This paper compares dust layer heights retrieved from various passive remote sensing algorithm to those detected by an active space lidar, the CALIOP. In particular, authors considered four IR algorithms applied to IASI measurements and two oxygen-band algorithms applied to GOME-2 and SCIAMACHY measurements in the O2 A bands. While aerosol height is the one of the most important variables that determine how aerosol affect climate and air quality, passive remote sensing of aerosol height is extremely more challenging than sensing the aerosol loading. This paper thus address an important study and falls into the scope of AMT journal. I agree with the first reviewer that, while the science and used approaches are sound, this manuscript still needs to improve its presentation of the results and discussion. And, a major revision is necessary to improve the logic and clearness of the paper.

This study presents an application of the Cytosense flow cytometer (CytoBuoy b.v., NL) for the analysis of the optical properties of phytoplankton cells. For the first time, the forward, sideward and backward cross sections (σFSC, σSSC and σbb ) were derived for two species morphologically different (Chlamydomonas concordia and Thalassiosira pseudonana). The objective of this work is to check the validity of the estimates before any applications in the frame of marine optics studies. Thus, estimates of σFSC and σSSC are tested with radiative transfer computations as no in situ measurements are available. A synthetic database is built considering homogeneous, multi-layered spheres, aggregates and cylinders. Scanning electron micrographs were performed to investigate the cell morphology to simulate particles as close as possible to the real cells. This set of numerical results represents a valuable database for many kinds of applications dealing with marine optics. Comparisons showed that the Cytosense estimates for the cultures are consistent with values predicted by the theory. In average, more than 92% of the Cytosense estimates were encompassed by predicted values. The backscattering cross section and the backscattering efficiency were compared with in situ measurements found in the literature wherever possible. Results showed that σbb and Qbb estimations fall within the range of the referenced values.

Abstract. Phytoplankton observation in the ocean can be a challenge in oceanography. Accurate estimations of its biomass and dynamics will help to understand ocean ecosystems and refine global climate models. Relevant data sets of phytoplankton defined at a functional level and on a sub-meso- and daily scale are thus required. In order to achieve this, an automated, high-frequency, dedicated scanning flow cytometer (SFC, Cytobuoy b.v., the Netherlands) has been developed to cover the entire size range of phytoplankton cells whilst simultaneously taking pictures of the largest of them. This cytometer was directly connected to the water inlet of a PocketFerryBox during a cruise in the North Sea, 08–12 May 2011 (DYMAPHY project, INTERREG IV A "2 Seas"), in order to identify the phytoplankton community structure of near surface waters (6 m) with a high spatial resolution basis (2.2 ± 1.8 km). Ten groups of cells, distinguished on the basis of their optical pulse shapes, were described (abundance, size estimate, red fluorescence per unit volume). Abundances varied depending on the hydrological status of the traversed waters, reflecting different stages of the North Sea blooming period. Comparisons between several techniques analysing chlorophyll a and the scanning flow cytometer, using the integrated red fluorescence emitted by each counted cell, showed significant correlations. For the first time, the community structure observed from the automated flow cytometry data set was compared with PHYSAT reflectance anomalies over a daily scale. The number of matchups observed between the SFC automated high-frequency in situ sampling and remote sensing was found to be more than 2 times better than when using traditional water sampling strategies. Significant differences in the phytoplankton community structure within the 2 days for which matchups were available suggest that it is possible to label PHYSAT anomalies using automated flow cytometry to resolve not only dominant groups but also community structure.

Abstract. Phytoplankton observation in the ocean can be a challenge in oceanography. Accurate estimations of their biomass and dynamics will help to understand ocean ecosystems and refine global climate models. This requires relevant datasets of phytoplankton at a functional level and on a daily and sub meso scale. In order to achieve this, an automated, high frequency, dedicated scanning flow cytometer (SFC, Cytobuoy, NL), has been developed to cover the entire size range of phytoplankton cells whilst simultaneously taking pictures of the largest of them. This cytometer was directly connected to the water inlet of a~pocket Ferry Box during a cruise in the North Sea, 8–12 May 2011 (DYMAPHY project, INTERREG IV A "2 Seas"), in order to identify the phytoplankton community structure of near surface waters (6 m) with a high resolution spacial basis (2.2 ± 1.8 km). Ten groups of cells, distinguished on the basis of their optical pulse shapes, were described (abundance, size estimate, red fluorescence per unit volume). Abundances varied depending on the hydrological status of the traversed waters, reflecting different stages of the North Sea blooming period. Comparisons between several techniques analyzing chlorophyll a and the scanning flow cytometer, using the integrated red fluorescence emitted by each counted cell, showed significant correlations. The community structure observed from the automated flow cytometry was compared with the PHYSAT reflectance anomalies over a daily scale. The number of matchups observed between the SFC automated high frequency in situ sampling and the remote sensing was found to be two to three times better than when using traditional water sampling strategies. Significant differences in the phytoplankton community structure within the two days for which matchups were available, suggest that it is possible to label PHYSAT anomalies not only with dominant groups, but at the level of the community structure.

