Evidence for a k^{-5/3} spectrum from the EOLE Lagrangian balloons in the low stratosphere

The EOLE Experiment is revisited to study turbulent processes in the lower stratosphere circulation from a Lagrangian viewpoint and resolve a discrepancy on the slope of the atmospheric energy spectrum between the work of Morel and Larcheveque (1974) and recent studies using aircraft data. Relative dispersion of balloon pairs is studied by calculating the Finite Scale Lyapunov Exponent, an exit time-based technique which is particularly efficient in cases where processes with different spatial scales are interfering. Our main result is to reconciliate the EOLE dataset with recent studies supporting a k^{-5/3} energy spectrum in the range 100-1000 km. Our results also show exponential separation at smaller scale, with characteristic time of order 1 day, and agree with the standard diffusion of about 10^7 m^2/s at large scales. A still open question is the origin of a k^{-5/3} spectrum in the mesoscale range, between 100 and 1000 km.Comment: 19 pages, 1 table + 5 (pdf) figure

Dispersion of passive tracers in model flows: effects of the parametrization of small-scale processes

Abstract. A set of numerical experiments is presented, in which we study the dynamics of passive particles advected by given two-dimensional velocity fields and perturbed by a non-white noise with a characteristic time τ. Data and model results have shown that this kind of random perturbation is able to represent subgridscale processes for upper ocean mesoscale turbulence for regions of the world ocean where turbulence can be assumed to be homogeneous. Extensive computations in different fields characterized by cell-like structure, both stationary and time-dependent, representing very idealized geophysical flow situations, show that the presence of a finite correlation time scale does lead to enhanced or arrested dispersion, depending on the considered flow; however, it does not seem to affect the gross qualitative behaviour of the dispersion processes, which is primarily affected by the large-scale velocity field

3D chaotic model for sub-grid turbulent dispersion in Large Eddy Simulations

We introduce a 3D multiscale kinematic velocity field as a model to simulate Lagrangian turbulent dispersion. The incompressible velocity field is a nonlinear deterministic function, periodic in space and time, that generates chaotic mixing of Lagrangian trajectories. Relative dispersion properties, e.g. the Richardson's law, are correctly reproduced under two basic conditions: 1) the velocity amplitudes of the spatial modes must be related to the corresponding wavelengths through the Kolmogorov scaling; 2) the problem of the lack of "sweeping effect" of the small eddies by the large eddies, common to kinematic simulations, has to be taken into account. We show that, as far as Lagrangian dispersion is concerned, our model can be successfully applied as additional sub-grid contribution for Large Eddy Simulations of the planetary boundary layer flow

Lagrangian drifter dispersion in the southwestern Atlantic Ocean

In the framework of Monitoring by Ocean Drifters (MONDO) Project, a set of Lagrangian drifters were released in proximity of the Brazil Current, the western branch of the Subtropical Gyre in the South Atlantic Ocean. The experimental strategy of deploying part of the buoys in clusters offers the opportunity to examine relative dispersion on a wide range of scales. Adopting a dynamical systems approach, we focus our attention on scale-dependent indicators, like the finite-scale Lyapunov exponent (FSLE) and the finite-scale (mean square) relative velocity (FSRV) between two drifters as function of their separation, and compare them with classic time-dependent statistical quantities like the mean square relative displacement between two drifters and the effective diffusivity as functions of the time lag from the release. We find that, dependently on the given observable, the quasigeostrophic turbulence scenario is overall compatible with our data analysis, with discrepancies from the expected behavior of 2D turbulent trajectories likely to be ascribed to the non stationary and non homogeneous characteristics of the flow, as well as to possible ageostrophic effects. Submesoscale features of O(1) km are considered to play a role, to some extent, in determining the properties of relative dispersion as well as the shape of the energy spectrum. We present, also, numerical simulations of an OGCM of the South Atlantic, and discuss the comparison between experimental and model data about mesoscale dispersion.Comment: 26 pages, 16 figure

