arXiv subjects

Alessandro Toffoli

Publications and source records attributed to Alessandro Toffoli.

At least 19 recordsLinked to original sources

arXiv2025

Fine-Scale Heterogeneity of Snow Cover in the Antarctic Marginal Ice Zone

Snow cover on Antarctic sea ice is highly heterogeneous, yet climate models have historically simplified it to a uniform layer in winter and bare ice in summer, introducing uncertainty in simulated surface energy fluxes. Here, using shipborne, high-resolution, close-range imagery of the sea ice surface, visual observations, and concurrent atmospheric data from five expeditions across the Antarctic marginal ice zone (2019-2024), we show that fine-scale (sub-grid) snow cover heterogeneity persists across all seasons, is largely independent of ice concentration, and is governed by ice morphology and regional accumulation history. When the heterogeneity is incorporated into flux calculations, it introduces systematic biases in surface energy fluxes relative to uniform-surface assumptions. These findings, based on observations from two Southern Ocean sectors, suggest that the inclusion of sub-grid snow distribution should be considered for simulating Antarctic sea ice evolution and its broader polar climate feedback.

physics.ao-ph↗

arXiv2025

Assessing the Ship Motion Prediction Capabilities of the Open-Source Model NEMOH Against Field Observations

Accurate ship motion prediction is critical for safe and efficient maritime operations, particularly in open ocean environments. This study evaluates the capability of NEMOH, an open-source potential flow boundary element solver, as an example of a ship motion prediction tool for real-world open ocean conditions. A linear model, known as the Response Amplitude Operator (RAO), is obtained using NEMOH, and is combined with the wave directional spectrum derived from the WaMoS-II marine radar on the research vessel Akademik Tryoshnikov during the Antarctic Circumnavigation Expedition (ACE) to predict ship motion responses in the frequency domain. Predictions of heave, pitch and roll are benchmarked against concurrent ship motion observations recorded by an onboard inertial measurement unit (IMU). The comparisons, based on the zeroth order moment of the ship motion spectrum, demonstrate a reliable heave prediction (Pearson correlation coefficient r=0.89, scatter index SI=0.41), a reasonable pitch prediction (r=0.80, SI=0.47), and an acceptable roll prediction (r=0.63, SI=0.84). More significant discrepancies for pitch and roll are identified under specific extreme sea conditions. The results demonstrate the capability of NEMOH, offering insights into its applicability for real-world maritime operations.

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arXiv2024

Sea ice floe segmentation in close-range optical imagery using active contour and foundation models

The size of sea ice floes in the marginal ice zone (MIZ) is a key factor influencing ice coverage, albedo, wave propagation, and ocean--atmosphere energy exchanges. Floe size can be observed by processing visual-range imagery from ships, aircraft, or satellites. However, autonomously capturing floe boundaries remains challenging, particularly due to sea ice heterogeneity, which impairs boundary definition and reduces image clarity. This study evaluates the accuracy of sea ice floe segmentation using the gradient vector flow (GVF) active contour method, the deep learning-based Segment Anything Model (SAM), and a hybrid approach combining GVF and SAM. Methods are evaluated on a representative subset of a large dataset of close-range, high-resolution imagery collected from cameras aboard an icebreaker during an Antarctic winter expedition. Spanning a wide range of ice conditions and image clarity in the MIZ, the subset provides a rigorous segmentation test bed. Performance is assessed in terms of floe detection accuracy, size distribution, and ice concentration, with results compared against a manually segmented benchmark. Results indicate SAM, in prompt-driven mode, offers the best balance between accuracy and computational efficiency. Its strong performance in estimating sea ice concentration and detecting floes, while maintaining close agreement with benchmark floe size distributions, makes it suitable for real-time applications and scalable analyses of large imagery datasets. Compared with SAM, the combined SAM-GVF method provides more accurate floe boundary delineation, although at much higher computational cost, and is therefore better suited for analyses requiring precise floe shapes.

physics.ao-ph↗

arXiv2023

Observations of rogue seas in the Southern Ocean

We report direct observations of surface waves from a stereo camera system along with concurrent measurements of wind speed during an expedition across the Southern Ocean in the austral winter aboard South African icebreaker S.A.~Agulhas~II. Records include water surface elevation across a range of wave conditions, spanning from early stages of wave growth to full development. We give experimental evidence of rogue seas, i.e., sea states characterizided by heavy tails of the probability density function well beyond the expectation based on bound mode theory. These conditions emerge during wave growth, where strong wind forcing and high nonlinearity drive wave dynamics. Quasi-resonance wave-wave interactions, which are known to sustain the generation of large amplitude rogue waves, capture this behaviour. Wave statistics return to normality as the wind forcing ceases and waves switch to a full developed condition.

