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Towards a Coastal Ocean data assimilation system of ocean surface data and vertical profiles of temperature in Brazil
Jean F. de Oliveira, Clemente A. S. Tanajura and Edmo J. D. Campos
Diretoria de Hidrografia e Navegação da Marinha do Brasil (DHN)
(Abstract received 05/08/2007 for session X)
ABSTRACT
Data assimilation methods are important numerical tools in meteorology and oceanography. They combine observational data and numerical model outputs to produce a new field in the model grid, the so-called objective analysis, with smaller errors than the model output. The analysis is used as initial condition for weather and climate models and as data for diagnostic studies. Also, it complements the monitoring of the physical state of the oceans and the atmosphere since it “extrapolates” information from regions with observations to regions in which there is lack of data. In order to support the activities of the off-shore oil industry along the Brazilian coast, the Brazilian oil company PETROBRAS is organizing a network of research institutions and universities to implement an operational coastal ocean forecast system along the Brazilian shore together with the Brazilian Navy (see http://labmon.io.usp.br/remo). One of the core ocean circulation models to be used in this system is HYCOM. This model uses hybrid vertical coordinates, i.e., it uses z-coordinates in the mixed layer, isopycnal coordinates in the deep ocean and sigma-z coordinates in the continental shelf. This flexibility is quite attractive since the properties of water masses in the deep ocean can be better maintained without compromising the representation of turbulent processes in the mixed layer and in the continental shelf. However, since vertical profiles of temperature are observed by moored and drifting buoys in z-coordinates, the assimilation of these data into HYCOM is not trivial. In the present work, a technique to transform vertical profiles of temperature from isopycnal coordinates to Cartesian coordinates and vice-versa is being applied. The transformation is required to the implementation of data assimilation in HYCOM and to validate the model simulations. The transformation uses Lagrangian multipliers and a minimization process that guarantees the conservation of the barotropic mass flux. The accuracy of the transformation is shown to be very high, with errors smaller than 0.1oC when HYCOM is discretized in 21 vertical layers and 125 z-layers are used in the transformation. For the assimilation of surface data, the scheme proposed by Ezer and Mellor (1997) is applied. The scheme uses statistical interpolation and correlations between the sea surface data – temperature and height – and subsurface temperature structure and the thickness of isopycnal layers. Preliminary results obtained in a climatological run show the data assimilation method is effective in the correction of the model output toward observations, by reducing the model error in more than 50%. Also, when both surface data are used, the efficiency of the assimilation is higher in relation to considering only one of the surface data.
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2007 LOM Workshop, Bergen, Norway, August 20-22, 2007