The ECMWF operationnal implementation of four dimensional variational assimilation. Part. I: experimental results with simplified physics
L'implémentation opérationnelle au CEPMMT implementation de l'assimilation variationnelle à quatre dimensions. Partie. I : résultats expérimentaux avec physique simplifiée
Rabier, F. ; Jarvinen, H. ; Klinker, E. ; Mahfouf, J.F. ; Simmons, A.
Année de publication
2000
This paper presents results of a comparison between four-dimensional <br>variational assimilation (4D-Var). using a 6-hour assimilation window <br>and simplified physics during the minimization, and three-dimensional <br>variational assimilation (3D-Var). Results have been obtained at <br>operational' resolution T213L31/T63L31. (T defines the spectral <br>triangular truncation and L the number of levels in the vertical, with <br>the first parameters defining the resolution of the model trajectory, <br>and the second the resolution of the inner-loop.) The sensitivity of the<br> 4D-Var performance to different set-ups is investigated. In particular,<br> the performance of 4D-Var in the Tropics revealed some sensitivity to <br>the way the adiabatic nonlinear normal-mode initialization of the <br>increments was performed. Going from four outer-loops to only one (as in<br> 3D-Var), together with a change to the 1997 formulation of the <br>background constraint and an initialization of only the small scales, <br>helped to improve the 4D-Var performance. Tropical scores then became <br>only marginally worse for 4D-Var than for 3D-Var. Twelve weeks of <br>experimentation with the one outer-loop 4D-Var and the 1997 background <br>formulation have been studied. The averaged scores show a small but <br>consistent improvement in both hemispheres at all ranges. In the short <br>range, each two- to three-week period has been found to be slightly <br>positive throughout the troposphere. The better short-range performance <br>of the 4D-Var system is also shown by the fits of the background fields <br>to the data. More results are presented for the Atlantic Ocean area <br>during FASTEX (the Fronts and Atlantic Storm-Track Experiment), during <br>which 4D-Var is found to perform better. In individual synoptic cases <br>corresponding to interesting Intensive Observing Periods, 4D-Var has a <br>clear advantage over 3D-Var during rapid cyclogeneses. The very <br>short-range forecasts used as backgrounds are much closer to the data <br>over the Atlantic for 4D-Var than for 3D-Var. The 4D-Var analyses also <br>display more day-to-day variability. Some structure functions are <br>illustrated in the 4D-Var case for a height observation inserted at the <br>beginning, in the middle or at the end of the assimilation window. The <br>dynamical processes seem to be relevant, even with a short 6-hour <br>assimilation period, which explains the better overall performance of <br>the 4D-Var system.</div>
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