physics.ao-ph↗

arXiv2022

Model predictions of wave overwash extent into the marginal ice zone

A model of the extent of wave driven overwash into fields of sea ice floes is proposed. The extent model builds on previous work modelling wave overwash of a single floe by regular waves by including irregular incoming waves and random floe fields. The model is validated against a laboratory experiment. It is then used to study the extent of wave overwash into marginal ice zones consisting of pancake and fragmented floe fields. The effects of wave conditions and floe geometry on predicted extents are investigated. Finally, the model is used to predict the wave overwash extent for the conditions observed during a winter (July) 2017 Antarctic voyage in which the sea surface was monitored by a stereo-camera system.

physics.ao-ph↗

arXiv2022

Three-dimensional imaging of waves and floe sizes in the marginal ice zone during an explosive cyclone

The marginal ice zone is the dynamic interface between the open ocean and consolidated inner pack ice. Surface gravity waves regulate marginal ice zone extent and properties, and, hence, fluxes between the atmosphere and ocean, and ice advance and retreat. Over the past decade, seminal experimental campaigns have generated much needed measurements of wave evolution in the marginal ice zone, which, notwithstanding the prominent knowledge gaps that remain, are underpinning major advances in understanding the region's role in the climate system. Here, we report three-dimensional imaging of waves from a moving vessel and simultaneous imaging of floe sizes, with the potential to enhance the marginal ice zone database substantially. The images give the direction--frequency wave spectrum, which we combine with concurrent measurements of wind speeds and reanalysis products to reveal the complex multi-component wind-plus-swell nature of a cyclone-driven wave field, and quantify evolution of large-amplitude waves in sea ice.

physics.ao-ph↗

arXiv2022

Interactions between irregular wave fields and sea ice: A physical model for wave attenuation and ice breakup in an ice tank

Irregular, unidirectional surface water waves incident on model ice in an ice tank are used as a physical model of ocean surface wave interactions with sea ice. Results are given for an experiment consisting of three tests, starting with a continuous ice cover and in which the incident wave steepness increases between tests. The incident waves range from causing no breakup of the ice cover to breakup of the full length of ice cover. Temporal evolution of the ice edge, breaking front and mean floe sizes are reported. Floe size distributions in the different tests are analysed. The evolution of the wave spectrum with distance into the ice-covered water is analysed in terms of changes of energy content, mean wave period and spectral bandwidth relative to their incident counterparts, and pronounced differences are found between the tests. Further, an empirical attenuation coefficient is derived from the measurements and shown to have a power-law dependence on frequency comparable to that found in field measurements. Links between wave properties and ice breakup are discussed.

physics.ao-ph↗

arXiv2021

A computational fluid dynamics model for the small-scale dynamics of wave, ice floe and interstitial grease ice interaction

The marginal ice zone is a highly dynamical region where sea ice and ocean waves interact. Large-scale sea ice models only compute domain-averaged responses. As the majority of the marginal ice zone consists of mobile ice floes surrounded by grease ice, finer-scale modelling is needed to resolve variations of its mechanical properties, wave-induced pressure gradients and drag forces acting on the ice floes. A novel computational fluid dynamics approach is presented, that considers the heterogeneous sea ice material composition and accounts for the wave-ice interaction dynamics. Results show, after comparing three realistic sea ice layouts with similar concentration and floe diameter, that the discrepancy between the domain-averaged temporal stress and strain rate evolutions increases for decreasing wave period. Furthermore, strain rate and viscosity are mostly affected by the variability of ice floe shape and diameter.

physics.flu-dyn↗

arXiv2020

Long-term and seasonal variability of wind and wave extremes in the Arctic Ocean

Over recent decades, the Arctic Ocean has experienced dramatic changes due to climate change. Retreating sea ice has opened up large areas of ocean, resulting in an enhanced wave climate. Taking into account the intense seasonality and the rapid changes to the Arctic climate, a non-stationary approach is applied to time-varying statistical properties to investigate historical trends of extreme values. The analysis is based on a 28-year wave hindcast (from 1991 to 2018) carried out with the WAVEWATCH III wave model forced by ERA5 wind speed. The results show notable seasonal differences and robust positive trends in extreme wave height and wind speed, especially in the Beaufort and East Siberian seas, with increasing rates in areal-average of the 100-year return period of wind speed of approximately 4\% and significant wave height up to 60%. It is concluded that the significant increases in extreme significant wave height are largely associated with sea-ice retreat and the enhanced fetches available for wave generation.

