Stefano Zecchetto

Sea Surface Winds over the Mediterranean Basin from Satellite Data (2000–04): Meso- and Local-Scale Features on Annual and Seasonal Time Scales

Abstract This paper investigates the mean spatial features of the winds in the Mediterranean and Black Seas using the wind fields observed by the SeaWinds scatterometer. Five years (2000–04) of data have been analyzed on annual and seasonal basis, with particular attention paid to the meso- and local scales. The fields show the existence of structured regional wind systems—in particular, the mistral in the western Mediterranean and the etesians in the Levantine Basin, which are characterized, respectively, by high wind variability and moderate steadiness and by low wind variability and high steadiness. Estimated seasonal mean wind stress τ fields show that the values falling in the top range 0.15 < τ < 0.20 N m−2 affect a large portion of the Mediterranean Basin in winter, in the belt extending from the Gulf of Lion up to the Levantine Basin and the northern Black Sea. In the other seasons, only few regions experience such high values of τ. The analysis of the wind vorticity shows and quantifies the main ...

Calculation of a mass-consistent two-dimensional wind field with divergence control

Abstract The aim of this paper is the calculation of mass-consistent wind velocity field in a two-dimensional domain Ω on the basis of sparse measurements collected by wind-measuring stations. Measured data are used to estimate an initial field. The result is obtained by a weighted interpolation method. An iterative scan procedure is used in which the radius of influence of each station over the surrounding grid points is decreased at each step of iteration. To adjust the initial field into a mass-consistent velocity wind field, a new technique is presented that is meant to extract only the purely divergent component of the wind velocity field. This component is then either suitably manipulated, reduced and reintroduced in the total wind field, or completely neglected. The main idea of the method is to obtain a mass-consistent wind by controlling the magnitude of the purely divergent component without completely destroying all the information connected with its spatial pattern. On the basis of a test case...

On shape, orientation, and structure of atmospheric cells inside wind rolls in two SAR images

Synthetic aperture radar (SAR) images of the sea surface often show roll-vortex structures and other features which, in general, are spread out over several length scales and may present spatial periodicity as well as intermittence. Standard techniques, such as two-dimensional (2-D) Fourier analysis, are unsuitable both when it is of interest to detect intermittent phenomena and to analyze the spatial disposition of the backscatter structures inside the SAR images. For the above reasons, the 2-D continuous wavelet transform analysis has been applied to two European Remote Sensing mission SAR images over the Mediterranean Sea, showing wind rolls and atmospheric gravity waves. Through the evaluation of the wavelet variance map, which ideally corresponds to the 2-D Fourier spectrum, it has been possible to assess the presence of two main energy areas at large (from 7-28 km) and small (from 0.5-2 km) spatial scales. While the large-scale fluctuations may be ascribed to atmospheric gravity waves and other features induced by the surroundings, the small-scale fluctuations reveal the inner structure of the atmospheric wind rolls. The SAR-like maps, obtained by adding the wavelet coefficient maps pertaining to the small scales, have permitted us to highlight the high- and low-intensity backscatter cells associated with the wind rolls. These cells have been statistically characterized by means of the frequency distributions of the size of the cells maximum and minimum axes, of the orientation of the maximum axis, and of their area. The results indicate that high- and low-intensity backscatter cells have similar characteristics in both cases studied: they appear of elliptic shape, with the major axis along the wind roll direction; the average axes ratio is 2.5:1. The frequency distributions of the cell area indicate a continuous distribution of sizes, without significant gaps.

Microwave remote sensing and hydrological modelling of snow melting cycle

A study of the melting cycle of snow was carried out by combining microwave active and passive measurements with meteorological data and snow modelling. The experiment took place in the eastern Italian Alps from mid February to late May 2003. Brightness temperature at C-, Ku- and Ka- bands (vertical and horizontal polarizations) and backscattering coefficient at Ku-band (VV), were continuously measured (24 h/day) with ground-based sensors. Remote sensing observations were supported by meteorological data, and snow measurements. A continuous simulation of the snow temperature, depth, and liquid water content was performed for the entire monitoring period by means of a physically based distributed snowmelt model. Both hydrological and remote sensing approaches gave useful and coherent results in describing the snow melting and refreezing cycles. Microwave active and passive data were consistent each other. During the melting cycle, the presence of liquid water caused an increase of absorption with a consequent increase of the brightness temperature and a decrease of the backscattering coefficient

A Wavelet Technique to Extract the Backscatter Signatures from SAR Images of the Sea

SAR 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 0.3km ¥ 4km are directly associated to the wind spatial structure deriving, in turns, from the turbulent characteristics of the wind ∞ow. They have an elliptic shape, with the major axis along the (aliased) wind direction. Those with size falling inside the spatial range 4km ¥ 20km 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 flelds 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.

