Ocean colour is recognised as an Essential Climate Variable (ECV) by the Global Climate Observing System (GCOS); and spectrally-resolved water-leaving radiances (or remote-sensing reflectances) in ...the visible domain, and chlorophyll-a concentration are identified as required ECV products. Time series of the products at the global scale and at high spatial resolution, derived from ocean-colour data, are key to studying the dynamics of phytoplankton at seasonal and inter-annual scales; their role in marine biogeochemistry; the global carbon cycle; the modulation of how phytoplankton distribute solar-induced heat in the upper layers of the ocean; and the response of the marine ecosystem to climate variability and change. However, generating a long time series of these products from ocean-colour data is not a trivial task: algorithms that are best suited for climate studies have to be selected from a number that are available for atmospheric correction of the satellite signal and for retrieval of chlorophyll-a concentration; since satellites have a finite life span, data from multiple sensors have to be merged to create a single time series, and any uncorrected inter-sensor biases could introduce artefacts in the series, e.g., different sensors monitor radiances at different wavebands such that producing a consistent time series of reflectances is not straightforward. Another requirement is that the products have to be validated against in situ observations. Furthermore, the uncertainties in the products have to be quantified, ideally on a pixel-by-pixel basis, to facilitate applications and interpretations that are consistent with the quality of the data. This paper outlines an approach that was adopted for generating an ocean-colour time series for climate studies, using data from the MERIS (MEdium spectral Resolution Imaging Spectrometer) sensor of the European Space Agency; the SeaWiFS (Sea-viewing Wide-Field-of-view Sensor) and MODIS-Aqua (Moderate-resolution Imaging Spectroradiometer-Aqua) sensors from the National Aeronautics and Space Administration (USA); and VIIRS (Visible and Infrared Imaging Radiometer Suite) from the National Oceanic and Atmospheric Administration (USA). The time series now covers the period from late 1997 to end of 2018. To ensure that the products meet, as well as possible, the requirements of the user community, marine-ecosystem modellers, and remote-sensing scientists were consulted at the outset on their immediate and longer-term requirements as well as on their expectations of ocean-colour data for use in climate research. Taking the user requirements into account, a series of objective criteria were established, against which available algorithms for processing ocean-colour data were evaluated and ranked. The algorithms that performed best with respect to the climate user requirements were selected to process data from the satellite sensors. Remote-sensing reflectance data from MODIS-Aqua, MERIS, and VIIRS were band-shifted to match the wavebands of SeaWiFS. Overlapping data were used to correct for mean biases between sensors at every pixel. The remote-sensing reflectance data derived from the sensors were merged, and the selected in-water algorithm was applied to the merged data to generate maps of chlorophyll concentration, inherent optical properties at SeaWiFS wavelengths, and the diffuse attenuation coefficient at 490 nm. The merged products were validated against in situ observations. The uncertainties established on the basis of comparisons with in situ data were combined with an optical classification of the remote-sensing reflectance data using a fuzzy-logic approach, and were used to generate uncertainties (root mean square difference and bias) for each product at each pixel.
A methodology is developed for deriving consistent ocean biological and biogeochemical products from multiple satellite ocean color sensors that have slightly different sensor spectral ...characteristics. Specifically, the required coefficients for algorithm modifications are obtained using the hyperspectral in situ optical measurements from the Marine Optical Buoy (MOBY) in the water off Hawaii. It is demonstrated that using the proposed approach for modifying ocean biological and biogeochemical algorithms, satellite-derived ocean property data over the global open ocean are consistent from multiple satellite sensors, although their corresponding sensor-measured normalized water-leaving radiance spectra nLw(λ) are different. Therefore, the proposed approach allows satellite-derived ocean biological and biogeochemical products to be consistent and can therefore be routinely merged from various satellite ocean color sensors. The proposed approach can be applied to any satellite algorithms that use the input of sensor-measured nLw(λ) spectra.
