Asymptotic solution of a weak nonlinear model for the mid-latitude stationary wind field of a two-layer barotropic ocean

A weak nonlinear model of a two-layer barotropic ocean with Rayleigh dissipation is built.The analytic asymptotic solution is derived in the mid-latitude stationary wind field,and the physical meaning of the corresponding problem is discussed.     

海洋激光雷达测量海中悬移质

研制了一套采用倍频YAG激光器的小型海洋激光雷达，用于测量海中悬移质浓度，2000年8月28日至9月5日，该系统安装于“东方红Ⅱ”号右舷下甲板，在渤海海区进行海中悬移质浓度现场测量，通过对18个站位的海洋激光雷达数据分析，获得的表层悬移质浓度与同步测量的海中悬移质浓度结果吻合较好。     

Energy-containing eddies in the strait of Messina

Summary The velocity field, measured by Vercelli (1922-23) in the Strait of Messina hourly at six depths, is analysed to determine the size and the intensity of the energy-containing eddies. The analysis is performed by separating the turbulent component of the velocity field from the deterministic tidal component. The intensity of the turbulence is given by the averaged square of the aleatoric velocities. The size of the eddies is computed through correlation of the aleatoric velocities at different depths. Results show that the intensity of the turbulent field velocity is of the order of (50⤜60) cm s⁻¹, while the size of the eddies is of the order of (20⤜45) m, reaching sometimes 70 m. These eddies are far beyond the inertial range predicted by the Kolmogorov theory. The eddies are due to Kelvin-Helmoltz shear instabilities between the surface and the deeper water.

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Riassunto Le velocità misurate da Vercelli (1922-23) nello Stretto di Messina con intervalli prari, a sei differenti profondità, sono usate per studiare la forma e l'intensità dei vortici portatori di energia. Si separa dapprima il campo deterministico delle velocità dal campo turbolento, la cui intensità è data dalla radice dell'energia dei fenomeni aleatori. La dimensione spaziale è data dalle correlazioni tra campi turbolenti osservati a profondità differenti. Ne risulta che l'intensità della velocità turbolenta raggiunge (50⤜60) cm s⁻¹ e le dimensioni sono (20⤜45) m ma possono arrivare a 70 m. Questi vortici sono lontani dalle dimensioni richieste dalla teoria di Kolmogorov e son dovuti a instabilità di Kelvin-Helmoltz tra gli strati di acqua che fluiscono a Messina.

     

Electromagnetic backscattering from one-dimensional drifting fractal sea surface I: Wave-current coupled model

To study the electromagnetic backscattering from a one-dimensional drifting fractal sea surface, a fractal sea surface wave-current model is derived, based on the mechanism of wave-current interactions. The numerical results show the effect of the ocean current on the wave. Wave amplitude decreases, wavelength and kurtosis of wave height increase, spectrum intensity decreases and shifts towards lower frequencies when the current occurs parallel to the direction of the ocean wave. By comparison, wave amplitude increases, wavelength and kurtosis of wave height decrease, spectrum intensity increases and shifts towards higher frequencies if the current is in the opposite direction to the direction of ocean wave. The wave-current interaction effect of the ocean current is much stronger than that of the nonlinear wave-wave interaction. The kurtosis of the nonlinear fractal ocean surface is larger than that of linear fractal ocean surface. The effect of the current on skewness of the probability distribution function is negligible. Therefore, the ocean wave spectrum is notably changed by the surface current and the change should be detectable in the electromagnetic backscattering signal.     

Evaporation and heat exchanges between the sea and the atmosphere in the Gulf of Trieste during 1988

Summary Daily mean values of meteo-oceanographic parameters are used to compute evaporation and heat fluxes between the sea and the atmosphere in the Gulf of Trieste during 1988. Incoming solar radiation, long-wave radiation, latent and sensible heat fluxes are considered in the total heat budget. The contribution of the single parameters entering each formula is examined and a comparison between the results from different evaporative formulae is performed. The total heat budget for the year considered gives a net loss of energy of 21 W/m² which must be supplied by the heat transported by marine currents. The balance is positive for the sea surface from April to July, negative for the remaining period. Long-wave radiation accounts for the 45% of the total heat loss, latent heat for 53% and sensible heat for 2%. Computed annual mean evaporation is 1058 mm, with high peaks of more than 15 mm/day during strong Bora wind events. Evaporation slightly prevails on precipitation giving a net water loss of 30 cm. A comparison with what is found in the literature shows these results representative of the entire North Adriatic Sea. They confirm that this basin represents a sink of heat whose role in the general Mediterranean circulation consists in trasforming surface warmer waters coming from South in denser ones.     

