Operational oceanography requires availability of remotely sensed data, for example sea surface temperature (SST), in near realtime (NRT). A system is presented that makes use of a combination of state of the art NASA Pathfinder SST (PFSST) algorithm and cloud detection procedures both adapted to operate in NRT. A novel cloud detection algorithm that makes use of a reference image based upon recent SST fields recovers data over coastal areas affected by sharp SST fronts that are discarded by the standard PFSST quality flag. The resulting increased SST coverage is visually checked, to remove residual cloud contamination, by a trained analyst prior to input to the objective analysis package in turn adapted for satellite-derived SST observations. The output daily gridded data in which the gaps due to clouds have been filled by the optimal interpolation module are assimilated into the Mediterranean Forecasting System Toward Environmental prediction (MFSTEP) ocean forecasting system on a weekly basis.
The present work develops in the framework of the EU-ADIOS project for providing an estimate of the seasonal occurrence of Saharan dust events over the Mediterranean Sea. SeaDAS True Color (TC) images are used for monitoring dust.
Dust identification has been carried out by visual inspection of TC images from 1998 up to 2002. The presence of dust, easily recognizable because of its color, is catalogued day by day. A grid representing a schematic division of the Mediterranean basin is overlaid to the TC images for a better identification and localization. The information retrieved by the cataloguing concerns space-time distribution of dust throughout the years examined.
It is not possible to retrieve the intensity of the event by looking only at the TC images. Monthly values of Saharan dust load observed for each sector of the Mediterranean Basin for the period 2001 to 2002 have been retrieved from the Aerosol Optical Thickness maps for each dust day identified by the previous analysis.
This is given in order to describe a spatial-temporal variability of the aerosol content. A brief description of the dataset, of the processing methodology and of the geophysical atmospheric products is also given.
Primary Production (PP) has been estimated in the Mediterranean Sea from SeaWiFS data for the years 1998-2001 adapting a model developed by Antoine et al. (1996) to Mediterranean conditions. The tuning is based on the use of a new large data set of chlorophyll profiles acquired during the last 10 years in the Mediterranean Sea. Moreover a new data set of cloud cover and temperature has been used and SeaWiFS derived chlorophyll has been estimated using a Mediterranean algorithm recently proposed by D’Ortenzio et al. (2002). Comparison between present model estimates, previous models estimates and in situ data gives satisfactory results and show that the adapted model better reproduces the PP seasonal trend of Mediterranean Sea. The application of the procedure to each pixel of single SeaWiFS images allows the creation of daily, weekly and monthly averaged estimates of depth-integrated PP.
KEYWORDS: Digital signal processing, Data modeling, Satellites, Image processing, Data acquisition, Satellite imaging, Data conversion, Data archive systems, Climatology, Meteorology
The Satellite Oceanography Group (GOS) of Rome developed a system that provides satellite ocean colour images and data on the web. This meets the growing demand for near real-time ocean colour products for applications in operational oceanography. The system has been developed to produce: 1) fast delivery images for monitoring
applications and operational support on oceanographic cruises; 2) accurate ocean colour products for data assimilation on ecosystem model. Real Time Images of SeaWiFS chlorophyll concentration, clouds/case I/case II water flags and true color images are obtained by processing the satellite passes using climatological ancillary data. These images are provided daily through an ad hoc automatic procedure that processes the raw satellite data and makes it available on the web within an hour after the acquisition. All the images are stored in a gallery archive organized in a calendar chart for the selection of the images to display. On the opposite, accurate chlorophyll maps for assimilation in numerical models are produced in near real time (typically after 4 days) as soon as daily meteorological ancillary data are made available on the NASA website. Each chlorophyll map is flagged for clouds or other contamination factors using the corresponding 24 quality flag maps. This implies that case-2 waters and possible contaminations of chlorophyll have
been implicitly removed. This final product is binned on Adriatic model grid and made available to ADRICOSM project on GOS web site. These daily chlorophyll maps are assimilated by ADRICOSM modeling group to provide the forecasting of Adriatic ecosystem.
Different methods for the extrapolation of vertical profiles from sea surface measurements are presented. The techniques, called Coupled Pattern Reconstruction (CPR) and multivariate EOF Reconstruction (mEOF-R) base on the multivariate analysis of the variability of vertical profiles from hydrographic data and on the hypothesis that only few modes are needed to explain most of the variance/covariance of the fields. The CPR and mEOF-R methods have been applied and tested on the data collected during SYMPLEX survey in a selected area of the Mediterranean sea (the Channel of Sicily), also using simultaneous altimeter measurements as surface input.
