TITLE "Theory and Applications of Variational, Sequential and Bayesian Hierarchical Methods in Data Assimilation" ABSTRACT Data assimilation methods that combine observations with dynamical models of the atmosphere and ocean rely on estimating the conditional probability distribution function (PDF) of the dynamical system conditioned on an observed state provided by measurements. Constructing such a PDF is impractical for most problems with high dimensions in the state vector and requires simplifications in the estimations of the conditional PDF. We compare the theories underlying several data assimilation methods (e.g. variational, ensemble, particle, bayesian hierarchical) and show how their estimate of the conditional PDF differs. We also present an example of a variational data assimilation experiment where the ocean surface winds associated with the transit of a hurricane are reconstructed using ocean buoy data. Emanuele Di Lorenzo School of Earth and Atmospheric Sciences Georgia Institute of Technology Atlanta, GA, 30332-0340