46 research outputs found
The Mediterranean Moored Multi-sensor Array (M3A): system development and initial results
International audienceOperational forecasting of ocean circulation and marine ecosystem fluctuations requires multi-parametric real-time measurements of physical and biochemical properties. The architecture of a system that is able to provide such measurements from the upper-thermocline layers of the Mediterranean Sea is described here. The system was developed for the needs of the Mediterranean Forecasting System and incorporates state-of-the-art sensors for optical and chemical measurements in the upper 100 m and physical measurements down to 500 m. Independent moorings that communicate via hydro-acoustic modems are hosting the sensors. The satellite data transfer and the large autonomy allow for the operation of the system in any open-ocean site. The system has been in pre-operational use in the Cretan Sea since January 2000. The results of this pilot phase indicate that multi-parametric real-time observations with the M3A system are feasible, if a consistent maintenance and re-calibration program is followed. The main limitations of the present configuration of M3A are related: (a) to bio-fouling that primarily affects the turbidity and secondarily affects the other optical sensors, and (b) to the limited throughput of the currently used satellite communication system. Key words. Atmospheric composition and structure (instruments and techniques.) Oceanography: general (ocean prediction) Oceanography: physical (upper ocean process
DATA BASE DEVELOPMENT AND EVALUATION OF EARTHQUAKE DAMAGE REPORTS UNDER THE SEISIMPACT-THES SYSTEM
The Permanent Facility for Satellite Altimetry Calibration in Gavdos/Crete, Greece: Fifteen years of Cal/Val Service.
Development of a Synthetic Earth Gravity Model by 3D mass optimisation based on forward modelling
Several previous Synthetic Earth Gravity Model (SEGM) simulations are based on existing information about the Earth’s internal mass distribution. However, currently available information is insufficient to model the Earth’s anomalous gravity field on a global scale. The low-frequency information is missing when modelling only topography, bathymetry and crust (including the Mohorovičić discontinuity), but the inclusion of information on the mantle and core does not seem to significantly improve this situation. This paper presents a method to determine a more realistic SEGM by considering simulated 3D mass distributions within the upper mantle as a proxy for all unmodelled masses within the Earth.The aim is to improve an initial SEGM based on forward gravity modelling of the topography, bathymetry and crust such that the missing low-frequency information is now included. The simulated 3D mass distribution has been derived through an interactive and iterative mass model optimisation algorithm, which minimises geoid height differences with respect to a degree-360 spherical harmonic expansion of the EGM2008 global external gravity field model. We present the developed optimisation algorithm by applying it to the development of a global SEGM that gives a reasonably close fit to EGM2008, and certainly closer than a SEGM based only on the topography, bathymetry and crust
A Synthetic Earth Gravity Model Designed Specifically for Testing Regional Gravimetric Geoid Determination Algorithms
A synthetic [simulated] Earth gravity model (SEGM) of the geoid, gravity and topography has been constructed over Australia specifically for validating regional gravimetric geoid determination theories, techniques and computer software. This regional high-resolution (1-arc-min by 1-arc-min) Australian SEGM (AusSEGM) is a combined source and effect model. The long-wavelength effect part (up to and including spherical harmonic degree and order 360) is taken from an assumed errorless EGM96 global geopotential model. Using forward modelling via numerical Newtonian integration, the short-wavelength source part is computed from a high-resolution (3-arc-sec by 3-arc-sec) synthetic digital elevation model (SDEM), which is a fractal surface based on the GLOBE v1 DEM. All topographic masses are modelled with a constant mass-density of 2,670 kg/m3. Based on these input data, gravity values on the synthetic topography (on a grid and at arbitrarily distributed discrete points) and consistent geoidal heights at regular 1-arc-min geographical grid nodes have been computed. The precision of the synthetic gravity and geoid data (after a first iteration) is estimated to be better than 30 μ Gal and 3 mm, respectively, which reduces to 1 μ Gal and 1 mm after a second iteration.The second iteration accounts for the changes in the geoid due to the superposed synthetic topographic mass distribution. The first iteration of AusSEGM is compared with Australian gravity and GPS-levelling data to verify that it gives a realistic representation of the Earth’s gravity field. As a by-product of this comparison, AusSEGM gives further evidence of the north–south-trending error in the Australian Height Datum. The freely available AusSEGM-derived gravity and SDEM data, included as Electronic Supplementary Material (ESM) with this paper, can be used to compute a geoid model that, if correct, will agree to in 3 mm with the AusSEGM geoidal heights, thus offering independent verification of theories and numerical techniques used for regional geoid modelling
A study on the evaluation of the geoid-quasigeoid separation term over Pakistan with a solution of first and second order height terms
Geoid and High Resolution Sea Surface Topography Modelling in the Mediterranean from Gravimetry, Altimetry and GOCE Data: Evaluation by Simulation
Abstract The determination of local geoid models has
traditionally been carried out on land and at sea using gravity
anomaly and satellite altimetry data, while it will be
aided by the data expected from satellite missions such as
those from the Gravity field and steady-state ocean circulation
explorer (GOCE). To assess the performance of heterogeneous
data combination to local geoid determination,
simulated data for the central Mediterranean Sea are analyzed.
These data include marine and land gravity anomalies,
altimetric sea surface heights, and GOCE observations processed
with the space-wise approach. A spectral analysis of
the aforementioned data shows their complementary character.
GOCE data cover long wavelengths and account for
the lack of such information from gravity anomalies. This is
exploited for the estimation of local covariance function models,
where it is seen that models computed with GOCE data
and gravity anomaly empirical covariance functions perform
better than models computed without GOCE data. The geoid
is estimated by different data combinations and the results
showthatGOCEdata improve the solutions for areas covered
poorly with other data types, while also accounting for any
long wavelength errors of the adopted reference model that
exist even when the ground gravity data are dense. At sea, the
altimetric data provide the dominant geoid information.However,
the geoid accuracy is sensitive to orbit calibration errors
and unmodeled sea surface topography (SST) effects. If such
effects are present, the combination of GOCE and gravity
anomaly data can improve the geoid accuracy. The present
work also presents results from simulations for the recovery
of the stationary SST, which show that the combination
of geoid heights obtained from a spherical harmonic geopotential
model derived from GOCE with satellite altimetry
data can provide SST models with some centimeters of error.
However, combining data from GOCE with gravity anomalies
in a collocation approach can result in the estimation
of a higher resolution geoid, more suitable for high resolution
mean dynamic SST modeling. Such simulations can be
performed toward the development and evaluation of SST
recovery methods