Satellite ocean-color project offices routinely generate Level 2 and Level 3 daily Photo-synthetically Available Radiation (PAR) products. Accuracy is currently evaluated against in-situ measurements from buoys and fixed platforms at a few locations, but specifying algorithm (and other) uncertainties on a pixel-by-pixel basis is needed to assess product quality. Expressing uncertainties requires modeling the measurement, identifying all possible error sources (e.g., noise in the input variables, imperfect/incomplete mathematical model), and determining the combined uncertainty. In the present study, algorithm uncertainties associated with PAR products are considered, i.e., those due to model approximations and parameter errors (e.g., decoupling effects of clouds and clear atmosphere, neglecting diurnal variability of clouds, using aerosol climatology) assuming that the input variables (TOA reflectance at wavelengths in the PAR spectral range) are known perfectly. A procedure is provided to estimate and provide, for each pixel of a product, this uncertainty component of the total uncertainty budget, which is expected to dominate. The bias and standard deviation of the daily PAR estimates are calculated as a function of clear sky PAR and cloud factor (i.e., the effect of clouds on daily PAR). The uncertainty characterization is accomplished using an extended simulation dataset covering the 2003–2012 time period using hourly MERRA-2 input data. The large number of data points allows one to sample well atmospheric variability and in particular many variations of daytime cloudiness, for all latitudes. Selected maps of global daily and monthly PAR and associated uncertainties (bias, standard deviation), obtained from MERIS data, are analyzed. Comparisons with match-up data at the COVE calibration/evaluation site reveal that experimental uncertainties are similar to the theoretical uncertainties obtained from simulated data.
The Copernicus programme brings a wealth of ocean colour data at medium and high spatial resolution with a full, free and open data access policy, allowing for unprecedented monitoring capabilities of the open ocean and coastal and inland waters. The POLYMER atmospheric correction algorithm, with its genericity and robustness to most atmospheric and surface perturbations (aerosols, sun glint, thin clouds, adjacency effect), allows to maximize these observation capabilities, in particular for Sentinel-2 MSI and Sentinel-3 OLCI. The algorithm is fully consistent between these sensors, which gives access to a unique product in terms of potential applications. The evolution of the POLYMER algorithm will be presented, with examples of applications and validation results for Sentinel-2 and Sentinel-3.
The Earth Polychromatic Imaging Camera (EPIC) onboard the Deep Space Climate Observatory (DSCOVR) in Lagrange-1 (L1) orbit provides observations of the Earth’s surface lit by the Sun at a cadence of 13 to 22 images/day and optical resolution of 16 km in 10 spectral bands from 317 to 780 nm. The EPIC data collected in the bands centered on 443, 551, and 680 nm are used to estimate daily mean photosynthetically available radiation (PAR) reaching the surface of the global, ice-free oceans. The solar irradiance reaching the surface is obtained by subtracting from the extraterrestrial irradiance (known), the irradiance reflected to space (estimated from the EPIC measurements), while taking into account atmospheric transmission (modeled). Clear and cloudy regions within a pixel do not need to be distinguished, i.e., the methodology is adapted to the relatively large EPIC pixels. A first daily mean EPIC PAR imagery is generated. Comparison with estimates from sensors in polar and geostationary orbits, namely MODIS and AHI, shows good agreement, with coefficients of determination of 0.79 and 0.92 and RMS differences of 8.2 and 5.7 E/m<sup>2</sup>/d, respectively, but overestimation by 1.08 E/m<sup>2</sup>/d (MODIS) and 3.44 E/m<sup>2</sup>/d (AHI). The advantages of using observations from L1 orbit are: 1) the daily cycle of cloudiness is well described (unlike from polar orbit) and 2) spatial resolution is not significantly degraded at high latitudes (unlike from geostationary orbit). The methodology can be easily extended to estimate ultraviolet (UV) surface irradiance using the spectral bands centered on 317, 325, 340, and 388 nm, all the more as ozone content, a key variable controlling atmospheric transmittance, is retrieved from the measurements.
Algorithms to retrieve ocean color from space, deterministic or statistical, often use a simplified water reflectance model, specified by a few parameters (e.g., chlorophyll concentration, backscattering and absorption coefficients at a given wavelength). The model, however, may not be representative of the worldwide ocean conditions, since many variables affecting reflectance are fixed at some average values. In this context, the semi-analytical model of Park and Ruddick (2005), PR05, used in the spectral matching POLYMER algorithm (Steinmetz et al., 2011), is examined in terms of its ability to represent properly water reflectance. The PR05 model depends on chlorophyll-a concentration, a parameter specifying the contribution of algal and non-algal particles to the backscattering coefficient, and a parameter allowing different absorption coefficients for dissolved organic matter. Model estimates at MODIS wavelengths, obtained for a representative set of Case 1 and Case 2 waters, are compared with Hydrolight calculations that include fluorescence and Raman scattering and AERONET-OC measurements. The accuracy of retrieving inherent optical properties (IOPs) using the reconstructed reflectance is also evaluated. The model parameters that give the best fit with the simulated data are determined. Agreement is generally good between the two- or three-parameter model results and Hydrolight/AERONETOC values, even in optically complex waters, with discrepancies much smaller than typical atmospheric correction errors. Significant differences exist in some cases, but having a more intricate model (i.e., using more parameters) might not guarantee convergence of the inversion scheme. The trade-off is between efficiency/robustness and accuracy. Significant errors are observed when using the model estimates to retrieve IOPs. Importantly, the model parameters that best fit the input data, in particular chlorophyll-a concentration, may not represent adequately actual values. The reconstructed water reflectance, not the retrieved model parameters, should be used in bio-optical algorithms.
