Excitation of Rossby-wave trains: optimal growth of forecast errors

Abstract

Singular vectors (SVs) with appropriate norms and time scales are used in this study to capture the excitation of Rossby-wave trains (RWTs). Using global analysis data provided by the Canadian Meteorological Centre, a long-lasting RWT is selected beginning on 19 Nov 2002. Ten SVs are calculated using an optimization time interval of 48 h and a tangent linear model including a complete set of simplified physical parameterizations. At initial time, the global total-energy norm is used. For the final time, the total energy (TE) and the rotational kinetic energy (RKE) norms over three different restricted horizontal and vertical domains are used. These different configurations are examined to enable an appropriate configuration to be selected for our future work on using SVs to study the excitation of RWTs. The SVs are computed and non-linear forecasts are performed using the Canadian Global Environment Multiscale model. Results obtained show that when the analysis is perturbed with the pseudo-inverse of the 48 h forecast error in the SV subspace, the perturbation propagates with the group velocity of the RWT and the error in the non-linear forecast over the life of the wave train (6 days) is significantly reduced. Comparison between using a final-time norm based on either TE or RKE showed little impact.