ABSTRACT
In this study, we present a radiative transfer model, so-called OSOAA, that is able to predict the radiance and degree of polarization within the coupled atmosphere-ocean system in the presence of a rough sea surface. The OSOAA model solves the radiative transfer equation using the successive orders of scattering method. Comparisons with another operational radiative transfer model showed a satisfactory agreement within 0.8%. The OSOAA model has been designed with a graphical user interface to make it user friendly for the community. The radiance and degree of polarization are provided at any level, from the top of atmosphere to the ocean bottom. An application of the OSOAA model is carried out to quantify the directional variations of the water leaving reflectance and degree of polarization for phytoplankton and mineral-like dominated waters. The difference between the water leaving reflectance at a given geometry and that obtained for the nadir direction could reach 40%, thus questioning the Lambertian assumption of the sea surface that is used by inverse satellite algorithms dedicated to multi-angular sensors. It is shown as well that the directional features of the water leaving reflectance are weakly dependent on wind speed. The quantification of the directional variations of the water leaving reflectance obtained in this study should help to correctly exploit the satellite data that will be acquired by the current or forthcoming multi-angular satellite sensors.
ABSTRACT
Since 18 December 2004, the PARASOL satellite is a member of the so-called A-train atmospheric orbital observatory, flying together with Aqua, Aura, CALIPSO, CLOUDSAT, and OCO satellites. These satellites combine for the first time a full suite of instruments for observing aerosols and clouds, using passive radiometer complementarily with active lidar and radar sounders. The PARASOL payload is extensively derived from the instrument developed for the POLDER programs that performs measurements of bidirectionality and polarization for a very wide field-of-view and for a visible/near-infrared spectral range. An overview of the results obtained during the commissioning phase and the reevaluation after one year in orbit is presented. In-flight calibration methods are briefly described, and radiometric and geometric performances are both evaluated. All algorithms are based on a panel of methods using mainly natural targets previously developed for POLDER missions and adapted or redeveloped in the PARASOL context. Regarding performances, all mission requirements are met except for band 443 (not recommended for use). After one year in orbit, a perfect geometrical stability was found while a slight decrease of the radiometric sensitivity was observed and corrected through an innovative multitemporal algorithm based on observations of bright and scattered convective clouds. The scientific exploitation of PARASOL has now begun, particularly by coupling these specific observations with other A-train sensor measurements.
ABSTRACT
A hand-held radiometer, called SIMBAD, has been designed and built specifically for evaluating satellite-derived ocean color. It provides information on the basic ocean-color variables, namely aerosol optical thickness and marine reflectance, in five spectral bands centered at 443, 490, 560, 670, and 870 nm. Aerosol optical thickness is obtained by viewing the Sun disk and measuring the direct atmospheric transmittance. Marine reflectance is obtained by viewing the ocean surface and measuring the upwelling radiance through a vertical polarizer in a geometry that minimizes glitter and reflected sky radiation, i.e., at 45 degrees from nadir (near the Brewster angle) and at 135 degrees in azimuth from the Sun's principal plane. Relative inaccuracy on marine reflectance, established theoretically, is approximately 6% at 443 and 490 nm, 8% at 560 nm, and 23% at 670 nm for case 1 waters containing 0.1 mg m(-3) of chlorophyll a. Measurements by SIMBAD and other instruments during the Second Aerosol Characterization Experiment, the Aerosols-99 Experiment, and the California Cooperative Oceanic Fisheries Investigations cruises agree within uncertainties. The radiometer is compact, light, and easy to operate at sea. The measurement protocol is simple, allowing en route measurements from ships of opportunity (research vessels and merchant ships) traveling the world's oceans.