Circulation and heat exchanges of the Northern Adriatic in the winter season 1992/93

Summary The sea currents, temperature and conductivity were measured continuously at two stations and two depths of the eastern part of the Northern Adriatic during the winter season 1992/93. Meteorological elements were measured simultaneously at the two nearest meteorological stations on the Croatian coast (Rovinj, Pula). The sea level was also measured in that period at Rovinj. One severe bora storm was observed in the period of measurements. Qualitative good agreement between observed currents and velocities of the earlier developed models for the response of the Northern Adriatic to the bora and sirocco wind forcing has been shown. The sea currents on the Rovinj-Po transect are of the opposite direction to the bora wind because of tis spatial heterogeneity. The sea level observed at Rovinj station rose during the bora blows and it was much higher than what expected in the developed models. The response time of the currents to the bora forcing is computed by using correlation function between currents and wind. The heat exchanges on the sea-atmosphere surface are calculated. The correlation between same wind components and between total heat exchanges for two different stations was made. The sea temperature evolution calculated from meteorological data agrees well with what observed, especially for the Pula station. At the end of the measurement period the new dense water (σ_t=29.3 kg m⁻) was observed. We dedicate this article to the memory of colleague Antonio Michelato. D. Morožin is not employed at the Institute R. Boŝković.     

On the baroclinic instability of nonplanar flows

Summary Different ocean models with one or two layers having constant static stability and supporting constant-shear flows, whose directions are allowed to change with depth, are examined in the frame-work of the linear nonzonal baroclinic stability theory and in the absence of the β-effect. The analysis is reduced to solving a simple Sturm-Liouville boundary value problem in one dimension. A fairly general dispersion relation is found which correctly reproduces several special cases analysed by other authors. This relation shows a fair variety of possible behaviours for stability curves of two-layer models. The results show that the presence of a nonplanar shear-flow may have profound consequences on the stability character of the stationary geostrophic flow. In fact, it appears that the stability properties are strongly dependent on the propagation angle of the disturbance so that wave numbers which appear stable in the usual zonal theory may result unstable on such a nonzonal flow andvice versa. Paper presented at the 1^o Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 19–22, 1984, Rome.     

Enrichment and transport of surfactants in marine aerosol formed during particular weather conditions

Summary Surfactants appear responsible for the presence and the enrichment of microfloatable components in marine aerosol. According to the first set of both surface thermodynamic and chemical data, a qualitative model of enrichment and mass transfer is proposed for marine aerosol, near the coast in particular weather conditions. The present data also suggest a quality test for coastal sea-water, integrated over large areas, in those weather conditions in which a direct surface water sampling is very difficult. Paper presented at the 1° Congresso del Gruppo Nazionale per la Fisica dell'atmosfera e dell'Oceano, June 19–22, 1984, Rome.     

Cylindrical solitons in shallow water of variable depth

Summary The propagation and the interaction of cylindrical solitons in shallow vater of variable depth are studied. Starting from the cylindrically symmetric version of the equations describing long waves in a beach, a Korteweg-de Vries equation type ψ_τ+6ψψ_ξ+ψ_ξξξ=−Γ(τ)ψ is derived. Since no exact analytical solution has been found to date for this equation, some remarkable cases in which the equation takes up a tractable form are analysed. Finally the interaction between cylindrical imploding and expanding waves is considered and the phase shifts caused by the head-on collision are given. This work was supported by CNR-GNFM.     

The impact of higher-order bragg terms on radar sea return

Summary The possibility that radar sea return observed using aKu band fan beam Doppler airborne scatterometer flown over crude oil artificial spills might have been back-scattered via the second-order Bragg interaction is surmised. An attempt is made to justify the absence of the first-order Bragg term.

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Riassunto Prendendo in considerazione l'effetto dei primi ordini di Bragg e sopprimendo il ter mine di ordine 1, si mostra come l'inesplicato andamento dell'eco del mare in presenza di film di spandimento, rilevato con radar doppler in bandaKu montato su aereo, coincida con misure effettuate altrove in presenza di analogo film. Si propone una teoria secondo cui l'assenza del primo ordine è dovuta ad una carenza dell'onda capillare pertinente.

