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.

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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.

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Ocean Color Chlorophyll Algorithms for SEAWIFS

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.

Estimation of aerosol columnar size distribution and optical thickness from the angular distribution of radiance exiting the atmosphere: simulations.

The reflectance of white caps in the open ocean is obtained with a 6-channel spectral radiometer, extended from the bow of a ship, at wavelengths; 410, 440, 510, 670 and 860 nm. In addition to reflectance data, air/water temperature, wind speed and direction are obtained as well as GPS information in order to characterize ocean surface conditions. A visual reference for sea surface conditions measured by the radiometer is recorded using a video camera mounted beside the radiometer. By measuring a small area of the water surface over time, the presence and spectral influence of whitecaps can be quantified.Using this technique the spectral reflectance of individual whitecaps can be measured and tracked through their complete life cycle. Also, integration over a larger time series of ocean surface reflectance data provides a measurement of the augmented or extra contribution from white water reflectance to the water leaving radiance observed by ocean color satellites. Estimates of the augmented reflectance are subsequently correlated to sea surface state, and include the frequency of occurrence and spectral contribution from various levels of white water reflectance as whitecaps grow and decay.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Whitecaps: spectral reflectance in the open ocean and their contribution to water-leaving radiance

The upwelling spectral radiance distribution is polarized, and this polarization varies with the optical properties of the water body. Knowledge of the polarized, upwelling, bidirectional radiance distribution function (BRDF) is important for generating consistent, long-term data records for ocean color because the satellite sensors from which the data are derived are sensitive to polarization. In addition, various studies have indicated that measurement of the polarization of the radiance leaving the ocean can used to determine particle characteristics (Tonizzo et al., 2007; Ibrahim et al., 2016; Chami et al., 2001). Models for the unpolarized BRDF (Morel et al., 2002; Lee et al., 2011) have been validated (Voss et al., 2007; Gleason et al., 2012), but variations in the polarization of the upwelling radiance due to the sun angle, viewing geometry, dissolved material, and suspended particles have not been systematically documented. In this work, we simulated the upwelling radiance distribution using a Monte Carlo-based radiative transfer code and measured it using a set of fish-eye cameras with linear polarizing filters. The results of model-data comparisons from three field experiments in clear and turbid coastal conditions showed that the degree of linear polarization (DOLP) of the upwelling light field could be determined by the model with an absolute error of ±0.05 (or 5% when the DOLP was expressed in %). This agreement was achieved even with a fixed scattering Mueller matrix, but did require in situ measurements of the other inherent optical properties, e.g., scattering coefficient, absorption coefficient, etc. This underscores the difficulty that is likely to be encountered using the particle scattering Mueller matrix (as indicated through the remote measurement of the polarized radiance) to provide a signature relating to the properties of marine particles beyond the attenuation/absorption coefficient.

/pdf/measuring-and-modeling-the-polarized-upwelling-radiance-3b656oh993.pdf

Measuring and Modeling the Polarized Upwelling Radiance Distribution in Clear and Coastal Waters

In the first 15 or so years of my 35-year involvement in ocean color remote sensing, I was fortunate to have witnessed and participated in much of the early development of this enterprise. In this chapter I relate those that had a significant impact on the subject and on my own work, and try to describe the historical setting in which they took place. These were very exciting and sometimes trying times for the members of the CZCS Experiment Team. I hope I can convey some of that excitement and frustration, and along the way, a few details about the subject.

Some Reflections on Thirty-Five Years of Ocean Color Remote Sensing

Many marine organisms ranging in size and complexity from bacteria and phytoplankton to zooplankton and fish possess the capability of emitting light. This is usually re­ ferred to as bioluminescence. The emission typically occurs when the organism is stimulated, e.g., mechanically by a swimmer or a ship; or photically by emission from other or­ ganisms. It can occur at the surface (e.g., Phosphorescent Bay, Puerto Rico), in the productive surface layers, or at great depth. This emission, which is typically centered near 470 nm, can often be detected visually from the sea surface. Since this emission is routinely observed (in efforts to locate fish schools at night) with instrumentation flown on aircraft at altitudes of 2-3 km, the question naturally arises: can the presence of bioluminescence be remotely detected and mea­ sured from earth-orbiting satellites? We do not attempt to answer this question here; rather, we shall discuss with the aid of radiative transfer theory the important optical parameters of the water that must be understood in order to address this question in a quantitative manner. Bioluminescence occurs as flashes with duration ranging from 0.2 sec to considerably longer than 1 sec. Because light travels with a speed of ~225 km/msec in water and 300 km/ msec in air, the transport of radiant energy through the ocean-atmosphere system from pulses of duration greater than a few hundred milliseconds will be governed by timeindependent transport theory. We shall limit ourselves to pulses which satisfy this criterion. The problem we wish to examine can be simply stated: given a distribution of isotropically emitting point sources embedded in the ocean, how does the associated radiance leaving the top of the atmosphere vary with the optical properties of the water? The steady-state radiative transfer equation (RTE) de­ scribing the transport of the radiance [L(r,ξ)] at r in a direc­ tion specified by the unit vector ξ is given by

Howard R. Gordon

Papers

Estimation of aerosol columnar size distribution and optical thickness from the angular distribution of radiance exiting the atmosphere: simulations.

Whitecaps: spectral reflectance in the open ocean and their contribution to water-leaving radiance

Measuring and Modeling the Polarized Upwelling Radiance Distribution in Clear and Coastal Waters

Some Reflections on Thirty-Five Years of Ocean Color Remote Sensing

Remote sensing marine bioluminescence: the role of the in-water scaler irradiance.