Dennis K. Clark

An emerging ground‐based aerosol climatology: Aerosol optical depth from AERONET

Long-term measurements by the AERONET program of spectral aerosol optical depth, precipitable water, and derived Angstrom exponent were analyzed and compiled into an aerosol optical properties climatology. Quality assured monthly means are presented and described for 9 primary sites and 21 additional multiyear sites with distinct aerosol regimes representing tropical biomass burning, boreal forests, midlatitude humid climates, midlatitude dry climates, oceanic sites, desert sites, and background sites. Seasonal trends for each of these nine sites are discussed and climatic averages presented.

Phytoplankton pigment concentrations in the Middle Atlantic Bight: comparison of ship determinations and CZCS estimates.

The processing algorithms used for relating the apparent color of the ocean observed with the Coastal-Zone Color Scanner on Nimbus-7 to the concentration of phytoplankton pigments (principally the pigment responsible for photosynthesis, chlorophyll a) are developed and discussed in detail. These algorithms are applied to the shelf and slope waters of the Middle Atlantic Bight and also to Sargasso Sea waters. In all, four images are examined, and the resulting pigment concentrations are compared to continuous measurements made along ship tracks. The results suggest that over the 0.08-1.5-mg/m3 range the error in the retrieved pigment concentration is of the order of 30-40% for a variety of atmospheric turbidities. In three direct comparisons between ship-measured and satellite-retrieved values of the water-leaving radiance the atmospheric correction algorithm retrieved the water-leaving radiance with an average error of approximately 10%. This atmospheric correction algorithm does not require any surface measurements for its application.

Clear water radiances for atmospheric correction of coastal zone color scanner imagery

The possibility that the inherent sea surface spectral radiance of clear water can be computed in the green, yellow, and red regions of the spectrum solely from a knowledge of the solar zenith angle is developed in detail. This concept is tested with experimental data, and the results indicate that the normalized inherent sea surface spectral radiance is constant to within ~10% for Morels Case 1 waters as long as the phytoplankton pigment concentration does not exceed 0.25 mg/m(3). In the same conditions the radiance at 670 nm is <0.012 mW/cm(2) microm sr. A scheme to implement this clear water radiance concept for atmospheric correction of CZCS imagery is presented.

Nimbus-7 Coastal Zone Color Scanner: System Description and Initial Imagery

The Coastal Zone Color Scanner (CZCS) on Nimbus-7, launched in October 1978, is the only sensor in orbit that is specifically designed to study living marine resources. The initial imagery confirms that CZCS data can be processed to a level that reveals subtle variations in the concentration of phytoplankton pigments. This development has potential applications for the study of large-scale patchiness in phytoplankton distributions, the evolution of spring blooms, water mass boundaries, and mesoscale circulation patterns.

Phytoplankton pigments from the Nimbus-7 Coastal Zone Color Scanner - Comparisons with surface measurements

The removal of atmospheric effects from Nimbus-7 Coastal Zone Color Scanner (CZCS) images reveals eddy-like ocean turbidity patterns not apparent in the original calibrated images. Comparisons of the phytoplankton pigment concentrations derived from the corrected CZCS radiances with surface measurements agree to within less than 0.5 log C, where C is the sum of the concentrations of chlorophyll a plus phaeopigments a (in milligrams per cubic meter).

Validation of atmospheric correction over the oceans

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.

Atmospheric effects in the remote sensing of phytoplankton pigments

We investigate the accuracy with which relevant atmospheric parameters must be estimated to derive phytoplankton pigment concentrations (chlorophyll a plus phaeophytin a ) of a given accuracy from measurements of the oceans apparent spectral radiance at satellite altitudes. The analysis is limited to an instrument having the characteristics of the Coastal Zone Color Scanner scheduled to orbit the Earth on NIMBUS-G. A phytoplankton pigment algorithm is developed which relates the pigment concentration (C) to the three ratios of upwelling radiance just beneath the sea surface which can be formed from the wavelengths (λ) 440, 520 and 550 nm. The pigment algorithm explains from 94 to 98% of the variance in log10 C over three orders of magnitude in pigment concentration. This is combined with solutions to the radiative transfer equation to simulate the oceans apparent spectral radiance at satellite altitudes as a function of C and the optical properties of the aerosol, the optical depth of which is assumed to be proportioned to λ-n. A specific atmospheric correction algorithm, based on the assumption that the ocean is totally absorbing at 670 nm, is then applied to the simulated spectral radiance, from which the pigment concentration is derived. Comparison between the true and derived values of C show that: (1) n is considerably more important than the actual aerosol optical thickness; (2) for C 0299-1 0.2 Μg l-1 acceptable concentrations can be determined as long as n is not overestimated; (3) as C increases, the accuracy with which n must be estimated, for a given relative accuracy in C, also increases; and (4) for C greater than about 0.5 Μg 1-1, the radiance at 440 nm becomes essentially useless in determining C. The computations also suggest that if separate pigment algorithms are used for C ≲ 1Μgl-1 and C ≳ 1 Μgl-1, accuracies considerably better than ±± in log C can be obtained for C ≲ 1 Μg l-1 with only a coarse estimate of n, while for C ≳ 10 Μgl-1, this accuracy can be achieved only with very good estimates of n.

Nimbus 7 CZCS: reduction of its radiometric sensitivity with time

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.

Phytoplankton Pigment Algorithms for the Nimbus-7 CZCS

Inherent sea surface spectral radiance ratios are related empirically to the sum of the photosynthetically active phytoplankton pigment chlorophyll a and phaeopigment a, the associated degradation product. The analysis of the in-water optical data is specifically focused on the spectral characteristics of the Nimbus-7 Coastal Zone Color Scanner instrument. In addition, an optically-dependent weighting function is applied to the vertical profile measurements of the pigments in order to accurately represent the condition of a remote sensor viewing a stratified ocean. The pigment algorithm formulated with these modifications and based on the inclusion of a larger and more diverse data base, from post-launch validation cruises, resulted in no statistically significant difference when compared to the preliminary form reported by Gordon and Clark (1980) or to Morel’s (1980) combined class 1 and class 2 waters. The pigment algorithm explains from 87 to 92% of the variance in log10 pigments over nearly four orders of magnitude in pigment concentration and is accurate to ±1/4 in log of pigment concentration.

Calibration of SeaWiFS. II. Vicarious techniques.

We present an overview of the vicarious calibration of the Sea-Viewing Wide Field-of-view Sensor (SeaWiFS). This program has three components: the calibration of the near-infrared bands so that the atmospheric correction algorithm retrieves the optical properties of maritime aerosols in the open ocean; the calibration of the visible bands against in-water measurements from the Marine Optical Buoy (MOBY); and a calibration-verification program that uses comparisons between SeaWiFS retrievals and globally distributed in situ measurements of water-leaving radiances. This paper describes the procedures as implemented for the third reprocessing of the SeaWiFS global mission data set. The uncertainty in the near-infrared vicarious gain is 0.9%. The uncertainties in the visible-band vicarious gains are 0.3%, corresponding to uncertainties in the water-leaving radiances of approximately 3%. The means of the SeaWiFS/in situ matchup ratios for water-leaving radiances are typically within 5% of unity in Case 1 waters, while chlorophyll a ratios are within 1% of unity. SeaWiFS is the first ocean-color mission to use an extensive and ongoing prelaunch and postlaunch calibration program, and the matchup results demonstrate the benefits of a comprehensive approach.