Intercomparison of satellite observations and atmospheric model simulations of a meso-scale cyclone in the Mediterranean Sea

The paper investigates the possibility of integrating satellite observations and limited-area atmospheric model simulations in the study of the meso-scale processes in the Mediterranean Sea. A tropical-like cyclone, which occurred on 9 and 10 December 1996 over the central Mediterranean Sea, has been used as a case study. The main features of the cyclone, derived from satellite wind and vertically integrated liquid water content (LWC) fields, have been compared with the results of a limited-area model, obtained with and without scatterometer data assimilation. The results indicate an acceptable comparability of the wind fields, despite the model‐scatterometer wind bias and the misplacement of the cyclone position in the simulations. The vertically integrated quantities, such as the LWC and the model vertically integrated cloud mixing ratio (CWINT), do not show the same degree of comparability. This is because they reflect only large-scale features, whereas the meso-scale features are only detected by the satellite observations. As expected, the results of assimilating the scatterometer data into the model show an improvement over the cyclone positioning but do not provide a better description of its meso-scale features. The lack of improvement in the determination of the CWINT horizontal structure after scatterometer data assimilation indicates the need to assimilate the LWC fields in future studies.

Wind Fields From C- and X-Band SAR Images at VV Polarization in Coastal Area (Gulf of Oristano, Italy)

This work deals with the spatial characteristics of the wind fields evaluated from synthetic aperture radar (SAR) images and simulated by the weather research and forecasting (WRF) atmospheric model in the Gulf of Oristano, a small coastal area about 10 km × 18 km wide in western coast of Sardinia (Western Mediterranean Sea). The SARderived wind fields have been obtained analyzing images of the COSMO-SkyMed, Radarsat-2, and Sentinel-1A satellites through a fully two-dimensional continuous wavelet transform (2-D-CWT) method. The analysis of the wind directions has shown that the model variability is limited if compared to that inferred by 2-D-CWT method, which mostly respects the variability evidenced by in situ data. As the use of model directions to compute the SAR wind fields is a standard in many studies, the impact on the SAR wind speed retrieval of using the model instead of the SARderived directions has been assessed: differences of wind speed greater than ±10% occur for about the 20% of data. The spatial variability of the SAR and model wind speed fields results quite different at both local and domain scales. The knowledge of the spatial variations of the surface wind fields can be very important for the oceanographic applications and constitutes the added value brought by SAR in the description of the coastal wind. For this reason, the SAR-derived wind fields should be taken as reference in many kind of applications.

Exploiting the Potential of Satellite Microwave Remote Sensing to Hindcast the Storm Surge in the Gulf of Venice

The potential of active microwave satellite observations of sea surface height (radar altimetry) and of sea surface wind (radar scatterometry) has been exploited for storm surge modeling purposes. The altimetry observations were assimilated into a storm surge model (SSM) with a dual 4D-Var system, in order to obtain the best possible surge level field as initial condition to reforecast runs. The scatterometer wind data were instead used to improve the accuracy of the wind fields of a global atmospheric model used as forcing to the SSM through a procedure that has been proved to be able to reduce the differences between the model winds and the scatterometer observations. Hindcast experiments were performed to test the sensitivity of the SSM to the altimetry data assimilation and to the modified wind forcing. Remarkable improvements of the storm surge level hindcast have been obtained for what concerns the modified model wind forcing, while encouraging results have been obtained with the altimeter data assimilation.

The wind-measuring system in the Gulf of Oristano: a support for coastal-scale oceanographic applications

ABSTRACT This paper describes a wind-measuring system (WMS) in the coastal area of the Gulf of Oristano, Sardinia, Italy. In this area, an oceanographic forecasting system based on a high-resolution finite-element hydrodynamic and wave model is operative. The system is forced by the wind fields provided by medium resolution (about 0.1×0.1 deg) atmospheric models. The high spatial resolution of the oceanographic prediction system (between 50 m and 1 km) is not accompanied by a similar resolution for atmospheric forcing. Therefore, the implementation of a WMS, aimed at accurately measuring the three-dimensional wind components, provides an essential tool to evaluate both the performance of the atmospheric models used to force the oceanographic models and the sensitivity of the oceanographic models to wind forcing of different qualities and spatial resolutions, the experimental fields included. This paper describes the WMS, the data collected and the geophysical quantities obtained from the time series of the three components of the wind sampled at high frequency. It also shows the results of comparisons between the experimental wind data and those from the atmospheric model at present used to force the oceanographic forecasting system. The information obtained from the WMS is unique, spanning from the spatial and temporal variability of the wind over the area of investigation to the evaluation of the basic quantities characterising the atmospheric boundary layer.

Predictability of wind‐induced sea surface transport in coastal areas

In this work we investigated the predictability of the wind-induced sea surface transport in coastal areas. The wind fields predicted by two state-of-the-art meteorological models, namely ECMWF and SKIRON, were used as forcing for a hydrodynamic and particle-tracking model applied to reproduce a set of observed drifters trajectories in a coastal area of the Mediterranean Sea. A set of anemometric data derived by in situ measurements were also adopted as model forcing to reproduce the observed drifter paths. This approach provided a baseline that was used as a reference for evaluating the effects of the predicted wind accuracy on the numerical model solution. The accuracy of the simulation results obtained using, as model forcing, the observed wind data was fair and suitable for most of the operational oceanographic purposes. It decreased when using the wind data predicted by the two meteorological models. In particular, the results obtained using ECMWF data were about 3 times more accurate than the ones obtained using SKIRON ones. The uncertainties were strongly dependent on the range of observed wind speed classes with a different behavior depending on the type of adopted wind data. Finally, the amplification of the errors in predicting the sea surface transport generated by the inaccuracies of the predicted wind fields was quantified.