COMPUTATION OF OPTIMAL UNSTABLE STRUCTURES FOR A NUMERICAL WEATHER PREDICTION MODEL

Numerical experiments have been performed to compute the fastest growi ng perturbations in a finite time interval for a complex numerical wea ther prediction model. The models used are the tangent forward and adj oint versions of the adiabatic primitive-equation model of the Integra ted Forecasting System developed at the European Centre for Medium-Ran ge Weather Forecasts and Meteo France. These have been run with a hori zontal truncation T21, with 19 vertical levels. The fastest growing pe rturbations are the singular vectors of the propagator of the forward tangent model with the largest singular values. An iterative Lanczos a lgorithm has been used for the numerical computation of the perturbati ons. Sensitivity of the calculations to different time intervals and t o the norm used in the definition of the adjoint model have been analy sed. The impact of normal mode initialization has also been studied. T wo classes of fastest growing perturbations have been found; one is ch aracterized by a maximum amplitude in the middle troposphere, while th e other is confined to model layers close to the surface. It is shown that the latter is damped by the boundary layer physics in the full mo del. The linear evolution of the perturbations has been compared to th e non-linear evolution when the perturbations are superimposed on a ba sic state in the T63, 19-level version of the ECMWF model.