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/m2/day and RMS errors of 8.5 (24.9%) and 4.5 (12.9%) E/m2/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 PM10 and PM2.5 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 PM10 and PM2.5, 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.
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
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 O2 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 O2 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.
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.