Particulate suspended matter concentration (SPM) were estimated over a 8 year time period (2002-2010) in the coastal waters of French Guiana from a regional algorithm applied to the MODIS monthly reflectance measurements. Comparison between SPOT images and MODIS-SPM maps revealed the strong spatiotemporal coupling between in water SPM and the dynamics of local (i.e. Kourou and Cayenne) mud banks. Highest MODIS SPM values (>13 g m(-3) approximately) can be significantly associated with the subtidal part of the banks as well as to the related turbid plume. The migration of these mud banks induces strong interannual changes in SPM reaching up to 6% year(-1) within increasing and decreasing patchy areas distributed alternatively along the coastline of French Guiana. Mud banks migration rates derived from MODIS SPM data reach in average about 2 km year(-1) in agreement with previous studies. The MODIS time series have allowed for a detailed description of the seasonality and interannual variations in the in-water SPM loads. Seasonal changes in SPM are related to the onset of the trade wind season. Marked non-linear patterns including a sharp evolution in the SPM values around 2005 as well as additional high frequency modulations have been emphasized within the upward and downward SPM trend regions. Concurrent temporal variations in the frequency of northward swells (favoring mud banks migration and reworking) as well as interannual changes in the amount of sediment delivered by the Amazon River have been shown to play a major role in the SPM temporal patterns observed in the French Guiana coastal waters. Our results clearly demonstrate the advantage for ocean color data to describe mud banks dynamics through the assessment of in water SPM temporal variability. (C) 2012 Elsevier Ltd. All rights reserved.

Past years have seen the development of different approaches to detect phytoplankton groups from space. One of these methods, the PHYSAT one, is empirically based on reflectance anomalies. Despite observations in good agreement with in situ measurements, the underlying theoretical explanation of the method is still missing and needed by the ocean color community as it prevents improvements of the methods and characterization of uncertainties on the inversed products. In this study, radiative transfer simulations are used in addition to in situ measurements to understand the organization of the signals used in PHYSAT. Sensitivity analyses are performed to assess the impact of the variability of the following three parameters on the reflectance anomalies: specific phytoplankton absorption, colored dissolved organic matter absorption, and particles backscattering. While the later parameter explains the largest part of the anomalies variability, results show that each group is generally associated with a specific bio-optical environment which should be considered to improve methods of phytoplankton groups detection.

Optical water types were identified from an in situ data set of concomitant biogeochemical and optical parameters collected in contrasted turbid coastal areas of the eastern English Channel, southern North Sea and French Guiana at different seasons (211 stations). Four optical classes have been defined using a clustering approach performed on the spectrally normalized reflectance spectra. Normalization of the reflectance spectra was carried out during the statistical analysis to emphasize the shape of the reflectances rather than their magnitude. Each optical water type is associated with a specific bio-optical environment, in agreement with previous works. Two classes present a very marked optical character, one being mostly determined by strong phytoplankton and dissolved material loads, and the other one by a high proportion of mineral particles. The two remaining classes are related to optically mixed situations although there are some differences in the relative proportion of particulate mineral material. Applying this optical typology to the SeaWiFS daily reflectance data, we emphasized the high representativeness of these 4 optical water types which allow to describe about two thirds of the reflectance spectra found within the development sites whatever the season. The adequacy of optical water type definition for monitoring the spatio-temporal variability of coastal water masses optical quality, which reflects the impact of biological and hydrodynamic processes occurring at different time scales (i.e. from high frequency to seasonal processes), has been demonstrated. The four optical classes' typology has been shown to remain highly representative at global scale. This underlines the effective optical vicinity of some parts of the coastal ocean during some periods of the year, in spite of the recognized high optical diversity of coastal waters. This further demonstrates the high pertinence of class-based approach for large scale coastal applications. Finally, the potential for class-based inversion algorithms for improving ocean color products retrieval, as well as the applicability of such class-specific algorithms with respect to satellite information have been illustrated from the estimation of the suspended matter concentration. This work provides very encouraging evidence of the potential and adequacy of class-based inversion methods for deriving bio-optical products in optically complex waters such as the coastal ocean.

