Monday, 12 January 2004: 2:00 PM
The impact of a wind-mass error analysis scheme on forecast skill
Room 6A
The data assimilation system of the Global Modeling and Assimilation Office
at NASA/GSFC applies a multivariate wind-height error covariance in the
analysis. This covariance model can in principle accommodate a flexible three-
dimensional specification of the error since the analysis equation is solved
in physical and observation space. There are two components of the wind error:
One is coupled to the geopotential height error through a basic mass-wind
balance relation, while the other component is mass-decoupled and thus by
definition uncorrelated with errors in the geopotential height field. The
uncoupled error is further specified as a sum of uncorrelated rotational
and divergent components, respectively. There are a couple of different
methods that are widely used to provide estimates of parameters used in
covariance models for meteorological analysis schemes. However, each of
these methods has its own limitations. It is therefore necessary to
test the estimates in assimilation/forecast experiments and modify them
based on empirical considerations where necessary. We describe an
improved wind-mass error analysis scheme in which the parameter estimates
are based on the statistics of the observation-minus-forecast residuals.
The impact of the scheme is illustrated by results from a series of
assimilation and forecast experiments. The assessment of these results
includes both statistical and dynamical aspects. The former is based on
the bias and standard deviation of the short-range forecast error as well
as standard medium-range skill scores, while the latter is based on the
representation of features and phenomena of the atmospheric circulation
in the analysis and forecast fields.
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