Paulo C. Leitão

3D modelling in the Sado estuary using a new generic vertical discretization approach

Abstract Sado estuary (Portugal) is a mesotidal well-mixed estuary. The flow however, displays strong three-dimensional features associated with bathymetry variations. The flow is simulated using a three-dimensional primitive equation model based on the finite-volume method and incorporating a new concept for the vertical discretization. The innovative features of the model are analysed and the advantages of the finite-volume method to implement the generic vertical discretization are put into evidence. The model was validated using water level, velocity and salinity measurements in several stations along the estuary. Results show the influence of main channels strong curvature on the generation of secondary flows inside the estuary. The steep bathymetry of the outer platform gives rise to a recirculation flow in the vertical plane that lasts for most of the tidal cycle. This structure is considered important for the sediment transport in that region. The above mentioned non-linear effects leave their print in both the transient and residual velocity fields. The results obtained help to understand the role of three-dimensional structures on the water exchange and sediment transport in the estuary. The results also confirm the suitability of the approach used in this model to simulate estuarine and coastal flows with strong three-dimensional effects.

A model for ocean circulation on the Iberian coast

We used a three-dimensional model with generic vertical coordinate to investigate the structure of flow and transports on the Iberian shelf-slope region. Two different types of experiments were conducted. The first was a process-oriented study to investigate the role of thermohaline forcing in the generation and meridional variability of the poleward slope current. The second experiment was a more realistic simulation for the year of 1994, a period for which there are several long-term current meter data sets available. Comparisons between model, data and other published works have shown a reasonable agreement both qualitatively and quantitatively. The results obtained included the seasonal cycle with upwelling during the summer and a winter surface poleward current over the shelf, with a permanent undercurrent transporting Mediterranean Water along the Portuguese and Spanish slopes. The along-slope transports were predominant especially in the OMEX II–II study area. The meridional transport is poleward all year round, decreasing from winter to summer and from the south to the north. D 2002 Elsevier Science B.V. All rights reserved.

A two-dimensional particle tracking model for pollution dispersion in A Coruna and Vigo Rias (NW Spain)

A two-dimensional hydrodynamic model coupled to a particle tracking model is applied to study the dispersion processes and residence time in two Galician rias (A Coruna and Vigo, NW Spain) under summer conditions. In A Coruna a long residence time was found in the harbour area due to the existence of a dock, and a short one in the river area. On the contrary, in Vigo, the residence time is smaller in the harbour area, due to the Rande Strait, beyond which the river effect is negligible.

Modelling the main features of the Algarve coastal circulation during July 2004: A downscaling approach

This article presents a methodology for simulating the Algarve coastal circulation using realistic forcing (e.g. low-frequency circulation, tide, high-resolution atmospheric forcing). Low-frequency open boundary conditions are defined via a downscaling of the HYCOM-US operational solution for the Gulf of Cadiz. Atmospheric forcing is imposed using the MM5 high-resolution mesoscale model (9 km resolution near the coast). A 3-level nesting system based on the MOHID numerical system is implemented. The higher nesting level has a horizontal resolution of 0.02° along the Algarve coast. The methodology is first validated qualitatively. A comparison between the numerical results and the conceptual model of the circulation described in the literature is presented. A quantitative validation is also performed, based mainly on remote sensing data (sea surface temperature and altimetry) available for July 2004. The numerical system is able to reproduce many of the circulation features described in the literature (e.g...

Short Note: Application of the Mohid-2D model to a mesotidal temperate coastal lagoon

1. IntroductionA mathematical model can be considered as anapproximate reconstruction of a real phenomenon.All parameterizations and approximations used inmodels lead to deviations of the model results fromnature. It is an accepted requirement that anumerical model of estuarine hydrodynamicsshould be verified, calibrated and validated beforeused in a practical application. However, theprocedures to perform these tasks are not widelyaccepted (Cheng et al., 1991). Calibration andvalidation methods appear in several forms, de-pending on data availability, water mass character-istics and researchers’ opinion (Hsu et al., 1999).In this work, the Mohid-2D model implementa-tion for the Ria de Aveiro lagoon is presented,describing its assessment through calibration andvalidation against several different data sets. Due tothe lagoon complex geometry and the large numberof calibration stations used, this goal constitutes avery challenging task.The model is calibrated using as a first approach aqualitative comparison of the temporal evolution ofsea surface elevation (SSE) data measured in 1987/1988 at several locations. When a good match isobtained for all stations, the model’s accuracy isevaluated through the determination of the rootmean square (RMS) error and also through thecomparison between amplitude and phase of themain tidal constituents determined from harmonicanalysis of the observed and computed data. Thevalidation procedure is performed using two in-dependent data sets, which includes observations ofcurrent velocities and SSE values (1997 data) andmeasured water fluxes at the lagoon’s inlet for theperiod of October 2002.2. The study areaRia de Aveiro (Fig. 1) is a shallow mesotidallagoon located in the Northwest coast of Portugalð40 38

