Some Experimental Lessons on Digital Filtering in the ALADIN-France 3DVAR Based on Near-Ground Examination.
Fischer, C. ; Auger, L.
This paper deals with the characteristics and effects of digital filter initialization, as implemented in the
operational three-dimensional variational data assimilation (3DVAR) system of the Aire Limite´e Adaptation<br>Dynamique De´veloppement International (ALADIN)-France regional weather forecast model.<br>First, a series of findings on the properties of the initialization of the model are discussed. Examples of initial<br>spinup linked with inertiagravity wave occurrence are shown, and the major sources for their generation are<br>listed. These experimental results are compared with past and present experiences concerning the use and<br>need for digital filter initialization. Furthermore, the impacts of switching to an incremental formulation of the<br>filter in data assimilation mode are demonstrated. Second, the effects of the filter formulation on the results of<br>an observation impact study are illustrated. The latter consists of implementing screen-level, 10-m horizontal<br>wind information into the ALADIN 3DVAR analysis. There can, indeed, be some delicate interference<br>between observation impact evaluation and the effects of filtering, at least on short-term forecasts.<br>The paper is concluded with some general considerations on the experimental evaluation of spinup and the<br>link between the assimilation system design and model state filtering.<br>
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