Samantha Lavender

Spectral signature of highly turbid waters: Application with SPOT data to quantify suspended particulate matter concentrations

An experimental method for determining water composition from ‘‘ocean colour’’ satellite data, in visible and near-infrared (NIR) wavelengths, is applied to highly turbid waters. Numerous spectroradiometric measurements are carried out in the Gironde estuary, for suspended particulate matter (SPM) concentrations ranging between 35 and more than 2000 mg l � 1 . Empirical relationships are established between remote-sensing reflectance (Rrs) in SPOT-HRV bands and SPM concentration through these numerous in situ measurements. We observed that remote-sensing reflectance increases with SPM concentration and that the SPOT bands saturate at the highest turbidities. The best correlations are obtained for the NIR band XS3 (790–890 nm) and for the reflectance ratios: Rrs(XS3)/Rrs(XS1) and Rrs(XS3)/Rrs(XS2). The XS1 and XS2 visible bands are only used to determine SPM concentrations in the lower part of the estuary (where the SPM concentrations are lower). As a result, SPM concentrations within the surface waters in the estuary are estimated up to 2000 mg l � 1 with an accuracy better than ±35%. The algorithm is finally applied to a SPOT scene. Satellite data are corrected for atmospheric effects using a radiative transfer code and in situ reflectance measurements; as a result, the horizontal distribution of SPM is retrieved. Moreover, the high spatial resolution HRV-SPOT sensor shows detailed sedimentary flows, especially in the visible XS1 and XS2 spectral bands. D 2002 Elsevier Science Inc. All rights reserved.

Importance of stirring in the development of an iron-fertilized phytoplankton bloom

The growth of populations is known to be influenced by dispersal, which has often been described as purely diffusive. In the open ocean, however, the tendrils and filaments of phytoplankton populations provide evidence for dispersal by stirring. Despite the apparent importance of horizontal stirring for plankton ecology, this process remains poorly characterized. Here we investigate the development of a discrete phytoplankton bloom, which was initiated by the iron fertilization of a patch of water (7 km in diameter) in the Southern Ocean. Satellite images show a striking, 150-km-long bloom near the experimental site, six weeks after the initial fertilization. We argue that the ribbon-like bloom was produced from the fertilized patch through stirring, growth and diffusion, and we derive an estimate of the stirring rate. In this case, stirring acts as an important control on bloom development, mixing phytoplankton and iron out of the patch, but also entraining silicate. This may have prevented the onset of silicate limitation, and so allowed the bloom to continue for as long as there was sufficient iron. Stirring in the ocean is likely to be variable, so blooms that are initially similar may develop very differently.

The atmospheric correction of water colour and the quantitative retrieval of suspended particulate matter in Case II waters: Application to MERIS

The remote sensing of turbid waters (Case II) using the Medium Resolution Imaging Spectrometer (MERIS) requires new approaches for atmospheric correction of the data. Unlike the open ocean (Case I waters) there are no wavelengths where the water-leaving radiance is zero. A coupled hydrological atmospheric model is described here. The model solves the water-leaving radiance and atmospheric path radiance in the near-infrared (NIR) over Case II turbid waters. The theoretical basis of this model is described, together with its place in the proposed MERIS processing architecture. Flagging procedures are presented that allow seamless correction of both Case I waters, using conventional models, and Case II waters using the proposed model. Preliminary validation of the model over turbid waters in the Humber estuary, UK is presented using Compact Airborne Spectrographic Imager (CASI) imagery to simulate the MERIS satellite sensor. The results presented show that the atmospheric correction scheme has superior perfo...

Sun Glint Correction of High and Low Spatial Resolution Images of Aquatic Scenes: a Review of Methods for Visible and Near-Infrared Wavelengths

Sun glint, the specular reflection of light from water surfaces, is a serious confounding factor for remote sensing of water column properties and benthos. This paper reviews current techniques to estimate and remove the glint radiance component from imagery. Methods for processing of ocean color images use statistical sea surface models to predict the glint from the sun and sensor positions and wind data. Methods for higher resolution imaging, used in coastal and shallow water mapping, estimate the glint radiance from the near-infrared signal. The effects of some current methods are demonstrated and possibilities for future techniques are briefly addressed.

Generalized ocean color inversion model for retrieving marine inherent optical properties

Ocean color measured from satellites provides daily, global estimates of marine inherent optical properties (IOPs). Semi-analytical algorithms (SAAs) provide one mechanism for inverting the color of the water observed by the satellite into IOPs. While numerous SAAs exist, most are similarly constructed and few are appropriately parameterized for all water masses for all seasons. To initiate community-wide discussion of these limitations, NASA organized two workshops that deconstructed SAAs to identify similarities and uniqueness and to progress toward consensus on a unified SAA. This effort resulted in the development of the generalized IOP (GIOP) model software that allows for the construction of different SAAs at runtime by selection from an assortment of model parameterizations. As such, GIOP permits isolation and evaluation of specific modeling assumptions, construction of SAAs, development of regionally tuned SAAs, and execution of ensemble inversion modeling. Working groups associated with the workshops proposed a preliminary default configuration for GIOP (GIOP-DC), with alternative model parameterizations and features defined for subsequent evaluation. In this paper, we: (1) describe the theoretical basis of GIOP; (2) present GIOP-DC and verify its comparable performance to other popular SAAs using both in situ and synthetic data sets; and, (3) quantify the sensitivities of their output to their parameterization. We use the latter to develop a hierarchical sensitivity of SAAs to various model parameterizations, to identify components of SAAs that merit focus in future research, and to provide material for discussion on algorithm uncertainties and future emsemble applications.

