Forecast skill of targeted observations: a singular vector based diagnostic
CARDINALI, C. ; BUIZZA, R.
Targeted dropsonde data have been assimilated using the operational ECMWF 4D-Var system for 10 cases of the NORth Pacific
EXperiment (NORPEX) campaign, and their impact on analyses and corresponding forecasts has been investigated. The 10 fastest<br>growing "analysis" singular vectors have been used to define a sub-space of the phase space where initial conditions are expected to<br>be modified by the assimilation of targeted observing. A linear combination of this vector basis is the pseudo-inverse, that is the<br>smallest perturbation with the largest impact on the forecast error. The dropsonde-induced analysis difference has been decomposed<br>into three initial perturbations, two belonging to the sub-space spanned by the leading 10 SVs and one to its complement. Differences<br>and similarities of the three analysis components have been examined, and their impact on the forecast error compared with the impact of the pseudo-inverse.<br>Results show that, on average, the dropsonde-induced analysis difference component in the sub-space spanned by the leading 10 SVs<br>and the dropsonde-induced analysis difference component along the pseudo-inverse directions are very small (6% and 15%,<br>respectively, in terms of total energy norm). In the only case where dropsonde data were exactly released in the area identified by the<br>SVs, the different components of the dropsonde-induced analysis difference and the pseudo-inverse had consistent impacts on the<br>forecast error. It is concluded that the poor agreement between the dropsonde location and the SV maxima is the main reason for the<br>relatively small impact of the NORPEX targeting observations on the forecast error.
Accès à la notice sur le site du portail documentaire de Météo-France