated important aerosol global maps and regional transport assessments. The new data will improve significantly atmospheric corrections for the aerosol effect on remote sensing of the oceans and be used to generate first real-time atmospheric corrections over the land. This special issue summarizes the science behind this change in remote sensing, and the sensitivity studies and applications of the new algorithms to data from present satellite and aircraft instruments. Background information and a summary of a critical discussion that took place in a workshop devoted to this topic is given in this introductory paper. In the discussion it was concluded that the anticipated remote sensing of aerosol simultaneously from several space platforms with different observation strategies, together with continuous validations around the world, is expected to be of significant importance to test remote sensing approaches to characterize the complex and highly variable aerosol field. So far, we have only partial understanding of the information content and accuracy of the radiative transfer inversion of aerosol information from the satellite data, due to lack of sufficient theoretical analysis and applications to proper field data. This limitation will make the anticipated new data even more interesting and challenging. A main concern is the present inadequate ability to sense aerosol absorption, from space or from the ground. Absorption is a critical parameter for climate studies and atmospheric corrections. Over oceans, main concerns are the effects of white caps and dust on the correction scheme. Future improvement in aerosol retrieval and atmospheric corrections will require better climatology of the aerosol properties and understanding of the effects of mixed composition and shape of the particles. The main ingredient missing in the planned remote sensing of aerosol are spaceborne and ground-based lidar observations of the aerosol profiles.

Simulated bidirectional reflectance distribution functions (BRDF) were compared with measurements made just beneath the water’s surface. In Case I water, the set of simulations that varied the particle scattering phase function depending on chlorophyll concentration agreed more closely with the data than other models. In Case II water, however, the simulations using fixed phase functions agreed well with the data and were nearly indistinguishable from each other, on average. The results suggest that BRDF corrections in Case II water are feasible using single, average, particle scattering phase functions, but that the existing approach using variable particle scattering phase functions is still warranted in Case I water., JRC.H.1-Water Resources

We present a simple modification to the standard coastal zone color scanner (CZCS) atmospheric correction algorithm for application to Sea-viewing-wide field-ofview-sensor (SeaWiFS). The modification reduces the error in the water-leaving reflectance using the standard algorithm by a factor of 2–6 when the aerosol behaves as predicted by the LOWTRAN-6 models. For many aerosol models likely to approximate aerosol properties over the oceans the error in the retrieved water-leaving reflectance is predicted to be < ± 0.002 at 443 nm for an aerosol load approximately 2 to 3 times that normally occurring in a maritime atmosphere. These errors in atmospheric correction lead to an error in the pigment concentration (C), retrieved using the blue-green ratio algorithm, of < 50% for more than 75% of the aerosol models tested, whenever the algorithm retrieves a “reasonable” pigment concentration, and when 0.1 ≤ C ≤ 1 mg / m3. This accuracy may be sufficient for some applications, for example, at-sea processing to guide ships to desirable sampling locations. An important feature of this algorithm is that, unlike more sophisticated and computational intensive algorithms, aerosol models are not required to effect the actual atmospheric correction. Investigators already having the CZCS algorithm implemented on an image processing system should be able to process SeaWiFS imagery by making very simple modifications to the code.

Typically, explanation/interpretation of observed light scattering and absorption properties of marine particles is based on assuming a spherical shape and homogeneous composition. We examine the influence of shape and homogeneity by comparing the optics of randomly-oriented cylindrically-shaped particles with those of equal-volume spheres, in particular the influence of aspect ratio (AR = length/diameter) on extinction and backscattering. Our principal finding is that the when AR > ~3–5 and the diameter is of the order of the wavelength, the extinction efficiency and the backscattering probability are close to those of an infinite cylinder. In addition, we show the spherical-based interpretation of extinction and absorption can lead to large error in predicted backscattering.

Sky-radiance measurements at the sea surface can be used to estimate radiative properties of aerosols over water. We demonstrate, through Monte Carlo simulations, that significant perturbations to sky radiance over the ocean can occur when measurements are carried out with radiometers located on islands. In particular, we present examples of the influence of the physical and optical thicknesses of an aerosol layer, the azimuth of observation relative to the Sun, the size of the island, the location of the radiometer on the island, and the albedo of the island on the magnitude of the perturbation for a circular island of uniform albedo. Relative errors in sky radiance of as high as 39% were found in the blue. Simulated (perturbed) sky radiances were combined with an algorithm for retrieving the aerosol phase function P(θ), where θ is the scattering angle, and with the single-scattering albedo ω(0), to demonstrate how the perturbation can influence the retrieved values. It was found that the fractional error in the retrieved values of the product ω(0)P(θ) can be significantly greater than the fractional error in the sky radiance, because of the effects of multiple scattering. This underscores the importance of removing the island perturbation before an inversion algorithm is used. A first-order procedure for removing the island perturbation based on the values of ω(0)P(θ) retrieved from the perturbed sky radiance is proposed and is found to be effective if the island perturbation is not too large. A simplified Monte Carlo procedure that is applicable to an island of arbitrary shape and albedo distribution is presented. The procedure could be used to assess the suitability of a given island as a measurement site, and to provide a first-order correction to actual experimental measurements.

