Change search
ReferencesLink to record
Permanent link

Direct link
Variational data assimilation in the tropics: the impact of a background error constraint
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
2004 (English)In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 130, 103-125 p.Article in journal (Refereed) Published
Abstract [en]

Geostrophic adjustment theory predicts that wind information is a primary information source in the tropics. We ask whether this is true when variational data assimilation including a background-error constraint is applied. The question is investigated by carrying out three- and four-dimensional variational (4D-Var) data assimilation experiments with nonlinear shallow-water equations and idealized waves. The contribution to the analysis from mass and wind field observations is contrasted to that of spectral characteristics and multivariate relationships of equatorial waves built into a background-error covariance matrix. A background-error term for the analysis is built by using the tropical eigenmodes and the observed variability is used to determine the relative weights for different modes.

Single observation sensitivity experiments illustrate an important impact of Kelvin and mixed Rossby-gravity waves in reducing the coupling between the mass and wind fields, found when only equatorial Rossby waves are utilized in the background-error constraint.

The assimilation experiments demonstrate two main features. First, the wind field information is of greater value than the mass field information, in spite of the known error statistics. A second intriguing feature is that the assimilation of wind observations better resolves smaller-scale features of the height field than the assimilation of height data. When height data are used, a somewhat different analysis response is obtained for the zonal and meridional wind components, due to the strong projection onto the Kelvin wave structure for height observations. Finally, a comparison of different time windows for 4D-Var illuminates the sensitivity of height field assimilation to the length of the assimilation window.

Place, publisher, year, edition, pages
2004. Vol. 130, 103-125 p.
Keyword [en]
Equatorial waves, Mass and wind fields, 4D-Var
URN: urn:nbn:se:su:diva-22780DOI: 10.1256/qj.03.13OAI: diva2:189436
Part of urn:nbn:se:su:diva-111Available from: 2004-04-28 Created: 2004-04-28 Last updated: 2010-08-03Bibliographically approved
In thesis
1. Dynamical aspects of atmospheric data assimilation in the tropics
Open this publication in new window or tab >>Dynamical aspects of atmospheric data assimilation in the tropics
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A faithful depiction of the tropical atmosphere requires three-dimensional sets of observations. Despite the increasing amount of observations presently available, these will hardly ever encompass the entire atmosphere and, in addition, observations have errors. Additional (background) information will always be required to complete the picture. Valuable added information comes from the physical laws governing the flow, usually mediated via a numerical weather prediction (NWP) model. These models are, however, never going to be error-free, why a reliable estimate of their errors poses a real challenge since the whole truth will never be within our grasp.

The present thesis addresses the question of improving the analysis procedures for NWP in the tropics. Improvements are sought by addressing the following issues:

- the efficiency of the internal model adjustment,

- the potential of the reliable background-error information, as compared to observations,

- the impact of a new, space-borne line-of-sight wind measurements, and

- the usefulness of multivariate relationships for data assimilation in the tropics.

Most NWP assimilation schemes are effectively univariate near the equator. In this thesis, a multivariate formulation of the variational data assimilation in the tropics has been developed. The proposed background-error model supports the mass-wind coupling based on convectively-coupled equatorial waves. The resulting assimilation model produces balanced analysis increments and hereby increases the efficiency of all types of observations.

Idealized adjustment and multivariate analysis experiments highlight the importance of direct wind measurements in the tropics. In particular, the presented results confirm the superiority of wind observations compared to mass data, in spite of the exact multivariate relationships available from the background information. The internal model adjustment is also more efficient for wind observations than for mass data.

In accordance with these findings, new satellite wind observations are expected to contribute towards the improvement of NWP and climate modeling in the tropics. Although incomplete, the new wind-field information has the potential to reduce uncertainties in the tropical dynamical fields, if used together with the existing satellite mass-field measurements.

The results obtained by applying the new background-error representation to the tropical short-range forecast errors of a state-of-art NWP model suggest that achieving useful tropical multivariate relationships may be feasible within an operational NWP environment.

Place, publisher, year, edition, pages
Stockholm: Meteorologiska institutionen (MISU), 2004. 45 p.
tropical data assimilation, variational methods, mass-wind coupling
National Category
Meteorology and Atmospheric Sciences
urn:nbn:se:su:diva-111 (URN)91-7265-867-3 (ISBN)
Public defence
2004-05-19, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 8 C, Stockholm, 10:00
Available from: 2004-04-28 Created: 2004-04-28Bibliographically approved

Open Access in DiVA

No full text

Other links

Publisher's full text

Search in DiVA

By author/editor
Källén, Erland
By organisation
Department of Meteorology
In the same journal
Quarterly Journal of the Royal Meteorological Society

Search outside of DiVA

GoogleGoogle Scholar
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Altmetric score

Total: 29 hits
ReferencesLink to record
Permanent link

Direct link