Many studies indicate that the atmosphere is a significant and in some cases the dominant pathway by which specific elements are transported from the land to the open sea. The Mediterranean Sea is a semi-enclosed basin, that continuously receives anthropogenic substances from the industrialized European country, and sporadically, from the arid region of the Sahara desert, nearly the 90% of the total amount of aerosols that reach the sea surface. The Mediterranean is a predominantly oligotrophic basin with areas of high productivity limited to areas influenced by runoff, rivers or upwelling. In situ biogeochemical measurements indicate that atmospheric deposition can induce significant productivity changes. The present work aims to use SeaWiFS satellite data and the SKIRON atmospheric model to provide an estimate of the temporal and spatial variability in the atmospheric forcing (dust events) and in the marine biological response (blooms), and to evaluate the overall contribution of these Saharan dust events to the fertility of the Mediterranean Sea. Although biological dynamic is meanly driven by the circulation features of the basin, results show that the atmospheric nutrient deposition gives some evident response in the biological activity.
Remote sensing instruments provide a huge volume of measurements of sea surface parameters which are of fundamental interest to the oceanographic community, mainly because of the high spatial and temporal coverage of satellite sensors respect to traditional techniques. In fact, the whole ocean circulation is actually driven by processes occurring at the air-sea interface, as the energy is mainly supplied to the ocean by atmosphere-ocean interactions. Consequently, altimeter derived SSH and SST obtained from AVHRR, that have already proved to be quit useful to evaluate the sea surface variability, can be though t as possible tools to investigate the ocean dynamics and atmosphere-ocean interaction more deeply. In this work, 2 years of Mediterranean Forecasting System Pilot Project dat over the Mediterranean basin are analyzed. The relationship between SST and SLA is investigated applying an objective method for coupled pattern detection, based on the singular value decomposition of the covariance of the two data-sets.
A Mediterranean data set containing coincident in situ chlorophyll and Sea WiFS remote sensing reflectance was used to evaluate the performances of several ocean color empirical algorithms. The validation of the algorithms is based on a match up file of 63 coincident in situ and Sea WiFS measurements selected on the basis of a sequence of mask criteria that prevents the use of contaminated pixels. All the algorithms calibrated to global or non-Mediterranean data sets overestimate the chlorophyll concentration in the Mediterranean Sea for low values of chlorophyll-a concentration. A total pigment concentration algorithm based on a regression with Mediterranean data, proposed by Gitelson et al. For CZCS, gives good results for low chlorophyll concentrations but exhibits a scatter larger than OC2 for concentrations higher than 0.2 mg/m3. In this paper we propose a preliminary version of a new Mediterranean ocean color algorithm based on the OC2 functional form with coefficients estimated from a regression with our Mediterranean in situ data.
An empirical method to estimate the solar incoming radiation has been improved using AVHRR-NOAA14 data in the framework of the ARTIST experiment. This method, using several bulk formulas, gives an estimation of the incoming solar radiation and results are compared with the surface-based and aircraft-based measurements. The first step of the method is the development of a cloud detection algorithm for the area and for the period of interest. The area is the Arctic Ocean near the Svalbard Islands and the period is from 15 March to 16 April 1998, the transition period form winter to sprint and in this particular case from the Polar Night to a period of quite normal insolation. Several bulk formulas, right for the Polar condition, are tested comparing the estimate insolation fluxes with the direct measurements made with a pyranometer mounted aboard the Polar2 aircraft and another pyranometer at the research base at Ny-Alesund. The comparison could define the best parameterization for the available data.