The goal of the Photosynthetically available radiation (PAR) for Primary Production (3P) project is to provide robust, complete, and user-friendly satellite radiation products for ecosystem modeling, carbon cycle investigations, and climate change monitoring. A specific objective is to design and distribute a daily PAR product from MERIS and potentially the recent OLCI. In view of this, a PAR algorithm, based on the NASA Ocean Biology Processing Group (OBPG) operational algorithm, has been developed. The algorithm takes into account statistical diurnal variability of clouds using 3-hourly International Satellite Cloud Climatology (ISCCP) data. The PAR modeling, simplified to accommodate the information available, is evaluated using a Monte Carlo tool that simulates the satellite radiance and corresponding daily PAR. The daily PAR estimates obtained from reduced resolution (i.e., 1 km) MERIS data are evaluated against in situ measurements routinely collected from fixed buoys and platforms, namely BOUSSOLE in the Mediterranean Sea, CCE- 1 and -2 off the West coast of the United States, and COVE in the coastal Atlantic Ocean. The agreement between estimated and measured values is good on a daily time scale and substantially improved on a monthly time scale, with a bias of 2.7 (7.7%) E/m<sup>2</sup>/day and RMS errors of 8.5 (24.9%) and 4.5 (12.9%) E/m<sup>2</sup>/day. The bias is reduced significantly (by 1.8%) when using diurnal cloud climatology. Overestimation in cloudy conditions is partly explained by decoupling the clear atmosphere from the cloud/surface layer. Large gaps in regions affected by sun glint (not processed because incorrectly interpreted as cloudy) are adequately filled in the monthly PAR imagery. The statistical performance is satisfactory for long-term studies of aquatic primary production, especially in view of the much larger uncertainties on the fraction of PAR absorbed by live algae and the quantum yield of carbon fixation.
Atmospheric correction of ocean-color imagery in the Arctic brings some specific challenges that the standard
atmospheric correction algorithm does not address, namely low solar elevation, high cloud frequency, multi-layered
polar clouds, presence of ice in the field-of-view, and adjacency effects from highly reflecting surfaces covered by
snow and ice and from clouds. The challenges may be addressed using a flexible atmospheric correction algorithm,
referred to as POLYMER (Steinmetz and al., 2011). This algorithm does not use a specific aerosol model, but fits
the atmospheric reflectance by a polynomial with a non spectral term that accounts for any non spectral scattering
(clouds, coarse aerosol mode) or reflection (glitter, whitecaps, small ice surfaces within the instrument field of
view), a spectral term with a law in wavelength to the power -1 (fine aerosol mode), and a spectral term with a law
in wavelength to the power -4 (molecular scattering, adjacency effects from clouds and white surfaces). Tests are
performed on selected MERIS imagery acquired over Arctic Seas. The derived ocean properties, i.e., marine
reflectance and chlorophyll concentration, are compared with those obtained with the standard MEGS algorithm.
The POLYMER estimates are more realistic in regions affected by the ice environment, e.g., chlorophyll
concentration is higher near the ice edge, and spatial coverage is substantially increased. Good retrievals are
obtained in the presence of thin clouds, with ocean-color features exhibiting spatial continuity from clear to cloudy
regions. The POLYMER estimates of marine reflectance agree better with in situ measurements than the MEGS
estimates. Biases are 0.001 or less in magnitude, except at 412 and 443 nm, where they reach 0.005 and 0.002,
respectively, and root-mean-squared difference decreases from 0.006 at 412 nm to less than 0.001 at 620 and 665
nm. A first application to MODIS imagery is presented, revealing that the POLYMER algorithm is robust when
pixels are contaminated by sea ice.
POLDER 3 is a multispectral and multidirectional imaging radiometer/polarimeter, the third instrument of the POLarization and Directionality of Earth Reflectances family. It is designed to collect global images of the earth/atmosphere reflectances with a wide field of view of 1600 km and a moderate spatial resolution of 6 km. It was developed and successfully launched onboard the PARASOL microsatellite by the Centre National d'Etudes Spatiales (the French Space Agency) on December 18, 2004, and the first images were acquired on January 7, 2005. The nominal acquisition phase has started on march 10, 2005 after a successful flight commissioning phase. The specificity of this instrument is to measure polarized reflectances for three out of the 10 spectral channels from 443 to 1020 nm, from 16 viewing directions during a single satellite pass.
The purpose of this paper is to present the preliminary ocean color scientific products: marine diffuse reflectances, amount and type of aerosols derived from the atmospheric correction scheme and concentration of chlorophyll pigments. The level 3 products will be compared to another ocean color instruments, MODIS.
