Relationship between singular vectors and transient features in the background flow

The relationships between singular vectors (SVs) and transient features in the background flow are examined through both composite techniques and case-studies. The SVs are derived using the NOGAPS forward and adjoint tangent models based on 48-hour forecasts during the NORPEX period. Composite results and case-studies both reveal significant spatial relationships between the SVs and transient features in the background flow. The SV perturbations often occur below distinctive high potential-vorticity (PV) features in the middle-to-upper troposphere. Case-studies reveal that the SVs propagate upward rapidly and have an impact on these PV features through the end of the optimization interval. In order to investigate how these small, initial perturbations have such a large impact on future development, the relationship between SVs and the quasi-geostrophic forcing is examined through the use of Q-vectors. The SV perturbations, while scaled to have a very small impact on the temperature and wind fields, have a very large impact on the mid-tropospheric Q-vectors. In contrast, the impact of the SV on the Q-vectors is negligible when added to an analysis field for which it is not optimal (e.g. the analysis field from another day). These findings establish a significant relationship between the SV perturbations and dynamically active regions in the middle troposphere, and point toward an integral link between SVs and upper-level PV precursors in synoptic development.