Changes in Atmospheric Constituents and in Radiative Forcing

The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard both NASA’s Terra and Aqua satellites is making near-global daily observations of the earth in a wide spectral range (0.41–15 m). These measurements are used to derive spectral aerosol optical thickness and aerosol size parameters over both land and ocean. The aerosol products available over land include aerosol optical thickness at three visible wavelengths, a measure of the fraction of aerosol optical thickness attributed to the fine mode, and several derived parameters including reflected spectral solar flux at the top of the atmosphere. Over the ocean, the aerosol optical thickness is provided in seven wavelengths from 0.47 to 2.13 m. In addition, quantitative aerosol size information includes effective radius of the aerosol and quantitative fraction of optical thickness attributed to the fine mode. Spectral irradiance contributed by the aerosol, mass concentration, and number of cloud condensation nuclei round out the list of available aerosol products over the ocean. The spectral optical thickness and effective radius of the aerosol over the ocean are validated by comparison with two years of Aerosol Robotic Network (AERONET) data gleaned from 132 AERONET stations. Eight thousand MODIS aerosol retrievals collocated with AERONET measurements confirm that one standard deviation of MODIS optical thickness retrievals fall within the predicted uncertainty of 0.03 0.05 over ocean and 0.05 0.15 over land. Two hundred and seventy-one MODIS aerosol retrievals collocated with AERONET inversions at island and coastal sites suggest that one standard deviation of MODIS effective radius retrievals falls within reff 0.11 m. The accuracy of the MODIS retrievals suggests that the product can be used to help narrow the uncertainties associated with aerosol radiative forcing of global climate.

/pdf/the-modis-aerosol-algorithm-products-and-validation-35oa3pb65x.pdf

The MODIS Aerosol Algorithm, Products and Validation

Aerosol radiative forcing is a critical, though variable and uncertain, component of the global climate. Yet climate models rely on sparse information of the aerosol optical properties. In situ measurements, though important in many respects, seldom provide measurements of the undisturbed aerosol in the entire atmospheric column. Here, 8 yr of worldwide distributed data from the AERONET network of ground-based radiometers were used to remotely sense the aerosol absorption and other optical properties in several key locations. Established procedures for maintaining and calibrating the global network of radiometers, cloud screening, and inversion techniques allow for a consistent retrieval of the optical properties of aerosol in locations with varying emission sources and conditions. The multiyear, multi-instrument observations show robust differentiation in both the magnitude and spectral dependence of the absorption—a property driving aerosol climate forcing, for desert dust, biomass burning, urban‐industrial, and marine aerosols. Moreover, significant variability of the absorption for the same aerosol type appearing due to different meteorological and source characteristics as well as different emission characteristics are observed. It is expected that this aerosol characterization will help refine aerosol optical models and reduce uncertainties in satellite observations of the global aerosol and in modeling aerosol impacts on climate.

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Variability of Absorption and Optical Properties of Key Aerosol Types Observed in Worldwide Locations

We assembled a dataset of 14C-based productivity measurements to understand the critical variables required for accurate assessment of daily depth-integrated phytoplankton carbon fixation (PP(PPeu)u) from measurements of sea surface pigment concentrations (Csat)(Csat). From this dataset, we developed a light-dependent, depth-resolved model for carbon fixation (VGPM) that partitions environmental factors affecting primary production into those that influence the relative vertical distribution of primary production (Pz)z) and those that control the optimal assimilation efficiency of the productivity profile (P(PBopt). The VGPM accounted for 79% of the observed variability in Pz and 86% of the variability in PPeu by using measured values of PBopt. Our results indicate that the accuracy of productivity algorithms in estimating PPeu is dependent primarily upon the ability to accurately represent variability in Pbopt. We developed a temperature-dependent Pbopt model that was used in conjunction with monthly climatological images of Csat sea surface temperature, and cloud-corrected estimates of surface irradiance to calculate a global annual phytoplankton carbon fixation (PPannu) rate of 43.5 Pg C yr‒1. The geographical distribution of PPannu was distinctly different than results from previous models. Our results illustrate the importance of focusing Pbopt model development on temporal and spatial, rather than the vertical, variability.

