1,720,992 research outputs found
A wavelet technique to extract the backscatter signatures from SAR images of the sea
No full textSAR images of the sea often show backscatter patterns linked to the horizontal structure of the Marine Atmospheric Boundary Layer (MABL) at the interface with the sea surface. In general, their dimensions are spread over a wide range of length scales, presenting spatial periodicity as well as intermittence. With the aim to isolate such backscatter structures, the two-dimensional Continuous Wavelet Transform (CWT2) analysis has been applied to SAR, images of the sea. The CWT2 analysis permits to highlight the backscatter cells associated to the structure of MABL, as well as to evidence the structure of the atmospheric gravity waves occurring at the lee side of islands and coast. The cells detected in the range 0.3 km divided by 4 km are directly associated to the wind spatial structure deriving, in turns, from the turbulent characteristics of the wind flow. They have an elliptic shape, with the major axis along the (aliased) wind direction. Those with size falling inside the spatial range 4 km divided by 20 km describe, instead, the atmospheric gravity waves structure (if present) and the structures linked to the wind shading. The technique developed is the background for several applications: it has been used to compute the wind fields without any a priori information, as well as to study the inner structure of the Langmuir atmospheric circulation. Other applications could be on the detection of sea surface oil slicks
Comparison of 10-m wind forecasts from a regional area model and QuikSCAT scatterometer wind observations over the Mediterranean Sea,
No full textSurface wind forecasts from a limited-area model [the Quadrics Bologna Limited-Area Model
(QBOLAM)] covering the entire Mediterranean area at 0.1° grid spacing are verified against Quick Scatterometer
(QuikSCAT) wind observations. Only forecasts within the first 24 h in coincidence with satellite
overpasses are used. Two years of data, from 1 October 2000 to 31 October 2002, have been considered,
allowing for an adequate statistical assessment under different wind conditions. This has been carried out
by analyzing the fields of the mean wind vectors, wind speed bias, correlation, difference standard deviation,
steadiness, gustiness, and mean wind direction difference, in order to investigate spatial variability. Statistics
have been computed on a seasonal basis. A comparison of satellite and forecast winds with measurements
from three buoys was also performed. Some critical areas of the Mediterranean Sea where wind forecast
quality is lower than average have been identified. Such areas correspond to semienclosed basins surrounded
by important orography and to small regions at the lee side of the main islands. In open-sea regions
the model underestimates wind strength from about 0.5 m s1 in spring and summer to 1.0 m s1 in winter,
as evidenced by the existing biases against scatterometer data. Also, a wind direction bias (scatterometer
minus model) generally between 5° and 15° exists. A survey of the identified and likely sources of forecast
error is performed, indicating that orography representation plays an important role. Numerical damping is
identified as a likely factor reducing forecast wind strength. The need for a correction scheme is envisaged
to provide more accurate forcing for numerical sea state forecasting models, wind energy evaluation, and
latent and/or sensible heat exchanges
Caratterizzazione della superficie del mare e riconoscimento delle sostanze tensioattive mediante telerilevamento a microonde
No full text
A joint analysis of radiometric and scatterometric simulations to tune an electromagnetic forward model within a common theoretical framework
No full textAn electromagnetic forward model originally developed to simulate the sea emission at microwave bands is extended to the active case. This work focuses on the tuning of the model to reproduce the behaviour of the geophysical model functions of ERS, NSCAT and QuikSCAT scatterometcrs (C and Ku band). In order to better match the model functions, we have modified some assumptions within the model concerning both the isotropic and anisotropic components of the sea wave spectrum and the hydrodynamic modulation. The preservation of the ability, to simulate passive radiometric data trough the same assumptions is also assessed
A joint analysis of microwave radiometer and scatterometer data to characterize meso-scale structures in the Mediterranean Sea
No full textAn analysis of a large set of data, collected from the Special Sensor Microwave/Imager (SSM/I) radiometer and from the NASA SeaWinds scatterometer over the Mediterranean Sea is presented, with the aim of studying the mean behavior of various geophysical parameters, such as water vapor, surface rain rate, wind speed and Ekman pumping for the years 2000 and 2001. This paper presents the results of the study of two different meteorological situations occurring, in the Mediterranean basin. Moreover, ability of these instruments for analyzing extreme events is shown through examples
Osservazioni della superficie marina con tecniche radar per l’estrazione dei parametri meteo-oceanografici
No full textAnalysis of ENVISAT RA2 altimeter sigma-nought bloom events for S and Ku bands backscattering intercomparison
No full textThe RA2 radar altimeter onboard the European Satellite Envisat is a double band microwave instrument working at Ku- and S-band. The S-band measurements provide appropriate atmospheric corrections for the more precise Ku-band measurements, but are also used in conjunction with the Ku-band data in order to investigate the underlying surface. In this study we try to identify differences and similarities of the backscatter signal in the two bands. We focus our attention on the condition of specular alike radar returns, which mostly occur on sea-ice regions. An attempt to find situations in which highly correlated radar waveforms in the two bands can be ascribed to simple scattering mechanisms is made in order to find indications on the mutual calibration of the two channels. The paper reports a number of cases ancountered while analyzing RA-2 data. Both level 2 (1 Hz geophysical parameters) and level 1 (18 Hz waveforms) have been used
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