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Balanced tropical data assimilation based on a study of equatorial waves in ECMWF short-range forecast errors
Stockholm University, Faculty of Science, Department of Meteorology .
2005 (English)In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 131, no 607, 987-1011 p.Article in journal (Refereed) Published
Abstract [en]

This paper seeks to represent the tropical short-range forecast error covariances of the European Centre for Medium-RangeWeather Forecasts (ECMWF) model in terms of equatorial waves. The motivation for undertaking this investigation is increasing observational evidence indicating that a substantial fraction of the tropical largescale variability can be explained by equatorially trapped wave solutions known from shallow-water theory. Shortrange forecast differences from a data-assimilation ensemble were taken to serve as a proxy for background errors.

It was found that the equatorial waves coupled to convection can explain on average 60–70% of the error variance in the tropical free atmosphere. The largest part of this explained variance is represented by the equatorial Rossby (ER) modes, and a significant percentage pertains to the equatorial inertio-gravity (EIG) modes. Eastwardpropagating EIG modes have maximum variance in the stratosphere, where the short-wave variance in westwardmoving waves is particularly small. This feature is most likely related to the phase of the quasi-biennial oscillation during the study period, suggesting that significant temporal variations could be present in longer-term time series of such statistics.

The vertical correlations for ER modes display characteristics similar to those of their extratropical counterparts: correlations narrow towards shorter scales and in the stratosphere. However, the present statistics do not display the significant increase with altitude of the horizontal correlation scale for the height field which is typical for global, quasi-geostrophic statistics commonly used in current data-assimilation schemes. Furthermore, tropospheric ER correlations are vertically asymmetric and deeper for the n=1 mode than for higher modes.Most likely, deep convection, acting as a generator of equatorial wave motion, is the dominant mechanism underlying these results.

In spite of its relatively small contribution to the tropospheric variance, the Kelvin-wave coupling plays a decisive role for determining the characteristics of the horizontal correlation near the equator. EIG modes also play an important role for the tropical mass–wind coupling; these waves have a major impact by reducing the meridional correlation scales and the magnitudes of the balanced height-field increments.

Place, publisher, year, edition, pages
2005. Vol. 131, no 607, 987-1011 p.
Keyword [en]
Covariance modelling, Ensemble methods, Mass–wind coupling, Tropics, Variational data assimilation
URN: urn:nbn:se:su:diva-22782DOI: 10.1256/qj.04.54OAI: diva2:189438
Part of urn:nbn:se:su:diva-111Available from: 2004-04-28 Created: 2004-04-28 Last updated: 2010-08-16Bibliographically 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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