1,720,975 research outputs found
Numerical study on signatures of atmospheric convective cells in radar images of the ocean
No full textCurrent and wind variations at the ocean surface can give rise to a modulation of the sea surface roughness and thus become visible in radar images. The discrimination between radar signatures of oceanic and atmospheric phenomena can be quite difficult, since signatures of different origin can have very similar shapes and magnitudes and are often superimposed upon each other. In this work we employ a numerical radar imaging model for an investigation of typical properties of radar signatures of atmospheric convective cells and of theoretical differences between such atmospherically induced radar signatures and those of oceanic phenomena. We show that main characteristics of observed multifrequency/multipolarization radar signatures of atmospheric convective cells over the Gulf Stream are reproduced quite well by the proposed model. This encourages us to vary wind and radar parameters systematically in order to get a general overview of the dependency of atmospherically induced radar signatures on these parameters. Finally, we compare typical characteristics of radar signatures of atmospheric and oceanic phenomena, and we present simulated radar images of a scenario of superimposed atmospheric convective cells and oceanic internal waves. We show that the proposed model supports the experimental finding that radar signatures of oceanic phenomena are stronger at horizontal (HH) than at vertical (VV) polarization, while atmospherically induced radar signatures are better visible at VV polarization
A new interpretation of multifrequency/multipolarization radar signatures of the Gulf Stream front
No full textRadar signatures which are observed on SIR-C/X-SAR multifrequency/multipolarization synthetic aperture radar images of the Gulf Stream off the U.S. east coast are compared with results of simulations with a numerical radar imaging model. Based on in situ data, current and wind variations are included into the model as well as a variation of the thermal stability of the marine atmospheric boundary layer across the Gulf Stream front. According to our model predictions, all of these parameter variations can cause radar signatures of similar shape and modulation depth. But, due to specific dependencies of radar signatures on variations of surface currents and winds, we show that it is possible to distinguish between radar signatures of oceanic and atmospheric origin in multifrequency/multipolarization images and to estimate the corresponding current and wind variations independently. For one set of radar images we derive a most likely scenario of oceanic and atmospheric parameters during the time of the image acquisition for which good overall agreement between observed and simulated radar signatures is obtained at most radar channels
The influence of data source characteristics on calculations of air-sea flux of carbon dioxide (abstract of poster presented at the EGU General Assembly 2005, Vienna, 24-29 April 2005)
No full textThe air-sea gas transfer velocity is commonly parameterised via the wind speed at neutral stratification of the marine atmospheric boundary layer at 10m height. This quantity known as u10 is available from a variety of sources, ranging from satellite scatterometers and passive microwave sensors to model re-analyses. Furthermore, more recent parameterisations of the gas transfer velocity involve parameters such as friction velocity, significant wave height and small-scale sea surface roughness, also obtainable from satellite measurements. This wealth of data together with publically available climatologies of air-sea concentration difference of carbon dioxide allows a calculation of global CO2 exchange at daily to monthly intervals.However, different sampling techniques and integration times of the various instruments or methodologies mean that parameter values, even if these claim to be measuring the same quantity, can disagree quite substantially. When these measurements are then used for calculations on a large scale such as for the determination of global air-sea gas fluxes, relatively small biases of data from different sources can result in large errors on the order of up to 20% in carbon dioxide fluxes. This is particularly true when considering parameterisations of the gas transfer velocity which are based on a squared or cubed function of u10.In this work we analyse data from a variety of sources and their sampling characteristics. For the calculation of air-sea gas flux we show how the choice of input data defines the uncertainty of the results. Further unknowns such as the parameterisation of gas transfer velocity are discussed. An accurate quantification of these uncertainties is one of the aims of the Centre for the observations of Air-Sea Interaction and fluXes (CASIX) in which this work is carried out
A sea-state dependent parameterization of whitecapping and air-sea gas transfer velocities (abstract of paper presented at the EGU General Assembly 2005, Vienna, 24-29 April 2005)
No full textA parameterization of whitecapping that depends on sea state in addition to the friction velocity of the wind is justified in terms of the energetics of wind waves. This new parameterization has implications for processes wholly or partly dependent on whitecapping including sea-salt production and air-sea gas transfer. A parameterization of gas transfer velocities is derived from a previous model modified by the sea-state dependent parameterization of whitecapping. This new model is evaluated. The new model provides a rationale for the divergence of earlier gas transfer coefficient models, giving due consideration to the sea-state conditions prevalent in the underlying data sets. Contemporary gas transfer is evaluated over the globe at seasonal and regional resolutions (up to monthly and 1 degree) for both the new and traditional parameterisations using both reanalysis products (ECMWF ERA40) and earth observation (scatterometer and altimeter) products. The new model implies mean global transfer velocities, mean global exchange coefficients and a net global carbon dioxide sink broadly in line with previous estimates but with significant differences in detail. The sensitivity of the net carbon dioxide sink to the balance of non-whitecapping and whitecapping components of gas transfer is high. Regional differences in gas transfer between traditional formulations and the new model are substantial. Whitecapping in the North Atlantic is notable for strong inter-annual variability driven primarily by the high sensitivity of wave heights to the North Atlantic Oscillation
Evaluation of a semi-analytical approach to the retrieval of water quality parameters from optical data in European coastal Case-II waters