physics.ao-ph↗

arXiv2020

Fourier Amplitude Distribution and Intermittency in Mechanically Generated Surface Gravity Waves

We examine and discuss the spatial evolution of the statistical properties of mechanically generated surface gravity wave fields, initialised with unidirectional spectral energy distributions, uniformly distributed phases and Rayleigh distributed amplitudes. We demonstrate that nonlinear interactions produce an energy cascade towards high frequency modes with a directional spread and triggers localised intermittent bursts. By analysing the probability density function of Fourier mode amplitudes in the high frequency range of the wave energy spectrum, we show that a heavy-tailed distribution emerges with distance from the wave generator as a result of these intermittent bursts, departing from the originally imposed Rayleigh distribution, even under relatively weak nonlinear conditions.

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arXiv2019

Ship resistance when operating in floating ice floes: a combined CFD&DEM approach

Whilst climate change is transforming the Arctic into a navigable ocean where small ice floes are floating on the sea surface, the effect of such ice conditions on ship performance has yet to be understood. The present work combines a set of numerical methods to simulate the ship-wave-ice interaction in such ice conditions. Particularly, Computational Fluid Dynamics is applied to provide fluid solutions for the floes and it is incorporated with the Discrete Element Method to govern ice motions and account for ship-ice/ice-ice collisions, by which, the proposed approach innovatively includes wave effects in the interaction. In addition, this work introduces two algorithms that can implement computational models with natural ice-floe fields, which takes randomness into consideration thus achieving high-fidelity modelling of the problem. Following validation against experiments, the model is shown accurate in predicting the ice-floe resistance of a ship, and then a series of simulations are performed to investigate how the resistance is influenced by ship speed, ice concentration, ice thickness and floe diameter. This paper presents a useful approach that can provide power estimates for Arctic shipping and has the potential to facilitate other polar engineering purposes.

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arXiv2019

Drift of pancake ice floes in the winter Antarctic marginal ice zone during polar cyclones

High temporal resolution in--situ measurements of pancake ice drift are presented, from a pair of buoys deployed on floes in the Antarctic marginal ice zone during the winter sea ice expansion, over nine days in which the region was impacted by four polar cyclones. Concomitant measurements of wave-in-ice activity from the buoys is used to infer that pancake ice conditions were maintained over at least the first seven days. Analysis of the data shows: (i)~unprecedentedly fast drift speeds in the Southern Ocean; (ii)~high correlation of drift velocities with the surface wind velocities, indicating absence of internal ice stresses $>$100\,km in from the edge in 100\% remotely sensed ice concentration; and (iii)~presence of a strong inertial signature with a 13\,h period. A Langrangian free drift model is developed, including a term for geostrophic currents that reproduces the 13\,h period signature in the ice motion. The calibrated model is shown to provide accurate predictions of the ice drift for up to 2\,days, and the calibrated parameters provide estimates of wind and ocean drag for pancake floes under storm conditions.

physics.ao-ph↗

arXiv2018

Directional Soliton and Breather Beams

Solitons and breathers are nonlinear modes that exist in a wide range of physical systems. They are fundamental solutions of a number of nonlinear wave evolution equations, including the uni-directional nonlinear Schrödinger equation (NLSE). We report the observation of slanted solitons and breathers propagating at an angle with respect to the direction of propagation of the wave field. As the coherence is diagonal, the scale in the crest direction becomes finite, consequently, a beam dynamics forms. Spatio-temporal measurements of the water surface elevation are obtained by stereo-reconstructing the positions of the floating markers placed on a regular lattice and recorded with two synchronized high-speed cameras. Experimental results, based on the predictions obtained from the (2D+1) hyperbolic NLSE equation, are in excellent agreement with the theory. Our study proves the existence of such unique and coherent wave packets and has serious implications for practical applications in optical sciences and physical oceanography. Moreover, unstable wave fields in this geometry may explain the formation of directional large amplitude rogue waves with a finite crest length within a wide range of nonlinear dispersive media, such as Bose-Einstein condensates, plasma, hydrodynamics and optics.