The diffuse attenuation coefficient,Kd(λ) is a fundamental radiometric parameter that is used to assess the light availability in the water column. A neural network approach is developed to assessKd(λ) at any visible wavelengths from the remote sensing reflectances as measured by the SeaWiFS satellite sensor. The neural network (NN) inversion is trained using a combination of simulated and in‐situ data sets covering a broad range ofKd(λ), between 0.0073 m−1at 412 nm and 12.41 m−1at 510 nm. The performance of the retrieval is evaluated against two data sets, one consisting of mainly synthetic data while the other one contains in‐situ data only and is compared to those obtained with previous published empirical (NASA, Morel and Maritorena (2001) and Zhang and Fell (2007)) and semi‐analytical (Lee et al., 2005b) algorithms. On the in‐situ data set from the COASTLOOC campaign, the retrieval accuracy of the present algorithm is quite similar to published algorithms for oligotrophic and mesotrophic ocean waters. But for Kd(490) > 0.25 m−1, the NN approach allows to retrieve Kd(490) with a much better accuracy than the four other methods. The results are consistent when compared with other SeaWiFS wavelengths. This new inversion is as suitable in the open ocean waters as in the turbid waters. The work here is straightforwardly applicable to the MERIS sensor and with few changes to the MODIS‐AQUA sensor. The algorithm in matlab and C code is provided as auxiliary material.

The variability of inherent optical properties is investigated in the ultra-oligotrophic waters of the Mediterranean Sea sampled during the BOUM experiment performed during early summer 2008. Bio-optical relationships found for ultra-oligotrophic waters of the three anticyclonic gyres sampled significantly depart from the mean standard relationships provided for the global ocean, confirming the peculiar character of these Mediterranean waters. These optical anomalies are diversely related to the specific biological and environmental conditions occurring in the studied ecosystem. Specifically, the surface specific phytoplankton absorption coefficient exhibits values lower than those expected from the general relationships mainly in relation with a high contribution of relatively large sized phytoplankton. Conversely, the particulate backscattering coefficient, bbp, values are much higher than the mean standard values for a given chlorophyll-a concentration, TChl-a. This feature can presumably be related to the relevant influence of highly refractive submicrometer particles of Saharan origin in the surface layer of the water column. The present measurements also show that the Mediterranean Sea is greener than TChl-a alone indicates, as already stressed in previous studies. This color anomaly is partly explained by the estimated colored dissolved organic matter and submicrometer particles absorption coefficients, and to a greater extent by the high bbp/TChl-a values assuming that these particles backscatter light similarly in the green and blue parts of the visible spectrum. The diel variation of both the particulate matter attenuation and backscattering coefficients were also investigated specifically. Despite some differences in the timing and the magnitude of the daily oscillations found for these optical parameters, potential for the backscattering coefficient daily oscillation to be used, similarly to that for the attenuation coefficient, as a proxy for estimating carbon community production budget has been highlighted for the first time. This result is particularly relevant for present and future geostationary spatial ocean color missions.

Knowledge of the oceanic particulate organic carbon (POC) pool and of its spatio-temporal variability is important for understanding the oceanic carbon cycle. Until now, POC estimates from space have been restricted to the surface layer. An empirical algorithm is developed to derive the POC content integrated over the euphotic layer (POCZeu) from the near-surface POC concentration (POCsurf). The relationship follows a power-law distribution, POCZeu=A×POCsurfB, with A and B depending on the type of waters (stratified or well-mixed) and on the near-surface chlorophyll concentration. A global climatology of POCsurf and POCZeu is generated from the SeaWiFS archive between 1998 and 2006. The global patterns of POCsurf and POCZeu follow the major gyre systems and other large-scale circulation features. High surface and integrated POC content, around 150mgm−3 and 6000mgm−2, are encountered at high latitude, whereas low content, <50mgm−3 and 2000mgm−2, are observed in subtropical gyres. The mean global values of POCsurf and POCZeu over the global ocean are 53mgm−3 and 3742mgm−2. The standing stocks of POC are 3.92×1014 and 1.19×1015g over the first penetration and euphotic depth, respectively.