We report the results of simulations in which an algorithm developed for estimation of aerosol optical properties from the angular distribution of radiance exiting the top of the atmosphere over the oceans [Appl. Opt. 33, 4042 (1994)] is combined with a technique for carrying out radiative transfer computations by synthesis of the radiance produced by individual components of the aerosol-size distribution [Appl. Opt. 33, 7088 (1994)], to estimate the aerosol-size distribution by retrieval of the total aerosol optical thickness and the mixing ratios for a set of candidate component aerosol-size distributions. The simulations suggest that in situations in which the true size–refractive-index distribution can actually be synthesized from a combination of the candidate components, excellent retrievals of the aerosol optical thickness and the component mixing ratios are possible. An exception is the presence of strongly absorbing aerosols. The angular distribution of radiance in a single spectral band does not appear to contain sufficient information to separate weakly from strongly absorbing aerosols. However, when two spectral bands are used in the algorithm, retrievals in the case of strongly absorbing aerosols are improved. When pseudodata were simulated with an aerosol-size distribution that differed in functional form from the candidate components, excellent retrievals were still obtained as long as the refractive indices of the actual aerosol model and the candidate components were similar. This underscores the importance of component candidates having realistic indices of refraction in the various size ranges for application of the method. The examples presented all focus on the multiangle imaging spectroradiometer; however, the results should be as valid for data obtained by the use of high-altitude airborne sensors.

Two satellite-borne ocean-color sensors scheduled for launch in the mid 1990's each have a spectral band (nominally 745-785 nm) that completely encompasses the O(2) A band at 762 nm. These spectral bands are to be used in atmospheric correction of the color imagery by assessment of the aerosol contribution to the total radiance at the sensor. The effect of the O(2) band on the radiance measured at the satellite is studied with a line-by-line backward Monte Carlo radiative transfer code. As expected, if the O(2) absorption is ignored, unacceptably large errors in the atmospheric correction result. The effects of the absorption depend on the vertical profile of the aerosol. By assuming an aerosol profile-the base profile-we show that it is possible to remove most of the O(2)-absorption effects from atmospheric correction in a simple manner. We also investigate the sensitivity of the results to the details of the assumed base profile and find that, with the exception of situations in which there are significant quantities of aerosol in the stratosphere, e.g., following volcanic eruptions or in the presence of thin cirrus clouds, the quality of the atmospheric correction depends only weakly on the base profile. Situations with high concentrations of stratospheric aerosol require additional information regarding vertical structure for this spectral band to be used in atmospheric correction; however, it should be possible to infer the presence of such aerosol by a failure of the atmospheric correction to produce acceptable water-leaving radiance in the red. An important feature of our method for removal of the O(2)-absorption effects is that it permits the use of lookup tables that can be prepared in the absence of O(2) absorption by the use of more efficient radiative transfer codes.

The second generation of ocean-color-analyzing instruments requires more accurate atmospheric correction than does the Coastal Zone Color Scanner (CZCS), if one is to utilize fully their increased radiometric sensitivity. Unlike the CZCS, the new instruments possess bands in the near infrared (NIR) that are solely for aiding atmospheric correction. We show, using aerosol models, that certain assumptions regarding the spectral behavior of the aerosol reflectance employed in the standard CZCS correction algorithm are not valid over the spectral range encompassing both the visible and the NIR. Furthermore, we show that multiple-scattering effects on the algorithm depend significantly on the aerosol model. Following these observations, we propose an algorithm that utilizes the NIR bands for atmospheric correction to the required accuracy. Examples of the dependence of the error on the aerosol model, the turbidity of the atmosphere, and surface roughness (waves) are provided. The error in the retrieved phytoplankton-pigment concentration (the principal product of ocean-color sensors) induced by errors in the atmospheric correction are shown to be