System Vicarious Calibration (SVC) ensures a relative radiometric calibration to satellite ocean color sensors that minimizes uncertainties in the water-leaving radiance Lw derived from the top of ...atmosphere radiance LT. This is achieved through the application of gain-factors, g-factors, to pre-launch absolute radiometric calibration coefficients of the satellite sensor corrected for temporal changes in radiometric sensitivity. The g-factors are determined by the ratio of simulated to measured spectral LT values where the former are computed using: i. highly accurate in situ Lw reference measurements; and ii. the same atmospheric models and algorithms applied for the atmospheric correction of satellite data. By analyzing basic relations between relative uncertainties of Lw and LT, and g-factors consistently determined for the same satellite mission using different in situ data sources, this work suggests that the creation of ocean color Climate Data Records (CDRs) should ideally rely on: i. one main long-term in situ calibration system (site and radiometry) established and sustained with the objective to maximize accuracy and precision over time of g-factors and thus minimize possible biases among satellite data products from different missions; and additionally ii. unique (i.e., standardized) atmospheric model and algorithms for atmospheric correction to maximize cross-mission consistency of data products at locations different from that supporting SVC. Finally, accounting for results from the study and elements already provided in literature, requirements and recommendations for SVC sites and field radiometric measurements are streamlined.
•System Vicarious Calibration (SVC) for satellite ocean color (OC) sensors.•Uncertainties in SVC correction factors supporting OC Climate Data Records (CDRs).•Recommendations for sites and in situ SVC measurements supporting OC CDRs.
The transmission coefficient,
, commonly used to propagate the upwelling nadir radiance, just below the ocean surface, to above the surface has been assumed to be a constant value of 0.543 in ...seawater. Because the index of refraction of seawater varies with wavelength, salinity, and temperature, the variation of
with these parameters should be taken into account, especially if low uncertainty is required for the quantities derived using
. In particular the wavelength dependence of this factor is important. For example at a salinity of 35 g/kg and a temperature of 26° C,
will be 1.3% lower at 380 nm and 1.1 % higher at 700 nm than the constant value (0.543) and should be taken into account when calculating the water leaving radiance and normalized water leaving radiance from in-water measurements.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
We challenge a recent paper in this journal suggesting that the well-established formula governing the transmittance of radiance across a refracting interface needs revision Optics Express, 25(22) ...27086 (2017). We provide a simple example of radiative transfer across an interface showing that the accepted formula is correct.
The spectral resolution requirements for in situ remote sensing reflectanceR
measurements aiming at supporting satellite ocean color validation and System Vicarious Calibration (SVC) were ...investigated. The study, conducted using sample hyperspectral R
from different water types, focused on the visible spectral bands of the ocean land color imager (OLCI) and of the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite sensors. Allowing for a ±0.5% maximum difference between in situ and satellite derived R
solely due to the spectral band characteristics of the in situ radiometer, a spectral resolution of 1 nm for SVC of PACE is needed in oligotrophic waters. Requirements decrease to 3 nm for SVC of OLCI. In the case of validation activities, which exhibit less stringent uncertainty requirements with respect to SVC, a maximum difference of ±1% between in situ and satellite derived data indicates the need for a spectral resolution of 3 nm for both OLCI and PACE in oligotrophic waters. Conversely, spectral resolutions of 6 nm for PACE and 9 nm for OLCI appear to satisfy validation activities in optically complex waters.
