Geoscience Reference
In-Depth Information
Chapter 15
A Weak Constraint 4D-Var Assimilation
System for the Navy Coastal Ocean Model
Using the Representer Method
Hans Ngodock and Matthew Carrier
Abstract A 4D-Variational system was recently developed for assimilating ocean
observations with the Navy Coastal Ocean Model. It is described here, along
with initial assimilation experiments in the Monterey Bay using a combination of
real and synthetic ocean observations. For testing a new assimilation system it is
advantageous to use this combination of real and synthetic data over simplified
cases of climatology and twin data. Assimilation experiments are carried out in
a weak constraint formulation, with the model's external forcing assumed to be
erroneous in addition to initial conditions. The system's ability to fit assimilated and
non assimilated observations is assessed, as well as the consistency and relevance of
the retrieved model forcing. Experiment results show that the assimilation system
fits the data with relatively high prior errors in the initial conditions and surface
forcing fluxes. However, the retrieved model forcing errors are well within the range
of acceptable corrections according to an independent study.
15.1
Introduction
This paper presents the development of a weak constraint 4D-Var data assimilation
system based on the representer method ( Bennett 1992 , 2002 ) for the Navy Coastal
Ocean Model (NCOM). NCOM is an operational ocean model that has been
validated ( Martin 2000 ; Barron et al. 2006 ). A major effort to implement state-of-
the-art assimilation schemes was undertaken a few years ago, with the development
of a 3DVAR, and a 4D-Var system based on the NCOM numerical code. The
3DVAR system is used for assimilation in global to regional scales, while the 4D-Var
is to be used in limited area models with in-situ observations, provided initial and
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