Mechanisms leading to singular‐vector growth for FASTEX cyclones

Calculations of localized singular vectors (SVs) are performed using the European Centre for Medium-Range Weather Forecasts prediction model and the dynamical properties responsible for the SVs' growth are investigated. Potential vorticity (PV) diagnostics are used to investigate the dynamical processes responsible for SV growth in terms of total energy and PV. The independent evolution of perturbations initially confined either in the lower or in the upper troposphere is considered in order to identify the more dynamically active regions within an SV. The extent to which balanced-flow theory, valid in idealized models of non-normal growth, can be applied during the evolution of SVs is also examined. It is found that the part of the SV located below 500 hPa plays a major role in the interaction processes between the perturbation and the basic-state fields and is responsible for the SVs' energy growth. SVs are also shown to grow in terms of PV due to SV advection of the basic-state PV horizontal gradients. Perturbations initially confined at low levels can propagate vertically more efficiently than those localized above 500 hPa and the interaction with the upper-level basic-state fields is much more effective, leading to PV growth. If SVs are calculated with the total-energy norm, their initial state is found to be somewhat out of balance, because of a relative lack of amplitude in the vorticity component. During their evolution, SVs achieve a more balanced configuration. Copyright © 2002 Royal Meteorological Society.