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Assimilation of equatorial waves by line of sight wind observations
Stockholm University, Faculty of Science, Department of Meteorology .
2004 (English)In: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 61, no 15, 1877-1893 p.Article in journal (Refereed) Published
Abstract [en]

This paper investigates the potential of line-of-sight (LOS) wind information from a spaceborne Doppler wind lidar to reduce uncertainties in the analysis fields of equatorial waves. The benefit of LOS winds is assessed by comparing their impact to that of a single wind component, full wind field information, and mass field data in three- and four-dimensional variational data assimilation.

The dynamical framework consists of nonlinear shallow-water equations solved in spectral space and a background error term based on eigenmodes derived from linear equatorial wave theory. Based on observational evidence, simulated wave motion fields contain equatorial Kelvin, Rossby, mixed Rossby–gravity, and the lowest two modes of the westward-propagating inertio–gravity waves. The same dynamical structures are included, entirely or partially, into the background error covariance matrix for the multivariate analysis. The relative usefulness of LOS data is evaluated by carrying out “identical twin” observing system simulation experiments and assuming a perfect model.

Results from the experiments involving a single observation or an imperfect background error covariance matrix illustrate that the assimilation increments due to LOS wind information rely more on the background error term specification than the full wind field information. This sensitivity is furthermore transferred to the balanced height field increments.

However, all assimilation experiments suggest that LOS wind observations have a capability of being valuable and need supplemental information to the existing satellite mass field measurements in the Tropics. Although the new wind information is incomplete, it has a potential to provide reliable analysis of tropical wave motions when it is used together with the height data.

Place, publisher, year, edition, pages
2004. Vol. 61, no 15, 1877-1893 p.
URN: urn:nbn:se:su:diva-22781OAI: diva2:189437
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

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