Frank E. Hoge

Satellite retrieval of inherent optical properties by linear matrix inversion of oceanic radiance models: An analysis of model and radiance measurement errors

The linear summability of inherent optical properties (IOPs) is exploited to provide matrix equations for retrieval of phytoplankton absorption, dissolved organic matter, and constituent backscatter. Spectral models for the phytoplankton pigment absorption coefficient, chromophoric dissolved organic matter (CDOM) absorption coefficient, and total constituent backscatter (TCB) coefficient are first used to calculate 5 × 105 water-leaving spectral radiances for a wide range of normally distributed IOP values at 410, 490, and 555 nm. Then, the spectral radiances are inverted to simultaneously provide phytoplankton pigment absorption coefficient, CDOM absorption coefficient, and total constituent backscatter coefficient on a pixel-by-pixel basis to demonstrate that (1) matrix inversion is very rapid and well conditioned, (2) the IOPs are exactly determined when the water-leaving radiances and IOP spectral models are error free, (3) for equal radiance errors sequentially occurring in one of three sensor bands, phytoplankton pigment retrieval errors are generally higher than CDOM and TCB coefficient errors, (4) retrieval errors for all the IOPs are strongly dependent on phytoplankton pigment spectral model Gaussian width errors, (5) phytoplankton pigment absorption and CDOM absorption retrieval errors are more sensitive to CDOM spectral slope errors than the TCB coefficient retrievals, and (6) TCB wavelength-ratio-exponent errors produce less impact on the phytoplankton absorption coefficient retrieval than upon the CDOM absorption or the TCB coefficient retrievals.

Mesoscale and upper ocean variabilities during the 1989 JGOFS bloom study

Abstract Altimetric data from Geosat and some critical hydrographic measurements were used to estimate in real time the mesoscale physical oceanographic environment surrounding the Joint Global Ocean Flux Study (JGOFS) 1989 North Atlantic Bloom Experiment. Three cyclonic eddies, including an exceptionally large one, evolved and interacted over the 10 weeks of observations. Subsequent analysis of all available hydrographic data confirmed the real time estimates and provided further quantitative information concerning the mesoscale and submesoscale structure of the upper ocean. Remotely sensed indicators of near-surface chlorophyll content reveal significant biological variability on these wavelengths. The altimetric and hydrographic data have been assimilated into a dynamical model to produce optimal estimates of physical fields of interest as they evolve in time for use in physical and biological process studies.

Feasibility of airborne detection of laser-induced fluorescence emissions from green terrestrial plants

Recent experiments conducted with the NASA airborne oceanographic lidar (AOL) have shown that laser-induced fluorescence (LIF) spectral emissions from green terrestrial plants are detectable from a remote platform. A 3-MW peak power frequency-doubled Nd:YAG laser at 532 nm was used from an altitude of 150 m to induce fluorescence from trees, bushes, and grasses growing on a barrier island. A companion 422-nm XeCl excimer pumped dye laser with a controlled maximum output power of 100 kW was used separately on additional passes in order to compare its effectiveness over the same test area. Slant range measurements obtained simultaneously from each laser on-wavelength return pulse provided valuable, comparative elevational information on the heights of plants and variations in terrain along the flight lines. Samples of airborne LIF color spectra obtained with 532-nm excitation and cross sectional profiles are given together with supporting spectral measurements performed on selected plant types with a laboratory laser system. While the results to date are very encouraging, additional laboratory and field tests are required to establish the utility of the airborne LIF technique for measuring the distribution of plant pigments and biomass remotely from an airborne platform.

Airborne dual laser excitation and mapping of phytoplankton photopigments in a Gulf Stream Warm Core Ring.