Over land, the Dense Dark Vegetation is used to derive in a first stage the aerosol path radiance and in a second stage to propose an aerosol product which consists of the aerosol type and of the aerosol optical thickness. Air quality monitoring of the particles is based on measurements of PM<sub>10</sub> and PM<sub>2.5</sub> which are respectively the density of particles of diameter lesser than 10μm, lesser than 2.5 μm, at the surface. The satellite aerosol product can be converted into PM<sub>10</sub> and PM<sub>2.5</sub>, based on different assumptions: particle density and vertical distribution mainly. This first attempt to monitor PM from space can be validated with in-situ data. An other approach will simply consist in using the in-situ PM measurements to calibrate the satellite imagery. With the frame of an European project, we generated, over an area centred on Lille (50'36° N, 3'08 E, North of France), a data base with the SeaWiFS archive, and the PM data collected by the regional air quality network. The above technique will be applied and validate using this data base.
We present the results of the first in-flight spectral calibration of the ENVISAT/MERIS instrument. An operational algorithm using the high sensitivity of absorption to the spectral location in the O<sub>2</sub> bands is briefly presented. We show that an accuracy of ± 0.02 nm can be reached with a careful analysis of the whole dataset. This method was successfully compared to other techniques and proved to have the lowest noise. Consequences for the MERIS Surface Pressure product are investigated and optimization of the O<sub>2</sub> bands setting is proposed.
Operational MERIS (MEdium Resolution Imaging Spectrometer) level 2 processing uses auxiliary data generated by two radiative transfer tools. These two codes simulate upwelling radiances within a coupled 'Atmosphere Land' system, using different approaches based on the matrix operator method (FUB), the discrete ordinate method and the successive orders technique (LISE). Intervalidation of these two radiative transfer tools was performed in order to implement them in the MERIS level 2 processing.. An extensive exercise was conducted for cases without gaseous absorption. The scattering processes both by the molecules and the aerosols were retrieved within few tenths of a percent. Nevertheless, some substantial discrepancies occurred if the polarization is not taken into account mainly in the Rayleigh scattering computations. Errors on the aerosol optical depth reach up to 30 percent in some geometries as observed in the SeaWiFS (Sea viewing Wide Field of view Sensor) images. The parameterization of the water vapor absorption defined for each of these two codes leads to a well agreement not only for the MERIS bands with residual absorption but also in the MERIS band centred at 900nm which is used for the water vapor retrieval. As for the strong oxygen absorption at the 760.625 nm MERIS wavelength, its parameterization varies between the two codes. Nevertheless, the systematic biases in the two codes will be removed thanks to the use of a differential method between two MERIS adjacent bands. For the oxygen absorption at 760.625 nm, a more exhaustive study need to be achieved.
The detection of Dense Dark Vegetation (DDV) using the Atmospherically Resistant Vegetation Index (ARVI) and then the aerosol retrieval over DDV is the critical point of the atmospheric correction scheme over land for MERIS implemented in the level 2 processor. We present here what we can expect from the MERIS product by applying a MERIS-like land algorithm to SeaWiFS data over Europe. It is shown that the DDV cover is sufficient in summer but not in winter where an extension of the concept of DDV is needed in order to enable an operational aerosol characterisation. A linear relationship between ARVI and reflectance of the extended DDV in the red should allow the use of such grey targets for the retrieval of aerosol optical properties (aerosol optical thickness at 550 nm and Angström coefficient) throughout the year with a little loss of accuracy
A balloon borne instrument BALLAD (balloon limb aerosol detection) has been developed at the LOA (Laboratoire d'Optique Atmospherique). It scans the Earth's limb at three wavelengths (450, 600, and 850 nm) from the float altitude between 30 - 35 km when the sun is low above the horizon; polarization is also measured in the 850 nm channel. A flight has been performed from the southwest of France on October 13, 1994 during the phase II of the SESAME (Second European Stratospheric Arctic and Mid-latitude Experiment) campaign. An analysis of the reflectances at 850 and 450 nm without polarization is presented.
The Polar Ozone and Aerosol Measurements II (POAM II) has been developed by the Naval Research Laboratory and launched end of September 1993 on the french satellite SPOT 3. The instrument observes solar occultations at 9 wavelength channels. The inversion algorithm allows the retrieval of the extinction vertical profiles and the separation of the species contributing to this extinction: air molecules, aerosols, ozone, water vapor and nitrogen dioxide; furthermore the retrieval of aerosol extinction at 4 wavelengths provides an information on the aerosol size distribution. An inversion algorithm has been developed at the LOA and preliminary comparisons with the NRL algorithm are presented. Two balloon-borne instruments have been flown from Kiruna (Sweden) in coincidence with POAM II observations: one of the instrument (RADIBAL) measures the radiance and polarization diagrams in the near infrared during the balloon ascent; the other instrument (BALLAD) observes the earth's limb at 3 wavelengths from the balloon ceiling altitude. Both instruments are complementary and provide the aerosol profile and the aerosol size distribution. Preliminary comparisons with POAM II data are presented.