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Bottom friction estimation in the context of regional barotropic modelling
Martial Boutet, Cyril Lathuiliere, Hong Son Hoang, Remy Baraille, Yves Morel
SHOM
(Abstract received 04/28/2015 for session X)
ABSTRACT
At regional scale, barotropic tides are the dominant source of variability of currents and water heights. A precise representation of these processes is essential because of their great impacts on human activities. This work focuses on the most promising improvement: the bottom friction representation. The method used to estimate bottom friction is the simultaneous perturbation stochastic approximation (SPSA) which consists in the approximation of the gradient and the Hessian matrix based on a fixed number of cost function measurements, regardless of the dimension of the control vector. Indeed, each cost function measurement is obtained by randomly perturbing every component of the control vector. An important feature of SPSA is its relative ease of implementation. In particular, the method does not require the development of the tangent linear model and its adjoint. Experiments are carried out to estimate bottom friction with HYCOM in barotropic mode (one isopycnal layer). The study area is the Northeastern Atlantic margin which is characterized by strong currents and an intense dissipation. Bottom friction is parameterized with a quadratic term and the friction coefficient is computed with the water height and the bottom roughness. The latter parameter is the one to be estimated. Assimilated data are the available tide gauge observations. First, the bottom roughness is estimated taking into account bottom sediment natures. Then, it is estimated with geographical degrees of freedom. Finally, the impact of the estimation of a mixed quadratic/linear friction is evaluated.
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2015 LOM Workshop, Copenhagen, Denmark June 2nd - 4th, 2015