     

On the eigenperiods in the Tyrrhenian Sea level oscillations

Summary In this paper the eigenperiods of the Tyrrhenian Sea are examined by a hydrodinamical model in two dimensions. The knowledge of these eigenperiods is important to evaluate the seiches that often appear, in some point or circumstance, stronger than the tides. This investigation is performed by energizing the basin through the Sardinia channel, in the same way as it was already done for the tides. This excitation simulates a large set of waves inducing oscillations inside the basin. The periods of the waves are narrow and their amplitude equal. An analogous method of investigation is that of the fluorescence. In addition, the results are compared with those previously obtained leaving the basin swinging, after having increased the sea level by a uniform displacement. The response of the bidimensional model agrees with that of a monodimensional one as far as possible; thus the last one can give useful information, also for a sea that does not have a channel shape, like the Tyrrhenian Sea. Nevertheless, this basin has other openings, besides the Sardinia channel, it seems that they do not influence the eigenperiod behaviour. This has been confirmed by performing the same computations over a sea 500 m less deep: the uniform lowering of the sea surface is equivalent to close the four minor openings. The results carried out in this case agree with the previous one. Among all the periods pointed out by our investigation, one is always exhibited in every point and by each system. The period is that of 5.70 h and it can be considered a fundamental mode of the Tyrrhenian Sea.     

Rising of gas bubbles in an aqueous medium in presence of surfactants

Summary Experiments based on the direct comparison of the gas-bubble rising times in a surfactant aqueous solution, in respect to those in pure water, give support to the hypothesis that a rising path of many metres is needed to reach the saturation of the adsorption process on bubbles (0.2÷0.3) cm in diameter. A maximum is evidenced in thet/t ₀ ratiovs. surfactant concentration at the highest concentrations tested. An explanation, on a qualitative basis, of the appearance of the observed maximum is proposed. Work presented at the IV Congress of the CNR (National Research Council) National Group of the Atmosphere and Ocean Physics, Rome, June 22–24, 1987.     

Thermal expansion of a rig

Summary We have estimated and measured the daily thermal expansion of an oceanographic platform. This has been found negligible as far as the calibration of a satellite altimeter is concerned.     

Linear absorbing boundary conditions for surface water wave propagation

Summary The meaning of linear absorbing boundary conditions for periodic water waves in elliptical approaches is discussed. It is found that the direction of propagation of the waves must be known for reliable boundary conditions and that the energy dissipation of waves running parallel to a boundary in general cannot be modelled in the framework of a linear theory. Work presented at the Euromech Colloquium 240 on ?Dispersive waves in dissipative fluids?, Bologna, August 30–September 2, 1988.     

Series solutions of the dispersion relation for linear water waves

Summary A review is given of various existing approximate solutions of the dispersion relation for linear water waves, along with some new series representations. A general method is also provided to get the wanted accuracy in the desired wave number range.

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Riassunto Si presenta una rassegna di varie formule atte ad approssimare la soluzione della relazione di dispersione per onde marine superficiali in regime lineare, insieme ad alcune nuove rappresentazioni in serie. Si fornisce anche un metodo generale per ottenere l'accuratezza desiderata nel campo di numeri d'onda voluti.

     

Local effects in wind and wave fields in the Tyrrhenian sea

Summary A very sophisticated wave model has been applied to the western Mediterranean Sea. The results show the importance of local effects that had not been taken into account by the input atmospheric model. The need is stressed for the use of local models nested in the large-scale ones. Paper presented at the IV Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 22–24, 1987, Rome.     

Some simple solutions concerning the Ligurian Sea vortex and its stability

Summary The cyclonic vortex of the Ligurian Sea is a very persistent feature of the Mediterranean Sea, even if its intensity varies seasonally. In this paper we examine the barotropic component of this vortex. In particular we study the influence of the bottom topography and of the coastlines in shaping the cyclonic flow. Furthermore we show that the barotropic part of the vortex is stable to finite perturbations, which may explain its persistency. Paper presented at the IV Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 22–24, 1987, Rome.     

Second-order analysis of shallow-water wave spectra

Summary This paper presents a study based on a second-order interpretation of wave ?energy? spectra, which may explain the build-up of low-and high-frequency peaks for wave spectra measured in shallow water. A comparison between this interpretative model and laboratory data, which were obtained in a bidimensional wave flume, is also given in the paper. Paper presented at the IV Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 22–24, 1987, Rome.     

Some remarks on the energetics of simple barotropic flows

Summary This note deals with the total energetics of simple barotropic flows in a shallow-water approximation. In particular the effect of the linearization procedure and the combined role of bottom variations, geostrophic and ageostrophic fields when the quasi-geostrophic context is assumed are discussed. Finally the results are compared with analogous ones reported in the literature. Paper presented at the IV Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 22–24, 1987, Rome.     

A process of thermocline erosion on the Sicilian continental shelf

Summary We study a process of thermocline erosion measured in the Channel of Sicily in the period 16 November–December 1985. Evidence is presented that the transition from Summer to Winter stratification has been an impulsive process. We show that the erosion of the seasonal thermocline and the deepening of the surface mixed layer were stimulated by the action of two strong atmospheric perturbations. Possible relations between mixing events and internal wave breaking are discussed. Paper presented at the IV Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 22–24, 1987, Rome.