Based on the fact that the part of downward radiance that depends on the optical properties of the aerosol in the atmosphere can be extracted from the measured sky radiance, a new scheme for retrieval of the aerosol phase function and the single-scattering albedo over the ocean is developed. This retrieval algorithm is tested with simulations for several cases. It is found that the retrieved aerosol phase function and the single-scattering albedo are virtually error free if the vertical structure of the atmosphere is known and if the sky radiance and the aerosol optical thickness can be measured accurately. The robustness of the algorithm in realistic situations, in which the measurements are contaminated by calibration errors or noise, is examined. It is found that the retrieved value of ω(0) is usually in error by ≲ 10%, and the phase function is accurately retrieved for θ ≲ 90°. However, as the aerosol optical thickness becomes small, e.g., ≲ 0.1, errors in the sky radiance measurement can lead to serious problems with the retrieval algorithm, especially in the blue. The use of the retrieval scheme should be limited to the red and near IR when the aerosol optical thickness is small.

The asymptotic daylight field in a homogeneous ocean is, like the absorption (a), scattering (b), and attenuation (c) coefficients and the volume scattering function (VSF), an inherent optical property (IOP) of the medium. A simple relationship in the spirit of the van de Hulst similarity relationships is developed from which the diffuse attenuation coefficient K(infinity) of the asymptotic light field can be obtained from a, b, and the VSF with an error of2%. In this relationship, the shape of the VSF is characterized by its asymmetry parameter g, whereas omega(0) = b/c characterizes the other IOP's. The relationship applies approximately to other quantities as well, particularly tau(x), which is the optical depth at which the downwelling irradiance attenuation coefficient K(d) can be replaced by K(infinity) with an error no greater than x%. Computations of tau(5) and tau(10) are presented as a function of g, omega(0) and the incident light field, and it is shown that for overcast conditions K(d) can be within 5% of K(infinity) at depths at which the downwelling irradiance is greater than 50% of its value at the surface. Simulations of radiative transfer in verticall inhomogeneous waters reveal that for sufficiently large depth (z), the value of K(infinity)(z) determined from the asymptotic theory that uses the values of the IOP's at z is a good approximation to K(d)(z). Thus our results suggest that in addition to being a pedagogically interesting concept, the asymptotic theory may actually be useful in ocean optics research. The influence of inelastic processes (fluorescence and Raman scattering) on K(infinity) are briefly examined, and it is shown that for an ocean of pure sea water, i.e., no particles or fluorescent compounds, K(infinity)(lambda) approximately 0.02 m(-1) for lambda/= 430 nm with little spectral variation.

For measurement of aerosols over the ocean, the total radiance L(t) backscattered from the top of a stratified atmosphere which contains both stratospheric and tropospheric aerosols of various types has been computed. A similar computation is carried out for an aerosol-free atmosphere yielding the Rayleigh scattered radiance L(r). The difference L(t) - L(r) is shown to be linearly related to the radiance L(as), which the aerosol would produce in the single scattering approximation. This greatly simplifies the application of aerosol models to aerosol analysis by satellite since adding to, or in some way changing, the aerosol model requires no additional multiple scattering computations. In fact, the only multiple computations required for aerosol analysis are those for determining L(r), which can be performed once and for all. The computations are explicitly applied to Band 4 of the CZCS, which, because of its high radiometric sensitivity and excellent calibration, is ideal for studying aerosols over the ocean. Specifically, the constant A in the relationship L(as) = A(-1)(L(t) - L(r)) is given as a function of position along the scan for four typical orbital-solar position scenarios. The computations show that L(as) can be retrieved from L(t) - L(r) with an average error of no more than 5-7% except at the very edges of the scan.

The addition of an El Chichon-like aerosol layer in the stratosphere is shown to have very little effect on the basic CZCS atmospheric correction algorithm. The additional stratospheric aerosol is found to increase the total radiance exiting the atmosphere, thereby increasing the probability that the sensor will saturate. It is suggested that in the absence of saturation the correction algorithm should perform as well as in the absence of the stratospheric layer.

Un modele bio-optique (Gordon H.R., 1987, Appl. Opt., 26) decrivant la distribution de l'eclairement a la surface marine resultant d'une source ponctuelle sous la surface d'un ocean homogene est etendu a un ocean stratifie; en particulier, la concentration des pigments associes au phytoplancton est variable avec la profondeur

