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Dynamical aspects of atmospheric data assimilation in the tropics
Stockholm University, Faculty of Science, Department of Meteorology.
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.
Keyword [en]
tropical data assimilation, variational methods, mass-wind coupling
National Category
Meteorology and Atmospheric Sciences
Identifiers
URN: urn:nbn:se:su:diva-111ISBN: 91-7265-867-3 (print)OAI: oai:DiVA.org:su-111DiVA: diva2:189439
Public defence
2004-05-19, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 8 C, Stockholm, 10:00
Opponent
Supervisors
Available from: 2004-04-28 Created: 2004-04-28Bibliographically approved
List of papers
1. Dynamical response of equatorial waves in variational data assimilation
Open this publication in new window or tab >>Dynamical response of equatorial waves in variational data assimilation
2004 (English)In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 56A, no 1, 29-46 p.Article in journal (Refereed) Published
Abstract [en]

In this study we question the relative importance of direct wind measurements in the tropics by investigating limits of four-dimensional variational assimilation (4D-Var) in the tropics when only wind or mass field observations are available. Typically observed equatorial wave motion fields (Kelvin, mixed Rossby-gravity and n= 1 equatorial Rossby waves) are assimilated in a non-linear shallow water model. Perfect observations on the full model grid are utilized and no background error term is used. The results illustrate limits of 4D-Var with only one type of information, in particular mass field information. First, there is a limit of information available through the internal model dynamics. This limit is defined by the length of the assimilation window, in relation to the characteristics of the motion being assimilated. Secondly, there is a limit related to the type of observations used. In all cases of assimilation of wind field data, two or three time instants with observations are sufficient to recover the mass field, independent of the length of the assimilation time window. Assimilation of mass field data, on the other hand, although capable of wind field reconstruction, is much more dependent on the dynamical properties of the assimilated motion system. Assimilating height information is less efficient, and the divergent part of the wind field is always recovered first and more completely than its rotational part.

Identifiers
urn:nbn:se:su:diva-22779 (URN)10.1111/j.1600-0870.2004.00036.x (DOI)
Note
Part of urn:nbn:se:su:diva-111Available from: 2004-04-28 Created: 2004-04-28 Last updated: 2017-12-13Bibliographically approved
2. Variational data assimilation in the tropics: the impact of a background error constraint
Open this publication in new window or tab >>Variational data assimilation in the tropics: the impact of a background error constraint
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.

Keyword
Equatorial waves, Mass and wind fields, 4D-Var
Identifiers
urn:nbn:se:su:diva-22780 (URN)10.1256/qj.03.13 (DOI)
Note
Part of urn:nbn:se:su:diva-111Available from: 2004-04-28 Created: 2004-04-28 Last updated: 2017-12-13Bibliographically approved
3. Assimilation of equatorial waves by line of sight wind observations
Open this publication in new window or tab >>Assimilation of equatorial waves by line of sight wind observations
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.

Identifiers
urn:nbn:se:su:diva-22781 (URN)
Note
Part of urn:nbn:se:su:diva-111Available from: 2004-04-28 Created: 2004-04-28 Last updated: 2017-12-13Bibliographically approved
4. Balanced tropical data assimilation based on a study of equatorial waves in ECMWF short-range forecast errors
Open this publication in new window or tab >>Balanced tropical data assimilation based on a study of equatorial waves in ECMWF short-range forecast errors
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.

Keyword
Covariance modelling, Ensemble methods, Mass–wind coupling, Tropics, Variational data assimilation
Identifiers
urn:nbn:se:su:diva-22782 (URN)10.1256/qj.04.54 (DOI)
Note
Part of urn:nbn:se:su:diva-111Available from: 2004-04-28 Created: 2004-04-28 Last updated: 2017-12-13Bibliographically approved

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