Utilization of a two-color airborne lidar system in the systematic study of a major oceanographic feature is reported here for the first time. An excimer pumped dye laser was optically and electronically integrated into the NASA Airborne Oceanographic Lidar for simultaneous use with a frequency doubled Nd:YAG laser. The output beams exit the laser system along parallel paths after being produced on an alternating pulse basis at a combined rate of 12.5 pps. Results are presented for missions flown over a Gulf Stream Warm Core Ring (WCR) as well as over shelf, slope, Gulf Stream, and Sargasso Sea waters. From the airborne data a high coherence is shown between the two-color chlorophyll a data and between the Nd:YAG chlorophyll a and phycoerythrin responses within each of these water masses. However, distinct differences in the response patterns of these photopigments are shown to exist between the differing water masses. At certain of the boundaries separating the water masses a sharp transition is seen to occur, while at others a wider transition zone was observed in which the correlation between the photopigments appears to degrade.

Detection of phytoplankton pigments from ocean color: improved algorithms

Passive ocean-color data at 32 wavelengths in the visible domain and laser-induced fluorescence line heights of chlorophyll and phycoerythrin, measured simultaneously from an aircraft in the New York Bight area, are used to examine the problem of developing algorithms for pigment retrieval from ocean-color data that would be capable of distinguishing between chlorophyll and phycoerythrin. Using factor analysis, it is shown that it is indeed possible to develop such algorithms. Furthermore, the wavelengths used in the algorithms can be reduced from 32 to 6 (similar to the SeaWiFS channels) without much loss in information. These multiwavelength algorithms yield significantly higher correlation coefficients for chlorophyll compared with the conventional blue-green ratio used for retrieval of this pigment. The Coastal Zone Color Scanner wavelengths appear to be inadequate for quantitative retrieval of the phycoerythrin signal.

Spatial variability in near-surface chlorophyll a fluorescence measured by the Airborne Oceanographic Lidar (AOL)

The primary purpose of the aircraft remote sensing component of the North Atlantic Bloom Experiment (NABE) was to: (1) quantify spatial patterns of surface Chl a variability and co-variability with temperature (T) within the NABE study regions along the 20°W meridian near 48 and 60°N; and (2) determine if the major NABE ship and mooring locations were representative of surrounding ocean waters with respect to large-scale distributions of surface Chl a and T. The sampling platform was a NASA P-3 aircraft equipped with the Airborne Oceanographic Lidar (AOL) system, which measures laser-induced Chl a fluorescence (LICF), upwelling spectral radiance and surface temperature (T). Results collected during nine AOL missions conducted between 26 April and 3 June show considerable mesoscale variability in LICF and T. Spatial statistics (structure functions) showed that the dominant scales of LICF and T were significantly correlated in the range 10–290 km. Spectral analysis of the results of long flight lines showed spectral slopes averaging −2 for both LICF and T for spatial scales in the range 1.2–50 km. As for previous investigations of this type, we interpret the correlation between LICF and T as evidence that physical processes such as upwelling and mixing are dominant processes affecting spatial variations in Chl a distributions in the North Atlantic during the period of our sampling. The minimum dominant T and LICF spatial scales (ca 10 km) we determined from structure functions are similar to minimum scales predicted from models (Woods, 1988, In: Toward a theory on biological-physical interactions in the world ocean, Kluwer Academic, Boston, pp. 7–30) of upwelling induced by vortex contraction on the anticyclonic side of mesoscale jets. The NABE experiment was planned with the explicit assumption that major biological and chemical gradients are in the north-south direction in the northeast Atlantic. Our results support this assumption, and we observed no large-scale (>200 km), east-to-west trends in surface Chl a in the two principal study areas. Our analyses show that satellite ocean color scanners with pixel resolution of 4 × 4 km will generally detect the major spatial patterns of Chl a distributions (at scales > 0.3 km), in near surface waters during the spring bloom in the North Atlantic.

