4D-Var analysis of potential vorticity pseudo-observation
Analyse 4D-Var des pseudo-observations de vorticité potentielle
Guérin, R. ; Desroziers, G. ; Arbogast, P.
It is shown that a poor forecast of a midlatitude cyclone developing over the western Atlantic Ocean can be
significantly improved by potential vorticity (PV) modifications in the vicinity of the dynamical tropopause in the<br>initial conditions, in order to modify the position of the tropopause. Manually built PV corrections are introduced<br>by using the ARPEGE-IFS 4D-Var assimilation system. For that purpose, a PV observation operator, its tangentlinear<br>and adjoint versions, based on a simplified form of Ertel PV, has been implemented. The impact of the<br>PV pseudo-observations in an optimal system (i.e. analysed using tuned observation-error statistics) is positive;<br>the maximum positive improvement of the forecast is obtained over western Europe when the cyclone reaches<br>its maximum amplitude. The forecast is never degraded during the model run, whereas another run forced by<br>the pseudo-observations is characterized by the development of a forecast error pattern downstream to the main<br>pattern of analysis increment. The conditioning of the minimization problem defined as the sparsity (for the nondiagonal<br>parts) of the Hessian matrix is weakly affected by the PV observation operator leading to quite good<br>convergence.<br>
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