Menghua Wang

Retrieval of water-leaving radiance and aerosol optical thickness over the oceans with SeaWiFS: a preliminary algorithm

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 <20% in approximately 90% of the cases examined. Finally, the aerosol thickness (τ(α)) is estimated through a simple extension of the correction algorithm. Simulations suggest that the error in the recovered value of τ(α) should be ≲ 10%.

Influence of oceanic whitecaps on atmospheric correction of ocean-color sensors.

The effects of oceanic whitecaps on ocean-color imagery are simulated and inserted into the proposed Sea-Viewing Wide-Field-of-View Sensor (SeaWiFS) atmospheric-correction algorithm to understand its tolerance to error in the estimated whitecap contribution. The results suggest that for wind speeds ≲ 10-12 m/s, present models that relate whitecap reflectance to wind speed are sufficiently accurate to meet the SeaWiFS accuracy goal for retrieval of the water-leaving radiance in the blue, when the aerosol scattering is weakly dependent on wavelength. In contrast, when the aerosol scattering has a strong spectral signature, the retrievals will meet the goal only when the whitecap reflectance is underestimated.

The United States' next generation of atmospheric composition and coastal ecosystem measurements : NASA's Geostationary Coastal and Air Pollution Events (GEO-CAPE) Mission

The Geostationary Coastal and Air Pollution Events (GEO-CAPE) mission was recommended by the National Research Councils (NRCs) Earth Science Decadal Survey to measure tropospheric trace gases and aerosols and coastal ocean phytoplankton, water quality, and biogeochemistry from geostationary orbit, providing continuous observations within the field of view. To fulfill the mandate and address the challenge put forth by the NRC, two GEO-CAPE Science Working Groups (SWGs), representing the atmospheric composition and ocean color disciplines, have developed realistic science objectives using input drawn from several community workshops. The GEO-CAPE mission will take advantage of this revolutionary advance in temporal frequency for both of these disciplines. Multiple observations per day are required to explore the physical, chemical, and dynamical processes that determine tropospheric composition and air quality over spatial scales ranging from urban to continental, and over temporal scales ranging from diu...

A simple, moderately accurate, atmospheric correction algorithm for SeaWiFS☆

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.

Retrieval of the columnar aerosol phase function and single-scattering albedo from sky radiance over the ocean - Simulations

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.

A Sensitivity Study of the SeaWiFS Atmospheric Correction Algorithm: Effects of Spectral Band Variations

With the success of launch and initial data processing of Sea-viewing Wide Field-of-view Sensor (SeaWiFS), there is a great interest in ocean color community to intercompare ocean color data between different ocean color sensors. It is well known that the atmospheric correction, which removes about 90% of sensor-measured signals contributed from atmosphere in the visible, is the key procedure in ocean color imagery data processing. Therefore, it is useful to evaluate the SeaWiFS atmospheric correction algorithm applying to various ocean color sensors. The SeaWiFS atmospheric correction algorithm uses lookup tables which were generated with over ∼25,000 radiative transfer model runs for different aerosol optical and microphysical properties, solar and viewing geometries, and, in particular, at the eight SeaWiFS spectral bands. Since different ocean color sensors usually have different band spectral characterizations, it is rather difficult to apply the SeaWiFS atmospheric correction algorithm to other sensors if one needs to regenerate lookup tables at spectral bands different from SeaWiFS. In this article, we evaluate the accuracy of the SeaWiFS atmospheric correction algorithm for various ocean color sensors using the current SeaWiFS lookup tables. The focus in on the following satellite ocean color sensors: the Modular Optoelectronic Scanner (MOS), the Ocean Color and Temperature Sensor (OCTS), and the Polarization and Directionality of the Earth’s Reflectances (POLDER). These sensors have a slightly different spectral bands compared with the SeaWiFS. It was found that, with an appropriate calculation of the Rayleigh scattering contributions at each sensor’s spectral band and a simple modification in computing the diffuse transmittance of the ocean-atmosphere system, the SeaWiFS atmospheric correction applied to other sensors is as accurate as for SeaWiFS for the solar zenith angles θ0⩽60°.