https://aslopubs.onlinelibrary.wiley.com/doi/epdf/10.4319/lo.1997.42.1.0001

Photosynthetic rates derived from satellite‐based chlorophyll concentration

A large data set containing coincident in situ chlorophyll and remote sensing reflectance measurements was used to evaluate the accuracy, precision, and suitability of a wide variety of ocean color chlorophyll algorithms for use by SeaWiFS (Sea-viewing Wide Field-of-view Sensor). The radiance-chlorophyll data were assembled from various sources during the SeaWiFS Bio-optical Algorithm Mini-Workshop (SeaBAM) and is composed of 919 stations encompassing chlorophyll concentrations between 0.019 and 32.79 μg L−1. Most of the observations are from Case I nonpolar waters, and ∼20 observations are from more turbid coastal waters. A variety of statistical and graphical criteria were used to evaluate the performances of 2 semianalytic and 15 empirical chlorophyll/pigment algorithms subjected to the SeaBAM data. The empirical algorithms generally performed better than the semianalytic. Cubic polynomial formulations were generally superior to other kinds of equations. Empirical algorithms with increasing complexity (number of coefficients and wavebands), were calibrated to the SeaBAM data, and evaluated to illustrate the relative merits of different formulations. The ocean chlorophyll 2 algorithm (OC2), a modified cubic polynomial (MCP) function which uses Rrs490/Rrs555, well simulates the sigmoidal pattern evident between log-transformed radiance ratios and chlorophyll, and has been chosen as the at-launch SeaWiFS operational chlorophyll a algorithm. Improved performance was obtained using the ocean chlorophyll 4 algorithm (OC4), a four-band (443, 490, 510, 555 nm), maximum band ratio formulation. This maximum band ratio (MBR) is a new approach in empirical ocean color algorithms and has the potential advantage of maintaining the highest possible satellite sensor signal: noise ratio over a 3-orders-of-magnitude range in chlorophyll concentration.

/pdf/ocean-color-chlorophyll-algorithms-for-seawifs-68regm2asj.pdf

Ocean Color Chlorophyll Algorithms for SEAWIFS

We investigate the influence of the curvature of the Earth on a proposed atmospheric-correction scheme for the Sea-Viewing Wide-Field-of-View Sensor (SeaWiFS) by simulating the radiance exiting the top of a spherical-shell atmosphere and inserting the result into the proposed correction algorithm. The error in the derived water-leaving reflectance suggests that the effects of the curvature are negligible for solar zenith angles (θ0) ≤ 70°. Furthermore, for θ0 > 70° the error in atmospheric correction can usually be reduced if the molecular-scattering component of the top of the atmosphere reflectance (ρr) is computed with a spherical-shell atmosphere radiative transfer code. Also, for θ0 > 70° the error in atmospheric correction in a spherical-shell atmosphere, when ρr is computed with a spherical-shell model, can be predicted reasonably well from computations made with plane-parallel atmosphere radiative transfer codes. This implies that studies aimed at improving atmospheric correction can be made assuming plane-parallel geometry and that the investigator can be confident when θ0 > 70° that any improvements will still be valid for a spherical-shell atmosphere as long as ρr is computed in spherical-shell geometry. Finally, a scheme for computing ρr in a spherical-shell atmosphere in a relatively simple manner is developed.

Atmospheric correction of ocean-color sensors: effects of the Earth’s curvature

http://oceancolor.gsfc.nasa.gov/staff/franz/papers/gordon_and_franz_2008_rse.pdf

Remote sensing of ocean color: Assessment of the water-leaving radiance bidirectional effects on the atmospheric diffuse transmittance for SeaWiFS and MODIS intercomparisons

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.

SeaWiFS observations of the Arabian Sea southwest monsoon bloom for the year 2000

It is shown that measurements of the absorption coefficient and the volume-scattering function for scattering angles ≳ 15° of ocean water are sufficient for predicting the transport of irradiance in the ocean. Thus, difficult-to-measure small-angle scattering is not necessary in many applications. Furthermore, the irrelevance of small-angle scattering suggests the irrelevance of the scattering coefficient and of the routinely measured beam-attenuation coefficient in many radiative-transfer problems. Finally, these observations provide a method for determining the adequacy of instruments in which small sampling volumes (~ few cm3) are analyzed when predicting irradiance attenuation and diffuse reflection for large volumes (~ 10–104 m3).

Sensitivity of radiative transfer to small-angle scattering in the ocean: Quantitative assessment

A methodology for determination of the effects of radiometric noise on the performance of ocean color sensors is developed and applied to the Coastal Zone Color Scanner on Nimbus 7 and the Moderate Resolution Imaging Spectrometer planned for the Earth Observing System.

Howard R. Gordon

Papers

Atmospheric correction of ocean-color sensors: effects of the Earth’s curvature

Remote sensing of ocean color: Assessment of the water-leaving radiance bidirectional effects on the atmospheric diffuse transmittance for SeaWiFS and MODIS intercomparisons

SeaWiFS observations of the Arabian Sea southwest monsoon bloom for the year 2000

Sensitivity of radiative transfer to small-angle scattering in the ocean: Quantitative assessment

Radiometric considerations for ocean color remote sensors