Diurnal warm‐layer events in the western Mediterranean and European shelf seas

We characterize near-surface ocean diurnal warm-layer events, using satellite observations and fields from numerical weather forecasting. The study covers April to September, 2006, over the area 11 degrees W to 17 degrees E and 35 degrees N to 57 degrees N, with 0.1 degrees cells. We use hourly satellite SSTs from which peak amplitudes of diurnal cycles in SST (dSSTs) can be estimated with error similar to 0.3 K. The diurnal excursions of SST observed are spatially and temporally coherent. The largest dSSTs exceed 6 K, affect 0.01% of the surface, and are seen in the Mediterranean, North and Irish Seas. There is an anti-correlation between the magnitude and the horizontal length scale of dSST events. Events wherein dSST exceeds 4 K have length scales of = 40 km. From the frequency distribution of different measures of wind-speed minima, we infer that extreme dSST maxima arise where conditions of low wind speed are sustained from early morning to mid afternoon.

SMOS first data analysis for sea surface salinity determination

Soil Moisture and Ocean Salinity (SMOS), launched on 2 November 2009, is the first satellite mission addressing sea surface salinity (SSS) measurement from space. Its unique payload is the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS), a new two-dimensional interferometer designed by the European Space Agency (ESA) and operating at the L-band frequency. This article presents a summary of SSS retrieval from SMOS observations and shows initial results obtained one year after launch. These results are encouraging, but also indicate that further improvements at various data processing levels are needed and hence are currently under investigation.

Validation of MERIS reflectance and chlorophyll during the BENCAL cruise October 2002: preliminary validation of new demonstration products for phytoplankton functional types and photosynthetic parameters

We measured water leaving reflectance, phytoplankton pigments, optical properties and photosynthetic parameters in the southern Benguela ecosystem in October 2002. These data were used to validate MERIS standard products: reflectance (MERIS wavelengths) and Case 1 Chlorophyll‐a. In this heterogeneous area, accurate validation required sampling within a few minutes of the satellite overpass. Inter‐pigment relationships e.g. Total Chlorophyll (TChla) to Total Pigment (TP) were robust (R2∼0.99) yet pigment ratios (TChla/TP) were not constant (range 0.44 to 0.62) increasing log‐linearly with biomass (R2∼0.7). Photosynthetic parameters (e.g. Photosynthetic Quantum Efficiency, PQE) and optical ratios (a676/a440) also increased log‐linearly with biomass (R2∼0.8). PQE, pigment and optical ratios were linearly inter‐correlated (R2∼0.7 to 0.8). From these data we derived the bio‐optical traits for several phytoplankton functional types (PFTs): micro‐plankton (diatoms and dinoflagellates) had high biomass, pigment ratios and PQE; nano‐flagellates had low to intermediate biomass, pigment ratios and PQE; prokaryotes had very low biomass, pigment ratios and PQE. We present MERIS data analysed for PFTs and new products (PQE).

Extending the SeaWiFS chlorophyll data set back 50 years in the northeast Atlantic

Phytoplankton play a key role in biogeochemical cycling and climate processes. Precise quantitative measurements of chlorophyll-a (Chl-a), a measure of phytoplankton biomass, have only been available globally since 1997 from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). In the North Atlantic, semi-quantitative measurements of chlorophyll ( Phytoplankton Color Index, PCI) for > 50 years have been collected by the Continuous Plankton Recorder. Here we demonstrate a significant correlation between PCI and SeaWiFS Chl-a from 1997-2002. Combining both time series allows quantification of the stepwise increase in biomass in the mid-1980s; this regime shift corresponded to a 60% increase in Chl-a. This was a result of an 80% increase in Chl-a during winter, alongside a smaller summer increase. This new high-resolution data set on the monthly variation of Chl-a in the North Atlantic since 1948 is now available for the development and validation of climate models, and for interpretation of ecological changes related to climate.

Remote sensing of sea surface temperature and chlorophyll during Lagrangian experiments at the Iberian margin

Satellite derived sea surface temperature (SST) and chlorophyll data are used to characterise the period of upwelling during a cruise on which two Lagrangian drift experiments were conducted off the Iberian Atlantic coast in August 1998. During the cruise there was a prolonged period of equator-ward winds which favour upwelling; three distinct maxima were observed in the meteorological data interspersed with periods of relaxation. The SST and chlorophyll imagery show upwelling to be active with distinct offshore filaments that are cooler and of higher chlorophyll concentration than the surrounding oceanic water; these filaments represent an important cross-shelf transport mechanism. A front detection methodology has been applied to satellite images and suggests that these filaments are distinct, long-lived features, characterised by enhanced primary production.