The significance of light backscattering in the ocean is wide ranging, especially in optical remote sensing. However, the complexity of natural seawater as an optical medium often obscures the ...measured optical signals to the point that our present-day interpretation and detailed understanding of major sources of backscattering and its variability in the ocean are uncertain and controversial. Here we review the roles played by various seawater constituents in light backscattering and we address a question of 'missing' backscattering. Historically, this question has resulted from a hypothesis that under non-bloom conditions in the open ocean, phytoplankton make a significantly smaller contribution to the particulate backscattering coefficient than to the particulate (total) scattering coefficient. By discussing the backscattering properties and potential contributions of the various water constituents (colloids, bacteria, phytoplankton, biogenic detritus, minerogenic particles, bubbles), we show that due to substantial variability in water composition, different types of constituents can explain the 'missing' backscattering. Under typical non-bloom conditions in the open ocean, the small-sized non-living particles appear to be the most important because of their high abundance relative to other particle types. These particles are believed to be primarily of organic origin but an important role of minerogenic particles cannot be excluded. Still, in the very clear ocean water the backscattering by water molecules themselves can contribute as much as 80% to the total backscattering coefficient in the blue spectral region. The general scenario of the dominance of molecules and small-sized particles can, however, be readily perturbed due to changes in local conditions. For example, bubbles entrained by breaking waves can intermittently dominate the backscattering at shallow depths below the sea surface, the calcifying phytoplankton (coccolithophores) producing calcite scales of high refractive index can dominate if present in sufficient concentration, and other plankton species can dominate during blooms. The role of phytoplankton could be generally greater than commonly assumed given the fact that real cells backscatter more light than predicted from homogeneous sphere models. In addition, high refractive index mineral particles can dominate in many coastal areas, and perhaps also in some open ocean areas during events of atmospheric dust deposition. It is likely that the different scenarios are quite widespread and frequent. Further improvements in quantitative understanding of the variability in light backscattering and its sources require an increased effort in basic research to better characterize the optical properties of the various seawater constituents and the variability in the detailed composition of seawater. Seawater is a complex optical medium containing a great variety of particle types and soluble species that vary in concentration and composition with time and location in the ocean, so ocean optics science must progress beyond the traditional overly simplified description, which has been based only on a few constituent categories defined broadly as molecular water, suspended particles (phytoplankton and non-algal particles), and dissolved organic matter. Keywords: Ocean optics; Optical properties; Seawater; Light scattering; Backscattering coefficient; Volume scattering function
Objective: Mobile health (mHealth) interventions show potential to broaden the reach of efficacious alcohol brief motivational interventions (BMIs). However, efficacy is mixed and may be limited by ...low participant attention and engagement. The present study examined the feasibility, acceptability, and preliminary efficacy of a live text-message delivered BMI in a pilot randomized clinical trial. Method: Participants were 66 college students (63.6% women; 61.9% White; Mage = 19.95, SD = 1.66) reporting an average of 11.88 (SD = 8.74) drinks per week, 4.42 (SD = 3.59) heavy drinking episodes (HDEs), and 8.44 (SD = 5.62) alcohol-related problems in the past month. Participants were randomized to receive either (a) education or (b) an alcohol BMI plus behavioral economic substance-free activity session (SFAS), each followed by 4 weeks of mini sessions. All sessions were administered via live text-message. Participants completed assessments postintervention (after the 4th mini session) and at 3-month follow-up. Results: 90.9% completed both initial full-length sessions and at least two of the four mini sessions with 87.9% retention at 3-month follow-up. Participants found the interventions useful, interesting, relevant, and effective, with no between-group differences. There were no statistically significant group differences in drinks per week or alcohol-related problems at follow-up, but BMI + SFAS participants reported fewer past-month HDEs than those who received education. Conclusions: Live text-messaging to deliver the BMI + SFAS is feasible and well-received. The preliminary efficacy results should be interpreted cautiously due to the small sample size but support further investigation.
Public Health Significance Statement
Live text-messaging with a counselor is a feasible and well-received way to deliver a brief alcohol intervention to reduce harmful alcohol use in emerging adults. This study found that live text-messaging with a counselor can reduce the frequency of binge drinking, a significant risk factor for experiencing alcohol-related problems. Live text-messaging may provide the benefits of interpersonal contact while retaining a sense of privacy and minimizing some burden associated with in-person sessions (e.g., travel costs, discomfort).
While the upwelling radiance distribution in the ocean can be highly polarized, there are few measurements of this parameter in the open ocean. To obtain the polarized in-water upwelling spectral ...radiance distribution data we have developed the POLRADS instrument. This instrument is based on the NuRADS radiance distribution camera systems in which linear polarizer's have been installed. By combining simultaneous images from three NuRADS instruments, three Stokes parameters (I, Q, U) for the water leaving radiance can be obtained for all upwelling angles simultaneously. This system measures the Stokes parameters Q/I and U/I with a 0.05-0.06 uncertainty and I with a 7-10% uncertainty.
PDF