High frequency sampling of the 1984 spring bloom within the mid-Atlantic Bight: Synoptic shipboard, aircraft, and in situ perspectives of the SEEP-I experiment

Abstract Moorings of current meters, thermistors, transmissometers, and fluorometers on the Mid-Atlantic shelf, south of Long Island, suggest a seaward export of perhaps 0.20 mg Chl m −3 day −1 at depths of 75–81 m, between the 80- and 120-m isobaths during February–April 1984. Using a C/Chl ratio of 45/1, such a horizontal loss of algal carbon over the lower third of the water column would be 19–67% of the March–April 1984 primary production within the overlying euphotic zone. This possible physical carbon loss is similar to daily grazing losses to zooplankton of 32–40% of the algal fixation of carbon. Metabolic demands of the benthos could be met by just the estimated fecal pellet flux, without direct consumption of the remaining algal carbon. Similarly bacterioplankton metabolism could be fueled by excretory release of dissolved organic matter during photosynthesis, rather than by consumption of particulate carbon. Sediment traps tethered 10 and 70 m off the bottom at the 120-m isobath caught as much as 0.10–0.16 g C m −2 during March–April 1984. This presumed vertical flux is about one-third to one-half of the horizontal flux of 0.30 g C m −2 day −1 estimated over the lower 33 m of the water column at the 100-m isobath. These estimates suggest that ∼50% of the carbon export at the shelf-break might be derived from the adjacent overlying water column, with the remainder from lateral injections of near-bottom particles originating on the inner shelf.

Airborne lidar detection of subsurface oceanic scattering layers

The airborne lidar detection and cross-sectional mapping of submerged oceanic scattering layers are reported. The field experiment was conducted in the Atlantic Ocean southeast of Assateague Island, VA. NASAs Airborne Oceanographic Lidar was operated in the bathymetric mode to acquire on-wavelength 532-nm depth-resolved backscatter signals from shelf/slope waters. Unwanted laser pulse reflection from the airwater interface was minimized by spatial filtering and off-nadir operation. The presence of thermal stratification over the shelf was verified by the deployment of airborne expendable bathythermographs. Optical beam transmission measurements acquired from a surface truthing vessel indicated the presence of a layer of turbid water near the sea floor over the inner portion of the shelf.

Active–passive airborne ocean color measurement. 2: Applications

Reported here for the first time is the use of a single airborne instrument to make concurrent measurements of oceanic chlorophyll concentration by (1) laser-induced fluorescence, (2) passive upwelling radiance, and (3) solar-induced chlorophyll fluorescence. Results from field experiments conducted with the NASA airborne oceanographic lidar (AOL) in the New York Bight demonstrate the capability of a single active–passive instrument to perform new and potentially important ocean color studies related to (1) active lidar validation of passive ocean color in-water algorithms, (2) chlorophyll a in vivo fluorescence yield variability, (3) calibration of active multichannel lidar systems, (4) effect of sea state on passive and active ocean color measurements, (5) laser/solar-induced chlorophyll fluorescence investigations, and (6) subsequent improvement of satellite-borne ocean color scanners. For validation and comparison purposes a separate passive ocean color sensor was also flown along with the new active–passive sensor during these initial field trials.

Inherent optical properties of the ocean: retrieval of the absorption coefficient of chromophoric dissolved organic matter from airborne laser spectral fluorescence measurements

The absorption coefficient of chromophoric dissolved organic matter (CDOM) at 355 nm has been retrieved from airborne laser-induced and water Raman-normalized CDOM fluorescence. Four combined airborne and ship field experiments have demonstrated that (1) the airborne CDOM fluorescence-to--water Raman ratio is linearly related to concurrent quinine-sulfate-standardized CDOM shipboard fluorescence measurements over a wide range of water masses (coastal to blue water); (2) the vicarious calibration of the airborne fluorosensor in units traceable to a fluorescence standard can be established and then maintained over an extended time period by tungsten lamp calibration; (3) the vicariously calibrated airborne CDOM fluorescence-to-water Raman ratio can be directly applied to previously developed shipboard fluorescence-to-absorption algorithms to retrieve CDOM absorption; and (4) the retrieval is not significantly affected by long-path multiple scattering, differences in attenuation at the excitation and emission wavelengths, or measurement in the 180° backscatter configuration. Airborne CDOM absorption measurements will find immediate application to (a) forward and inverse modeling of oceanic water-leaving radiance and (b) validation of satellite-retrieved products such as CDOM absorption.