The ability of four-dimensional variational (4DVAR) assimilation of data to
reduce various observational error structures in a quasigeostrophic model
is studied. It is found that 4DVAR with assimilation periods on the order o
f a week is very efficient at reducing error in phase space directions that
have not amplified in the past, that is, those phase space directions that
do not lie on the unstable manifold of the system. This is particularly tr
ue for observational errors that project in rapidly growing singular vector
phase space directions.
In general, long period 4DVAR changes the forecast error growth rates to ra
tes similar to the leading Lyapunov exponents for the system. However, erro
r structures that grow significantly faster than the leading Lyapunov vecto
r and are not readily reduced by long period 4DVAR can be constructed by do
ing a singular vector decomposition in the subspace of growing backward Lya
punov vectors. This procedure is an approximation to calculating the singul
ar vectors using an appropriate analysis error covariance metric for the as
similation technique. 4DVAR acting on observational errors constructed in t
his manner yields forecast error growth a factor of 5 larger than that of t
he leading Lyapunov vector over a 4-day forecast period.
The addition of model error places limits on the application of long assimi
lation period 4DVAR. Model error adds a background level of error to the as
similated solution that cannot be reduced, and also limits how far into the
past the assimilation period can be extended. These effects combine to red
uce the quality of the optimal assimilated state that can obtained by apply
ing 4DVAR. However, model error does not diminish the ability of long assim
ilation period 4DVAR to reduce rapidly growing singular vector-type error c
omponents. Since long assimilation periods can potentially produce large an
alysis errors if model error exists, the relative benefit of extending the
assimilation period to reduce forecast error growth rates must be weighed i
n a given situation.