Scientific strategy for the implementation of a 3D-VAR data assimilation scheme for a double-nested limited area model

Steluta ALEXANDRU, updated on November 16, 2001


The three-dimensional variational data assimilation (3D-VAR) scheme for ALADIN was first implemented in 1999 in Toulouse Siroka and Horanyi, 1999). Then in 2000 the scheme was successfully ported to Budapest (Horanyi, 2000) for the Origin 2000 machine of the Hungarian Meteorological Service (HMS). Recently AL13 code version of ALADIN is used for 3D-VAR taking into account classical NMC statistics for the background term and SYNOP and TEMP observations for the observational term of the cost function. It is mentioned that 3D-VAR is acting for the upper-air meteorological fields and optimal interpolation (CANARI) analysis is applied for the surface variables. Recently new background error statistics were computed based on the lagged constant coupling formulation of the NMC method (Horanyi and Boloni, 2001). Intensive sensitivity experiments were performed with the lagged method in order to see the effect of the different forecast differences and integration lengths on the computed background error covariances. All the ingredients mentioned above will serve as a basis for the work to be carried out in Budapest by Steluta.


Design and implementation of a 3D-VAR data assimilation system around the ALADIN/HU (Horanyi et al, 1996) mesoscale numerical weather prediction model. The initial version of the system should be not worse than the recently operational dynamical adaptation system and should contain several promising developmental areas for the further perfecting of the system. For the end of the work in Budapest the developed 3DVAR system should be robust and superior to the dynamical adaptation technique in terms of consecutive forecasts originating from the 3D-VAR provided initial conditions. The system should be capable for the easy and smooth transition towards 4D-VAR. The research and development areas needed for 4D-VAR should be clearly distinguished and possible solutions should be addressed.


1. Observations

Recently only SYNOP and TEMP observations are taken into account in the course of the 3D-VAR scheme in Budapest. New data sources should be investigated (e.g. satellite and/or radar data, AIREP data etc.) and impact studies should performed. Alternatively observation information from Toulouse can be taken and then only additional local information can be added to that file. The recent version using SYNOP and TEMP measurements is considered to be the reference and it should be carefully tested whether adding new data the overall quality of the forecasts starting from 3D-VAR analysis is improved. It means that this study should be performed in a successive way. A possible direction of attack is as follows:

It is noted that there are also several technical issues related to the observation handling. Roger Randriamampianina will be in charge of supporting Steluta in the technical problems related to observations.

2. Background error statistics

Several sets of background error statistics are available for the ALADIN/HU domain. Choices should be done between classical (standard) statistics and lagged statistics. In the latter case the best variant of the lagged statistics should be applied. This part of the work will be carried out together with Gergely Boloni, who would complete the started work on the lagged statistics. It shouldn't be forgotten that there is a strong inter-relation between the choice of background errors and first guess (for instance in case of lagged statistics, when large scales are removed from the covariances the first guess should contain information coming from the large scales). See more details in the next point, where the different possibilities for the first guess is explained.

3. First guess

It is not obvious at all what first guess to use in the context of mesoscale data assimilation. It is rather clear for global models, because there only the 6h forecast from a previous model state can be applied. In mesoscale one of the following three possibilities might be chosen:

4. Role of initialisation

The first tests with initialisation and 3D-VAR was performed by Adam Dziedzic (Dziedzic, 2000) in Toulouse. Adam found that DFI in its classical formulation might destroy some useful signal of the 3D-VAR increments. At that time it was decided to try to use the incremental version of DFI and see if it is a more appropriate filter. Based on this work the following questions should be addressed:

5. Effect of double nesting

It is of scientific interest to study the effect of double nesting in the 3D-VAR data assimilation scheme. Experiments can be performed with the systematic comparison of 3D-VAR ALADIN, which is coupled directly from ARPEGE and coupled from ALADIN/LACE. Sensitivity tests should be carried out with respect to the domain size, resolution and coupling frequency.

6. Validation and tuning tools

It is of great importance to create an effective validation framework on the testing of all the different options tried for ALADIN 3D-VAR. The following main tools are proposed:

7. Towards 4D-VAR

At the end of the work on 3D-VAR some outlook should be given on the tasks to be carried out for the 4D-VAR implementation.


Hereafter some schedule is planned to be drawn with special emphasis and details on the first stay of Steluta.

1. 1st of November 2001 - 31st of May, 2002 (7 months)

Outcome: Replacement of dynamical adaptation with a scientifically sound 3D-VAR scheme.

2. 1st of September, 2002 - 31st of March, 2003

3. 1st of June, 2003 - 30th of November 2003


The work will be conducted by Andras Horanyi.

Strong cooperation with the local NWP team of HMS, especially with Gergely Boloni for background error statistics related questions and Roger Randriamampianina with observation handling related matters. Sandor Kertesz will give some support on screening questions and Gabor Radnoti on blending and code related issues. Afterall it is expected that a strong team (with Steluta, Gergely and Roger in its core) will be out together and this team will be capable to carry out together all the scientific and technical work described above.

It is highly expected that frequent e-mail discussions should take part with other ALADIN colleagues working on 3D-VAR. Hereafter the main subjects and persons to discuss with will be summarised:



All matters

Claude Fischer, Maria Siroka, Andras Horanyi

Strategic decisions

Jean-Francois Geleyn


Loik Berre, Gergely Boloni, Simona Stefanescu


Gabor Radnoti, Radmila Brozkova


Patrick Moll, Roger Randriamampianina


Parick Moll, Sandor Kertesz

Posteriori validation

Wafaa Sadiki

Code related matters

Gabor Radnoti