The use of satellites to monitor the color of the ocean requires effective removal of the atmosphere signal. The methods for treating the atmosphere have depended on the high absorption of red and near-infrared (NIR) light by water. In coastal waters, the presence of water-leaving radiances in the NIR introduces two sources of error into the removal of the aerosol. As a result, the atmospheric radiances will be overestimated at all bands with increasing severity for shorter wavelengths, even leading to negative radiances in the blue bands in coastal water. This results in severe errors, if not complete failure, of various algorithms for chlorophyll-a concentration and inherent optical properties.

A methodology is presented to estimate aerosol altitude from reflectance ratio measurements in the 02 absorption A-band. Previous studies have shown the impact of the vertical distribution of scatterers on the reflectance ratio. The reflectance ratio is defined as the ratio of the reflectance in a first spectral band, strongly attenuated by 02 absorption, to the reflectance in a second spectral band, minimally attenuated. First, a sensitivity study is performed to quantify the expected accuracy for various aerosol loadings and models. An accurate, high spectral resolution, radiative transfer model that fully accounts for interactions between scattering and absorption is used in the simulations. Due to their adequate spectral characteristics, POLDER and MERIS instruments are considered for simulations. For a moderately loaded atmosphere (i.e., aerosol optical thickness of 0.3 at 760 nm), the expected error on aerosol altitude is about 0.3 km for MERIS and 0.7 km for POLDER. More accurate estimates are obtained with MERIS, since the spectral reflectance ratio is more sensitive. Second, the methodology is applied to MERIS and POLDER imagery. Estimates of aerosol altitude are compared with lidar profiles of backscattering coefficient acquired during the AOPEX-2004 experiment. Retrievals are consistent with measurements and theory. These comparisons demonstrate the potential of the differential absorption methodology for obtaining information on aerosol vertical distribution.

A methodology is presented and evaluated to retrieve vertically integrated water vapor content over the ocean in any viewing geometry from POLDER data. The methodology is based on differential absorption by water vapor in the near‐infrared. Over the ocean, except in sun glint conditions, surface reflectance is small, and interaction between aerosol scattering and water vapor absorption is exploited to estimate total water vapor content. A sensitivity study performed with an accurate radiative transfer code (GAME) shows that a determination of total water vapor content is theoretically possible if the optical thickness and scale height of aerosols are known. An inaccuracy of 0.1 to 0.6 g cm−2 is expected, depending on water vapor content. The aerosol optical thickness δa at 865 nm is available from POLDER standard products. The aerosol scale height Ha can be estimated from the surface pressure derived from POLDER oxygen channels at 763 and 765 nm. A retrieval scheme is devised using parameterizations calculated using GAME for water vapor and surface pressure. The inversion scheme is applied to 20 POLDER orbits with observed water vapor contents ranging from 0.2 to 6 g cm−2. Comparisons with SSM/I estimates indicate a RMS error ranging from 0.21 to 0.58 g cm−2, depending on water vapor content, with a mean RMS error of 0.44 g cm−2. This result is slightly larger than deviations obtained between POLDER and SSM/I during the POLDER standard product validation over ocean in sun glint conditions (mean RMS error of 0.42 g cm−2).

A methodology for water vapor retrieval over ocean, using a differential absorption technique from near-IR channels, is presented. Over ocean, surface reflectance is close to zero and aerosol scattering is used to estimate water vapor contents, from an accurate radiative transfer code to account for optical thickness and scale height of aerosols. This method has been applied to POLDER data and comparisons with ECMWF (European Centre for Medium-Range Weather Forecasts) data are presented over the Straits of Gibraltar.

A method for the in-flight spectral calibration of the oxygen A-band channel of MERIS is presented. This calibration is based on measurements of the atmospheric transmission in the oxygen A-band, using the relation between oxygen transmission and surface pressure. The accuracy of the method has been estimated by using radiative transfer calculations for realistic atmospheric conditions, assuming spectral shifts on the MERIS response. When the calibration is applied over bright surfaces, the expected accuracy of the spectral shift determination is about - 0.01 nm.