Investigation of indiscriminate nudging and predictability in a nested quasi-geostrophic model

International audienceIn this work, we consider the effect of indiscriminate nudging time on an idealized high-resolution global model (GM) and limited-area model (LAM) simulations. The model used is a two-layer quasi-geostrophic model on the beta-plane. The effect of nudging is studied as a function of the predictability time, following a 'Big Brother' experimental approach: a high-resolution 'global' model is used to generate a 'reference run'. These fields are filtered afterwards to remove small scales and provide the coarse-resolution fields which are used to drive the high-resolution GM and the LAM. Comparison of the reference fields and the high-resolution runs over the same region allows the estimation of the ability of the high-resolution GM and LAM to regenerate the removed small scales. This fully nonlinear set-up mimics the configuration used for regional high-resolution atmospheric modelling. For the high-resolution GM, the results show that the behaviour of the nudged model depends primarily on the ratio of the nudging time to the predictability time. When the nudging time is very small compared to the predictability time, the model reproduces the large scale used to drive the model. On the other hand, if the nudging time is close to or larger than the predictability time, the nudging effect is weak and both large and small scales are poorly reproduced compared to the reference fields. The best result is obtained with a nudging time close to half the predictability time. This technique clearly improves the model capacity to reproduce the reference fields. For the high-resolution LAM, our results show that for a sufficiently small domain the simulation is largely controlled by the lateral boundary conditions (LBCs) and is quasi-insensitive to nudging. However, if the domain size exceeds a few Rossby radii, the high-resolution LAM becomes sensitive to initial conditions and the control exerted by LBCs becomes insufficient to prevent a divergence from the driving fields. Although the reconstructed fine scales are significantly damped, they are surprisingly well correlated to their reference values in a deterministic sense, not a statistical sense. © 2011 Royal Meteorological Society