As assessment of the singular-vector approach to targeted observing using the FASTEX dataset

Ln this study, we investigate whether the results of assimilating special t argeted observations from the Fronts and Atlantic Storm-Track EXperiment (F ASTEX) in an operational forecast model support the underlying principles o f the singular-vector (SV) approach to targeted observing. A simple framewo rk is presented that allows explicit examination of the changes made to the analysis in the subspace of the leading SVs from assimilation of the obser vations. The impact of this component on the forecast provides a key measur e of the effectiveness of SV-based targeting. Results confirm that the impact of the additional observations occurs prima rily as a result of changes to the analysis in the subspace of the leading SVs. These changes account for a small fraction of the total targeting incr ement at initial time, but explain a large fraction of the response of the forecast at the verification time. The results also confirm that analysis e rrors in the middle and lower troposphere are an important source of error in forecasts of extratropical cyclones. While moist processes can play an important role in the forecast-error evol ution, SVs that exclude these processes can remain an effective targeting t ool. This is because the location of maximum sensitivity will not necessari ly differ from that identified by the dry SVs. It is also shown that the lo cations of the leading (target) SVs can be computed accurately with lead ti mes of up to 48 hours, allowing ample time for the deployment of observatio nal resources.