P46 The impact of selecting different cut-off wave numbers and spin-up times in Spectral Nudging applications

Gómez, Breogán, Universidade de Santiago de Compostela (USC), Spain and Met Office, UK, Gonzalo Miguez-Macho, Carlos Otero-Casal and Miguel Ángel Prósper, USC, Spain

We present here a study of the impact of selecting different cut-off wave numbers and spin-up times when using Spectral Nudging. Four-day long simulations are performed using the WRF model for three one-month periods with various Spectral Nudging configurations that have ten different cut-off wave numbers ranging from the smallest (Free Run) to the largest possible (Grid Nudging). In addition, we have implemented a humidity nudging capability in Spectral Nudging, allowing for a more comprehensive comparison with Grid Nudging. Results show that the model error diminishes rapidly as the nudging expands over broader parts of the spectrum, due to correction of shifts in the synoptic patterns. However, this decreasing trend ceases sharply at cut-off wave numbers equivalent to a length scale of about 1000 km, where the model high frequency contribution starts to be significantly damped, and the error magnitude changes minimally thereafter. This scale corresponds to the Rossby Radius of deformation, separating synoptic from convective scales in the flow, thus making 1000 km the appropriate scale threshold to nudge to balance both effects. Finally, we note that longer spin-up times, between 36 and 48 hours, are needed for model errors to stabilize when using Spectral Nudging than with Grid Nudging.