 |
| Titel |
Representing model error in ensemble data assimilation |
| VerfasserIn |
C. Cardinali, N. Žagar, G. Radnoti, R. Buizza |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1023-5809
|
| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 21, no. 5 ; Nr. 21, no. 5 (2014-09-23), S.971-985 |
| Datensatznummer |
250120942
|
| Publikation (Nr.) |
 copernicus.org/npg-21-971-2014.pdf |
|
|
|
|
|
| Zusammenfassung |
The paper investigates a method to represent model error in the ensemble data
assimilation (EDA) system. The ECMWF operational EDA simulates the effect of
both observations and model uncertainties. Observation errors are represented
by perturbations with statistics characterized by the observation error
covariance matrix whilst the model uncertainties are represented by
stochastic perturbations added to the physical tendencies to simulate the
effect of random errors in the physical parameterizations (ST-method). In
this work an alternative method (XB-method) is proposed to simulate model
uncertainties by adding perturbations to the model background field. In this
way the error represented is not just restricted to model error in the usual
sense but potentially extends to any form of background error. The
perturbations have the same correlation as the background error covariance
matrix and their magnitude is computed from comparing the high-resolution
operational innovation variances with the ensemble variances when the
ensemble is obtained by perturbing only the observations (OBS-method). The
XB-method has been designed to represent the short range model error relevant
for the data assimilation window. Spread diagnostic shows that the XB-method
generates a larger spread than the ST-method that is operationally used at
ECMWF, in particular in the extratropics. Three-dimensional normal-mode
diagnostics indicate that XB-EDA spread projects more than the spread from
the other EDAs onto the easterly inertia-gravity modes associated with
equatorial Kelvin waves, tropical dynamics and, in general, model error
sources.
The background error statistics from the above described EDAs have been
employed in the assimilation system. The assimilation system performance
showed that the XB-method background error statistics increase the
observation influence in the analysis process. The other EDA background error
statistics, when inflated by a global factor, generate analyses with
30–50% smaller degree of freedom of signal. XB-EDA background error
variances have not been inflated.
The presented EDAs have been used to generate the initial perturbations of
the ECMWF ensemble prediction system (EPS) of which the XB-EDA induces the
largest EPS spread, also in the medium range, leading to a more reliable
ensemble. Compared to ST-EDA, XB-EDA leads to a small improvement of the EPS
ignorance skill score at day 3 and 7. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|