Coupling Lagrangian simulation models and remote sensing to explore the environmental effect on larval growth rate: The Mediterranean case study of round sardinella (Sardinella aurita) early life stages

The relationship between environmental conditions and early life-history traits of Sardinella aurita are investigated using material collected in two sites of the Central Mediterranean Sea. Individual mean daily growth during the planktonic phase has been determined by using otolith microstructure analysis, while Lagrangian simulation models allowed to estimate the daily position in space and time of each specimen from the hatching to the catch. Generalized Additive Mixed Models (GAMMs) have been implemented to explore the impact of environmental conditions at time t, t-1 day and t-2 days on the mean daily growth rate occurring at time t. Spatial analysis evidenced a wide dispersion of eggs and larvae in the coastal area of both sampling sites in correspondence to relatively warmer and chlorophyll-a enriched waters. Lagrangian simulations detected a complementary larval dispersal pathway able to transport larvae to a known retention area. Temperature at time t was the most important driver affecting the mean daily larval growth, followed by the food availability. On the other hand, models performed on lagged environmental covariates (t-1 and t-2) did not show any significant effect on the growth rate at time t. In addition to the sub-linear positive correlation between temperature and mean daily larval growth, model highlighted a decrease in the otolith core width at higher temperature that can be linked to an earlier stage of ontogeny at hatching. This study provided a useful methodological approach that takes advantage of available remote sensing data to perform ecological studies in support to fisheries management

Characterization of non asymptotic properties Lagrangian transport

Dottorato di ricerca in fisica. 12. ciclo. A.a. 1998-99. Coordinatore Sergio Petrera. Tutore Guido ViscontiConsiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7 , Rome; Biblioteca Nazionale Centrale - P.za Cavalleggeri, 1, Florence / CNR - Consiglio Nazionale delle RichercheSIGLEITItal

Anisotropic Dispersion in Rotating Fluids. A Laboratory Model of Large Scale Flows

The variation of the Coriolis parameter with the latitude (beta effect) strongly affects the turbulent inverse cascade. In fact, when this effect holds, energy is preferentially transferred towards zonal modes. The consequent emergence of flow structures along the zonal direction can strongly impact transport and modify both zonal and meridional scalar diffusion. We performed a detailed analysis of Lagrangian tracers dispersion on rotating flows simulated in laboratory. A relatively wide range of the zonostrophy index, a parameter used to characterize flow regimes in the presence of a beta effect, is investigated. The degree of anisotropy and the characteristic scales of the flow have been estimated by means of the zonal and radial components of the Finite Scale Lyapunov Exponents. Moreover, we introduced the Lagrangian Anisotropy Index is in order to describe the onset of anisotropy and to verify the agreement with the theoretical predictions

Chaotic Lagrangian models for turbulent relative dispersion

A deterministic multiscale dynamical system is introduced and discussed as a prototype model for relativedispersion in stationary, homogeneous, and isotropic turbulence. Unlike stochastic diffusion models, heretrajectory transport and mixing properties are entirely controlled by Lagrangian chaos. The anomalous “sweepingeffect,” a known drawback common to kinematic simulations, is removed through the use of quasi-Lagrangiancoordinates. Lagrangian dispersion statistics of the model are accurately analyzed by computing the finite-scaleLyapunov exponent (FSLE), which is the optimal measure of the scaling properties of dispersion. FSLE scalingexponents provide a severe test to decide whether model simulations are in agreement with theoretical expectationsand/or observation. The results of our numerical experiments cover a wide range of “Reynolds numbers” and showthat chaotic deterministic flows can be very efficient, and numerically low-cost, models of turbulent trajectoriesin stationary, homogeneous, and isotropic conditions. The mathematics of the model is relatively simple, and, ina geophysical context, potential applications may regard small-scale parametrization issues in general circulationmodels, mixed layer, and/or boundary layer turbulence models as well as Lagrangian predictability studie