A model of the optical properties of the ocean, providing the absorption and scattering coefficients of the medium as nonlinear functions of the concentration of pigments associated with phytoplankton and their immediate detrital material, is presented. Monte Carlo computations of the attenuation coefficient of downwelling irradiance K(d) for an ocean-atmosphere system illuminated by the sun at zenith, agree well with experimental data and demonstrate the validity of such a model for studying the influence of phytoplankton biomass on the propagation to the surface of light generated through bioluminescence. The radiative transfer equation for the irradiance at the sea surface resulting frow illumination by a point source embedded in the water is solved by Monte Carlo techniques. The solution technique is validated through comparison with an asymptotic analytic solution for isotropic scattering. The computations show that the irradiance distribution just beneath the surface as a function of R, the distance measured along the surface from a point vertically above the source, is described by two regimes: (1) a regime in which the irradiance is governed mostly by absorption and geometry with scattering playing a negligible role-the near field; (2) a regime in which the light field at the surface is very diffuse and the irradiance decays approximately exponentially in R and is a very weak function of the source depth-the diffusion regime. The near field is of primary interest because it contains most of the power reaching the sea surface. An analytical model of the irradiance distribution just beneath the surface as a function of R, the source depth, and the pigment concentration for the near field is presented. This model is based on the observation that at most scattering events the change in the photon's direction is slight, and therefore, scattering is rather ineffective in attenuating the irradiance. An analytic solution for the irradiance from the point source, then, is first carried out ignoring scattering altogether; however, recognizing that backscattering will attenuate the irradiance, the absorption coefficient is replaced by an effective attenuation coefficient k. This effective attenuation coefficient is determined by fitting the total power just beneath the surface determined from the Monte Carlo computations to the analytical model. The resulting k is closely related to K(d), and the Monte Carlo irradiance as a function of R and source depth in the near-field regime can be approximated with high accuracy using the model. These results indicate K(d) can be estimated at night by releasing a point source in the water, measuring the irradiance at the surface as it sinks, and fitting the measurements to the relationships developed here to determine k. The analytic model also enables estimation of the source depth and power from the irradiance distribution just beneath the surface.

Preliminary results are described for an effort to quantify the sensitivity decay of a radiometry sensor (the Coastal Zone Color Scanner or CZCS aboard Nimbus 7). The method used in the study is to (1) compute the water-leaving radiance for imagery acquired in regions where this radiance is known or can be independently estimated, and (2) adjust the sensor calibration to force agreement between the two radiances. Decay factors for orbit numbers from 0 to 20,000 are plotted, and surface and space measurements are compared for the Gulf Stream and the Northern Sargasso Sea at different seasons. The fact that a seasonal variability in the chlorophyll a concentration in the Sargasso Sea was found in the sensor analysis (apparently the first such satellite observation) increases confidence in the method.

The possibility of computing the inherent sea surface radiance for regions of clear water from coastal zone color scanner (CZCS) imagery given only a knowledge of the local solar zenith angle is examined. The inherent sea surface radiance is related to the upwelling and downwelling irradiances just beneath the sea surface, and an expression is obtained for a normalized inherent sea surface radiance which is nearly independent of solar zenith angle for low phytoplankton pigment concentrations. An analysis of a data base consisting of vertical profiles of upwelled spectral radiance and pigment concentration, which was used in the development of the CZCS program, confirms the virtual constancy of the normalized inherent sea surface radiance at wavelengths of 520 and 550 nm for cases when the pigment concentration is less than 0.25 mg/cu m. A strategy is then developed for using the normalized inherent sea surface radiance in the atmospheric correction of CZCS imagery.

The radiative transfer equation is modified to include the effect of fluorescent substances and solved in the quasi-single scattering approximation for a homogeneous ocean containing fluorescent particles with wavelength independent quantum efficiency and a Gaussian shaped emission line. The results are applied to the in vivo fluorescence of chlorophyll a (in phytoplankton) in the ocean to determine if the observed quantum efficiencies are large enough to explain the enhancement of the ocean's diffuse reflectance near 685 nm in chlorophyll rich waters without resorting to anomalous dispersion. The computations indicate that the required efficiencies are sufficiently low to account completely for the enhanced reflectance. The validity of the theory is further demonstrated by deriving values for the upwelling irradiance attenuation coefficient at 685 nm which are in close agreement with the observations.

The influence of the ocean's optical properties and wind induced sea surface foam (white caps) on the short-wave albedo of the ocean-atmosphere system is studied by solving the radiative transfer equation using a Monte Carlo method. It is found that for a foam free ocean, the planetary albedo of a very clear ocean is at most 10 percent greater than that for a totally absorbing ocean. However, the introduction of a relatively small amount of sea foam on the surface can produce a considerable increase in the albedo, especially if the foam is highly reflecting. For example, it is shown that for foam which is totally reflecting (the foam albedo is 1), an increase in the wind speed from 6 to 14 m/sec would double the planetary albedo for small solar zenith angles.