The impact of wave number selection and spin-up time in spectral nudging

Resumo

The present work studies the spectral nudging technique aiming to improve the accuracy of the forecasts of atmospheric limited area models. It allows to impose a forcing over a subset of the spatial spectral scale, typically the largest one, while it allows the limited area model to develop its own dynamics in the unconstrained ones. Despite a substantial number of works has been published on the topic, not many have studied the technique in a systematic way. A great part its extension studies two relevant parameters from spectral nudging. Firstly, the cut-off wave number, which effectively separates the nudged scales from the free running ones. And, secondly, the spin-up time, which determined how much time is needed to reach a balance between the nudging force and the model internal climate. Our results show that the optimal cut-off wave number coincides with the Rossby Radius of Deformation, both in mid and tropical latitudes, suggesting that this parameter is related with the dynamic characteristics of the modelled area, and not with features of the experiment design. The optimal spin-up time is found to be 24/48 h for mid latitudes and 72/96 h for tropical latitudes. At the same time, we have studied the suitability of spectral nudging as a poor man’s data assimilation technique, which are commonly used to improve the initialization of the numerical weather prediction models. Our results indicate that it is beneficial to use some nudging technique at the beginning of the simulation, but we have not found relevant differences between grid nudging and spectral nudging.