Development of NAVDAS-AR: formulation and initial tests of the linear problem

A 4-D implementation of an observation space variational data assimilation system is under development at the Marine Meteorology Division of the Naval Research Laboratory (NRL). The system is an extension of the current US Navy 3-D operational data assimilation system, the NRL Atmospheric Variational Data Assimilation System (NAVDAS). The new system, NAVDAS-AR, where AR stands for accelerated representer, is similar in many respects to the European Centre for Medium-Range Weather Forecasts (ECMWF) 4DVAR system. However, NAVDAS-AR is based on a weak constraint observation space, while the ECMWF system is based on a strong constraint model space. In this paper the formulation of NAVDAS-AR is described in detail and preliminary results with a perfect model assumption and comparisons with the operational NAVDAS are presented.