MOHID 2000 - A coastal integrated object oriented model

Mathematical modeling, although a recent science, can be considered old when the time scale used to measure its age is the information technology evolution. FORTRAN 77 has been the most popular programming language among hydrodynamic modelers over the last two decades. This language was adequate to the (low) complexity of the models, limited by the exiting computers. The fast increase of the computing capacity allowed the development of more complex and multidisciplinary models. The necessity of new programming techniques became clear. It was started the era of the Object Oriented programming. Object Oriented programming isolates different parts of the code and allows communication among them using simple and robust interfaces. MOHID 2000 is an integrated modeling tool programmed in FORTRAN 95, using an Object Oriented strategy. It is the evolution of a set of models originally programmed in FORTRAN 77. Taking advantage of the new language possibilities, with a single code it is possible to perform different simulations (1D, 2D or 3D) and use a variety of vertical discretizations. This technique has also turned easy to integrate hydrodynamic, water quality, sediment transport and pollutants dispersion simulations using eulerian or lagrangian formulations. Having in mind the development of a tool accessible to the final user, the modeler must think about on the algorithms and code, but also on pre and post processing of the model data. MOHID 2000 input data files can be constructed by a graphical user interface. Output files use the standard HDF format making results exploring simple and allows the use of different graphic packages.

A circulation model for the European ocean margin

Abstract This paper presents a new numerical model for the study of circulation in coastal and deep ocean regions. Sigma coordinate models are appropriate where currents follow topography, but there are problems of numerical diffusion in stratified areas. Some of these problems can be partly overcome by applying instead a cartesian model, whose outcome, however, is dependent on variable ocean depth. As a compromise between these different models with their varying advantages and disadvantages, a double sigma transformation model offers perhaps the best solution. Accordingly, in the simulations presented in this paper, the double sigma coordinate model was applied as a diagnostic tool to calculate features of ocean circulation off the coast of Western Europe, particularly in the area between La Chapelle Bank and Porcupine Bank. Calculations derived from the model indicated density-driven poleward currents continuous from the Portuguese to the Irish shelves in accordance with actual observations and numerical results available. The model also indicates that seasonal winds strongly modify flow over the shelf and upper slope.

The object-oriented design of the integrated water modelling system MOHID

With increasing computer power, the modelling tools for water resources, nowadays, not only integrate physical-based transport models with biogeochemical process models, but also couple surface water body models, groundwater models and hydrographic basin models. This paper describes how the MOHID Water Modelling System achieves the mentioned integration using object-oriented programming in FORTRAN 95. After a short historical overview of MOHIDs evolution, this paper focuses on the object-oriented design of the water modelling system MOHID and the way how object-oriented features are implemented in FORTRAN 95. Attention is also given to the way how the numerical software code interacts with the graphical user interface. It follows an exemplification of how the object-oriented design is useful for implementing complex system for two cases: the operational model of the Tagus Estuary and the coupling of the Trancao Basin model to the Tagus Estuary model.

Simulating vertical water mixing in homogeneous estuaries: the SADO Estuary case

In this article, a three-dimensional primitive equation model is applied to Sado Estuary, which is a meso-tidal homogeneous estuary. Despite the homogeneity of its density field, the flow presents important vertical structures. This article shows that the common practice of using two-dimensional depth-integrated models in homogeneous estuaries can filter important vertical hydrodynamic features, such as secondary and circulating flows. The physical processes associated with these flows are analysed. The strong curvature of the channels and the strong bathymetry gradients were identified as the principal morphologic features of the estuary responsible for the generation of the flows. The model possesses a generic vertical mesh geometry approach, which is used to test the sensibility of the model to the vertical discretization when simulating these flows.

From regional to local scale modelling on the south-eastern Brazilian shelf: case study of Paranaguá estuarine system

A aplicabilidade de um modelo numerico seguindo uma metodologia de downscaling foi avaliada para a plataforma sudeste brasileira (modelo regional) e o sistema estuarino de Paranagua (modelo local). Esta abordagem permite simular fenomenos de escalas diferentes, como mare meteorologica e ressurgencia costeira, e e apropriada para fins de previsao operacional. Quando grandes areas sao cobertas por modelos regionais, a propagacao da mare no interior do dominio pode ser afetada significativamente pelo potencial local da mare, principalmente onde o fenomeno de ressonância e observado. A plataforma sudeste brasileira e conhecida pela ressonância da constituinte da mare lunar principal terci-diurna (M3), com as maiores amplitudes encontradas no sistema estuarino de Paranagua. Por este motivo, a significância do potencial local da mare foi verificada neste estudo para as constituintes da mare mais importantes dentro do sistema estuarino (incluindo a M3). A validacao do modelo foi realizada com dados de maregrafos, perfis de boias Argo e medicoes de satelite da temperatura da superficie do mar. A metodologia descrita neste estudo pode ser replicada para outros sistemas estuarinos importantes localizados na plataforma sudeste brasileira. Alem disso, o modelo numerico foi desenvolvido na perspectiva de simulacoes operacionais em nowcast/forecast, uteis para diversas atividades, como navegacao e resposta a emergencias (e.g., derrames de petroleo).