A new method to estimate radiation budgets at air-sea interface by means of AVHRR data has been developed and tested in the framework of the Arctic Radiation and Turbulence Interaction Study (ARTIST). Main goal of the ARTIST project is assess the effects of clouds and of Arctic Haze on the radiative fluxes at the surface and in the atmospheric column in the European Arctic. One month of Advanced Very High Resolution Radiometer (AVHRR) data relative to the period March - April 1998 has been processed and analyzed in order to evaluate short and long wave radiation budgets in the Arctic Sea during the experiment. Remote sensing data (NOAA 14 satellite) have been acquired at the Tromso station and then processed at the Istituto Fisica dell'Atmosfera (IFA) to produce maps of surface albedo and brightness temperature of the experiment's zone. These maps were used to develop a new cloud detection algorithm for the region. Image pixels then have been classified as ice, clouds or water. The method was applied to 151 available AVHRR scenes. The pixels classification performance was verified against the analysis of an expert in satellite image. The cloud classification results to be quite accurate. In fact 99 images are classified as 'very good' by the expert and 37 images the accuracy is a little lower. The radiation budgets are then estimated using several available empirical formulae for clear sky and overcast conditions. The results were compared with in situ measurements made during the ARTIST experiment in order to define the best parameterization of the fluxes. The best estimates of shortwave incoming radiation results from the Bennet (1982) formula for clear sky condition with the Laevastu (1960) correction for overcast condition. The more accurate estimate of incoming long wave radiation in clear sky has been obtained using the Swinbank (1963) parameterization. Finally, averaged map of total radiation budget are calculated for the time period of the ARTIST experiment in the Arctic region.
The monitoring of organic matter, suspended or dissolved in the water column, is relevant for the study of the aquatic environment. Actually, the Dissolved Organic Matter (DOM) represents a major reservoir of reactive carbon in the global carbon cycle, thus influencing significantly the marine ecosystem. Due to the strong absorption in the near ultraviolet, DOM reduces considerably the extinction path of solar light in the water column, affecting phytoplankton population and its vertical distribution. The measurement of the DOM absorption coefficient has to be regarded as a good parameter for the monitoring of water quality. This paper deals with the measurements carried out during the oceanographic campaign 'Marine Fronts,' which took place in the Western Mediterranean Sea and Atlantic Ocean from July 14 to August 5, 1998. In this measurement campaign, a high spectral resolution fluorescence lidar (FLIDAR) was installed on the rear-deck of the O/V 'Urania,' acquiring remote fluorescence spectra both in ship motion and in stations. A particular attention was devoted to the monitoring of DOM distribution in the different water masses in marine frontal areas. The lidar data were compared and integrated with SST satellite data and biological samplings. The results show that FLIDAR data agree with satellite imagery, particularly for marine front detection. The comparison with water sample data gave indications for retrieving the DOM absorption coefficient directly from fluorescence remote spectra. In addition, a protein like fluorescence band was detected in the measurements carried out on total suspended matter filtered from the water samplings.
Advanced Very High Resolution Radiometer (AVHRR) and Sea viewing Wide Field of view Sensor (SeaWiFS) data from January 1998 to June 1999 are used to examine spatial and temporal variability of sea surface temperature (SST) and apparent chlorophyll (AChl) in the Adriatic Sea. Flows long the Albanian coast and the Italian can be distinguished year-round in the monthly averaged AChl, but only in the colder months in the monthly averaged SST's. The AChl averaged fields supply less information on circulation features away from coastal boundaries and where conditions are generally oligotrophic except for the early spring bloom in the Southern Adriatic Gyre. The winter-spring SST and chlorophyll distributions are very different between the two years, particularly in the Northern Adriatic shelf and the Southern Adriatic Gyre. It is hypothesized that this difference may be related to dense water formation that occurs only in the northern and southern Adriatic Seas. The time series of daily SST indicate that dense water formation was favored in 1999 by episodes of cooler winter temperatures in the southern gyre (less than 13.5 degrees Celsius) and on the northern Adriatic shelf. Blooms in 1999 may have been delayed due to surface replacement flows driven by sinking of dense water.