No full textThis work addresses the retrieval of the three water quality parameters chlorophyll-a, yellow substance and suspended particulate matter from spectra of remote sensing reflectance in European coastal waters. We study the suitability of a semi-analytical algorithm for the retrieval of these parameters in coastal waters to investigate the validity of radiative transfer theory and bio-optical models that have been developed primarily for open ocean waters. To obtain water quality parameters from reflectance measurements we employ a non-linear inversion method (Gauss-Newton). Algorithm parameters are established to ensure convergence of the method and reduce trapping by local minima. The developed algorithm is then evaluated with the help of a case-specific sensitivity analysis that reveals strengths and weaknesses with respect to measurement errors and inaccuracies of the bio-optical models on which the algorithm is based. In order to establish the validity of the results, a second sensitivity analysis is carried out based on the analysis of normalised partial derivatives of the algorithm's central equation. The algorithm is then applied to an extensive in situ data set consisting of 447 high-resolution spectra of remote sensing reflectance and water quality parameters from a range of European coastal waters, acquired in the framework of three different projects. Given the different measurement techniques within the various projects, it is not surprising that the algorithm performs poorly for the complete data set. Studying the regional subsets individually yields improved results in some cases, suggesting potential for developing regionally specific algorithms on the basis of dedicated tuning. The complete failure of the algorithm in other regions displays the shortcomings of the methodology. It is shown that, in some cases, the forward model fails to describe the optical characteristics encountered producing a pronounced mismatch between calculated and measured reflectance spectra in both spectral shape and magnitude. In other regions the spectral shape is largely reproduced by the model but a mismatch in magnitude results in failure of the inversion procedure. However, the most fundamental problem encountered is the non-uniqueness of the reflectance inversion process for some spectra. Improved bio-optical models and dedicated measurement campaigns in coastal waters are a crucial requirement to resolve this problem for future regional applications of semi-analytical algorithms. We point out the optical characteristics of favourable and unfavourable conditions for the retrieval of water quality parameters and provide some guidelines to future measurements of optical properties of coastal waters
On the sensitivity of semi-analytical algorithms for the retrieval of water quality parameters from optical measurements in coastal waters
No full textSemi-analytical algorithms which are used for the retrieval of water quality parameters from optical measurements of the ocean surface layer are based on empirical bio-optical models. We investigate the sensitivity of a semi-analytical algorithm to systematic errors in the bio-optical models and study the resulting errors in the retrieval of water quality parameters
Validation of SAR ocean data products and verification of process models used for SAR interpretation
No full textThis paper examines the issue of the quality analysis of ocean data products derived from SAR. It is based on work in the EU MARSAIS Project. The principles of quality assessment are addressed, distinguishing between the tasks of calibration, process verification and validation. These principles are applied to four classes of SAR ocean data product; wind, waves, features measured by their surface current signatures, and oil slick monitoring. Wind and wave products are shown to be quite well validated, but the retrieval of information about current-based features or about oil slicks are poorly validated, even though the process models used have been verified. The reason is the scarcity, or absence, of any independent measurements of the ocean variables retrieved from SAR. This shortcoming must be addressed if SAR is to be widely accepted as a tool for measuring ocean phenomena
Synthetic aperture radar signatures of marine atmospheric boundary layer cellular convection and longitudinal roll vortices
No full textRemote sensing of oceanic current features by synthetic aperture radar - achievements and perspectives
No full textIt is generally accepted that synthetic aperture radar (SAR) images can be quite useful for a better understanding of hydrodynamic processes in the ocean, because they provide valuable information on the location and spatial scales of oceanic features such as fronts, internal waves, and eddies. However, the retrieval of actual surface current fields from the shape and modulation depth of radar signatures is a much more challening problem, since the imaging mechanism is a complex and nonlinear two-step mechanism which cannot be inverted easily. In this article we review the state-of-the-art in modeling radar signatures of current features and we present the concept of an iterative scheme for inverting radar images into current fields, which will be implemented within the framework of the European project MARSAIS. We estimate the accuracy and spatial resolution of the proposed remote sensing system on the basis of findings from recent case studies and some dedicated simulations. According to the results of our analyses, it should be possible to retrieve spatial surface current variations and current gradients from a typical spaceborne C band SAR image with an accuracy on the order of 20% and a spatial resoution on the order of 50m
Status report on the remote sensing of current features by spaceborne synthetic aperture radar
No full textSpatial variations in ocean surface currents can become visible in synthetic aperture radar (SAR) images via hydrodynamic modulation of the surface roughness. The interpretation of such SAR signatures is a challenging problem, since the imaging mechanism is quite complex and nonlinear and cannot be inverted easily. Furthermore, the distinction between SAR signatures of current features and other phenomena can be difficult. However, SAR is the only existing technique for the observation of current variations on spatial scales of tens of meters from satellites. There is a vital demand for such information, particularly in coastal regions. A variety of algorithms have been developed for the retrieval of information on current features from SAR images for different purposes. We give an overview of the state of the art, existing and potential applications, and future perspectives and requirements
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