nlin.PS↗

arXiv2018

Hydroelastic interactions between water waves and floating freshwater ice

Hydroelastic interactions between regular water waves and floating freshwater ice are investigated using laboratory experiments for a range of incident wave periods and steepnesses. It is shown that only incident waves with sufficiently long period and large steepness break up the ice cover, and that the extent of breakup increases with increasing period and steepness. Further, it is shown that an increasing proportion of the incident wave propagates through the ice-covered water as period and steepness increase, indicating the existence of a positive feedback loop between ice breakup and increased wave propagation.

physics.ao-ph↗

arXiv2018

Wave turbulence and intermittency in directional sea states

The evolution of surface gravity waves is driven by nonlinear interactions that trigger an energy cascade similarly to the one observed in hydrodynamic turbulence. This process, known as wave turbulence, has been found to display anomalous scaling with deviation from classical turbulent predictions due to the emergence of coherent and intermittent structures on the water surface. In realistic oceanic sea states, waves are spread over a wide range of directions, with a consequent attenuation of the nonlinear properties. A laboratory experiment in a large wave facility is presented to discuss the effect of wave directionality on wave turbulence. Results show that the occurrence of coherent and intermitted structures become less likely with the broadening of the wave directional spreading. There is no evidence, however, that intermittency completely vanishes.

physics.flu-dyn↗

arXiv2017

Spontaneous emergence of rogue waves in partially coherent waves: a quantitative experimental comparison between hydrodynamics and optics

Rogue waves are extreme and rare fluctuations of the wave field that have been discussed in many physical systems. Their presence substantially influences the statistical properties of an incoherent wave field. Their understanding is fundamental for the design of ships and offshore platforms. Except for very particular meteorological conditions, waves in the ocean are characterised by the so-called JONSWAP (Joint North Sea Wave Project) spectrum. Here we compare two unique experimental results: the first one has been performed in a 270-meter wave tank and the other in optical fibers. In both cases, waves characterised by a JONSWAP spectrum and random Fourier phases have been launched at the input of the experimental device. The quantitative comparison, based on an appropriate scaling of the two experiments, shows a very good agreement between the statistics in hydrodynamics and optics. Spontaneous emergence of heavy tails in the probability density function of the wave amplitude is observed in both systems. The results demonstrate the universal features of rogue waves and provide a fundamental and explicit bridge between two important fields of research. Numerical simulations are also compared with experimental results.

nlin.PS↗

arXiv2017

An experimental comparison of velocities underneath focussed breaking waves

Nonlinear wave interactions affect the evolution of steep wave groups, their breaking and the associated kinematic field. Laboratory experiments are performed to investigate the effect of the underlying focussing mechanism on the shape of the breaking wave and its velocity field. In this regard, it is found that the shape of the wave spectrum plays a substantial role. Broader underlying wave spectra leads to energetic plungers at a relatively low amplitude. For narrower spectra waves break at a higher amplitudes but with a less energetic spiller. Comparison with standard engineering methods commonly used to predict the velocity underneath extreme waves shows that, under certain conditions, the measured velocity profile strongly deviates from engineering predictions.

physics.ao-ph↗

arXiv2017

Observation of turbulence in wave-induced oscillatory flows

The dynamic and thermal regimes of climate are regulated by an exchange of energy and momentum between the atmosphere and the ocean. The role exerted by surface waves on this interchange is particularly enigmatic. Waves induce turbulence in the upper ocean by breaking and through Langmuir circulations. However, waves can directly inject energy into subsurface layers. This relates to waves not being truly irrotational and therefore the induced orbital motion being turbulent. The existence, extent and properties of this turbulent oscillatory flow still remain uncertain. Here we present measurements of the velocity field of oscillatory flows, which are induced by mechanically generated random wave fields in a large scale experimental facility. Velocities were recorded at a depth sufficiently far from the water surface to rule out effects of wave breaking. We demonstrate that the spectral tail of the velocity field follows a power-law scaling close to $ω^{-5/3}$. The turbulent behaviour is investigated via rigorous statistical analysis of the structure functions to highlight the emergence of intermittency in oscillatory flows. The results show that wave motion is turbulent and can contribute to ocean mixing. By deepening of the mixed layer, wave induced motion affects cyclogenesis and sediment resuspension.

physics.ao-ph↗