Estimating aerosol optical properties over the oceans with the multiangle imaging spectroadiometer: some preliminary studies.

The multiangle imaging spectroradiometer (MISR) scheduled to be flown on the first platform of the Earth Observing System in 1998 provides an opportunity to enhance considerably the accuracy with which aerosol properties over the ocean can be retrieved through passive sensing from Earth orbit. As opposed to most radiometers in space that scan the earth in a plane normal to the subsatellite path, the MISR will scan the earth simultaneously in nine planes and thus provide the radiance exiting the atmosphere over a given pixel in nine different directions and at four wavelengths. We examine the problem of extracting the aerosol optical thickness (τ(a)) over the oceans from MISR data, and we produce two algorithms, a single-band algorithm and a spectral or two-band algorithm, for deriving τ(a). The algorithms are based on the use of realistic aerosol models as candidates on which to base an estimation of the aerosol optical properties. They take into account all orders of multiple scattering. Simulations suggest that for nonabsorbing or mildly absorbing aerosol (single-scattering albedo ω(a) > 0.90) the error in the recovered τ(a) is ≲ 10%, as long as the candidate models adequately cover the size refractive index distribution range of the expected aerosols. In the special case of a strongly absorbing aerosol (ω(a) ≍ 0.75), the error in τ(a) becomes large; however, the combination ω(a)τ(a) (the scattering optical thickness) can still be recovered with an error of ≲ 20%, although it is always underestimated. The reason for this decrease in accuracy is that multiple-scattering effects are a strong function of ω(a). A simple extension of the two-band algorithm permits the retrieval of the aerosol scattering phase function with surprising accuracy.

Radiance reflected from the ocean–atmosphere system: synthesis from individual components of the aerosol size distribution

We describe a method by which the aerosol component of the radiance at the top of the atmosphere (TOA) can be synthesized from the radiances generated by individual components of the aerosol size-refractive-index distribution. The method is exact in the single-scattering approximation. For regimes in which the single-scattering approximation is not valid, the method usually reproduces the aerosol contribution with an error ≲2-3% (and only rarely >3-4%) for Sun and viewing angles as large as 80° and 70°, respectively, and for aerosol optical thicknesses as large as 0.50. In the blue, where molecular scattering makes a dominant contribution to the TOA radiance, the percent error in the synthesized total radiance is significantly less than in the synthesized aerosol component and typically will be less than the radiometric calibration uncertainties of Earth-orbiting sensors. When the aerosol is strongly absorbing, the method can fail; however, the potential for failure is easy to anticipate a priori. An obvious application of our technique is to provide a basis for the estimation of aerosol properties with Earth-orbiting sensors, e.g., the Multiangle Imaging Spectroradiometer.

Calibration of ocean color scanners: how much error is acceptable in the near infrared?

Abstract 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

Surface-roughness considerations for atmospheric correction of ocean color sensors. II: Error in the retrieved water-leaving radiance

In the algorithm for the atmospheric correction of coastal zone color scanner (CZCS) imagery, it is assumed that the sea surface is flat. Simulations are carried out to assess the error incurred when the CZCS-type algorithm is applied to a realistic ocean in which the surface is roughened by the wind. In situations where there is no direct Sun glitter (either a large solar zenith angle or the sensor tilted away from the specular image of the Sun), the following conclusions appear justified: (1) the error induced by ignoring the surface roughness is less, similar1 CZCS digital count for wind speeds up to approximately 17 m/s, and therefore can be ignored for this sensor; (2) the roughness-induced error is much more strongly dependent on the wind speed than on the wave shadowing, suggesting that surface effects can be adequately dealt with without precise knowledge of the shadowing; and (3) the error induced by ignoring the Rayleigh-aerosol interaction is usually larger than that caused by ignoring the surface roughness, suggesting that in refining algorithms for future sensors more effort should be placed on dealing with the Rayleigh-aerosol interaction than on the roughness of the sea surface.