Upper ocean dynamics is characterized by a strong variability, at different scales, both in direction and structure of the flow. Mesoscale variability, which is ubiquitous in the world ocean, is often the dominant component in the variance spectrum of velocity with relevant implications on water mass mixing and transformation and on the carbon transfer in the marine food web. Mesoscale activity is manifested through the formation of instabilities, meanders and eddies. Eddies generate either a doming of isopycnals (cyclones) or a central depression (anticyclones). This in turn modifies, among the others, nutrient and organism distributions in the photic zone eventually enhancing or depressing photosynthetic activity and other connected biological responses. The mechanism is similar to what has been thoroughly studied for the warm and cold core rings but at different spatial and temporal scales. The enhancement of phytoplankton growth and the modification of photosynthetic parameters has been shown to occur in situ by means of a modulated fluorescence probe. More recently, an attempt to estimate the magnitude of this specific forcing on nutrient fluxes and primary production has also been conducted at different scales by modeling exercises, though with contrasting estimates the relative importance concerns. Because phytoplankton growth takes place when light, nutrients and cells are found at the same place, the increase in primary production favored by mesoscale eddies cannot be easily predicted. The incident light, the seasonality, the life-time of the structure, its intensity etc. can all influence the final yield. In addition, it has still to be determined which component of the community reacts faster and takes advantage of the new nutrients and how efficiently the new carbon is channeled in the food web. For what remote sensing is concerned, the detectability form the space of such structures is certainly dependent on the depth at which the upward distortion of isopycnals takes places. It can be supposed that a change in bio-optical signature of the whole structure could occur because of the 3-D dynamics of the eddy. If this holds true, then color remote sensing coupled with sea level topography and sea surface temperature should be a powerful tool to track such transient structures. The ALT-SYMPLEX program has been designed to better understand the relationship between short living eddies and carbon transfer in the food web. This is based on several experiments aimed to integrate remote sensing data (ocean color and surface topography) and in situ data in order to evaluate the relationship between surface and sub-surface physical dynamics and its relations on chemical and biological aspects in presence of mesoscale features.
OGCM's numerical experiments for climatic studies can be performed only after testing their capability to well represent the today ocean circulation. In order to improve the model results we assimilated satellite SST (from NOAA-AVHRR sensors) by means of a nudging method. Daily 1987 - 1992 NASA Pathfinder SST have been interpolated in time and space by means of an objective analysis algorithm to fill gaps in the time series. These data were analyzed to investigate the main variability of the Mediterranean SST field. The 1988 data was selected to be assimilated in the OGCM. To relate more closely the assimilation of the SST data to the physics of the air-sea interaction, here we propose a new parameterization of the surface relaxation time. Actually the use of realistic surface forcing strongly contribute to reproduce the correct hydrological values of the Mediterranean water masses, the right energetic basin budgets and the mesoscale features of the Mediterranean circulation.
TOPEX/Poseidon and ERS-1 altimeter data have successfully been used to study the mesoscale field in the Mediterranean Sea and to investigate the seasonal variability of the Sea Level and eddy statistics in this basin. The comparison between the two altimeter performance is very good and underline importance to combine the information of TOPEX/Poseidon and ERS-1 to study the Mediterranean mesoscale eddy field. Sea level anomalies maps every five days were produced using the both data sets by means of sub-optimal interpolation. A comparison between mesoscale features detected by the two altimeters and contemporaneous features observed using Sea Surface Temperature maps definitively proves the direct relation between sea level anomalies and the Mediterranean eddy field. The data collected ERS-SYMPLEX cruise have been used to verify the performance of ERS-1/2 and TOPEX/POSEIDON altimeters in the Mediterranean Sea. The ERS-SYMPLEX experiment has been carried out in the channel of Sicily during spring 1996 in order to make a direct comparison between SLA from ERS1/2 and TOPEX/POSEIDON altimeters and in situ data. During the cruise CTD and XBT casts have been done in the central and eastern regions of the channel, and particularly densely along selected altimeters' tracks at the same time of the satellite pass. Sea level anomalies have been computed and compared with the dynamic heights and with the main circulation features resulting from SYMPLEX cruise. The results definitively prove the capability of the two altimeters to correctly detect both basin and mesoscale features of the Mediterranean circulation. Collinear analysis along a repeated T/P track has also suggested potential use of SLA in order to detect barotropic component of currents in the channel of Sicily.
Seasonal and interannual variability of the sea surface temperature field in the Adriatic Sea is analyzed using a five years time series of daily AVHRR (advanced very high resolution radiometer) data. The data used are the 9 km resolution sea surface temperature maps obtained in the framework of the Pathfinder project. This spatial resolution allowed analysis of only basin and sub-basin scale features. Average monthly sea surface temperature maps for the entire studied period (1987 - 1991) are discussed. The analysis shows the absence of any permanent surface feature in the Adriatic Sea. The cyclonic circulation of the south Adriatic Sea is marked by a surface temperature minimum that recurrently appears in late autumn or early winter, i.e. in the preconditioning and deep water formation phase. During autumn the surface inflow of Ionian water is observed while the Adriatic outflow is more evident in winter. Results of an EOF analysis suggest that the intensity of the western coastal cold water plume and the north Adriatic thermal front vary from year to year. Other sub-basin features like the south Adriatic gyre also display year-to-year variability.
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