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

Abstract

Numerous studies have been presented describing the benefits of using Nudging to constrain the evolution of a numerical model. Recently, a variation of this approach called Spectral Nudging has gained popularity for its ability to maintain the higher temporal and spatial variability of the model results, while forcing the large scales in the solution with a coarser‐resolution field. Here, we focus on a not‐much‐explored aspect of this technique: the impact of selecting different cut‐off wave numbers and spin‐up times. We perform 4‐day‐long simulations with the WRF model, daily for three different 1‐month periods including a free run and several Spectral Nudging experiments with cut‐off wave numbers ranging from the smallest to the largest possible (Grid Nudging). Results show that Spectral Nudging is very effective at imposing the selected synoptic scales onto the solution, while allowing the limited‐area model to incorporate finer‐scale features. The model error diminishes rapidly as the nudging expands over broader parts of the spectrum, but this decreasing trend ceases sharply at cut‐off wave numbers equivalent to a length‐scale of about 1000 km, and the error magnitude changes minimally thereafter. This scale corresponds to the Rossby radius of deformation, separating synoptic from convective scales in the flow. When nudging is applied for scales larger than the Rossby radius of deformation, a shifting of the synoptic patterns can occur in the solution, yielding large model corrections towards the analysis. However, when selecting smaller scales, the fine‐scale contribution of the model is damped, thus making 1000 km the appropriate scale threshold to nudge in order to balance both effects. Finally, we note that longer spin‐up times are needed for model errors to stabilize when using Spectral Nudging than with Grid Nudging. Results suggest that this time is between 36 and 48 h.

Precipitation Dynamical Downscaling Over the Great Plains

“…Stronger nudging with larger nudging coefficients on more wave numbers was shown to have a detrimental effect on downscaled precipitation over the Great Plains, particularly on the detailed structures of precipitation, since it may destroy the mesoscale features simulated by the dynamic model (Tian et al, ; Wang & Kotamarthi, ). Gomez and Miguez‐Macho () explicitly suggest that 1,000 km is the optimal scale threshold to nudge in order to balance the constraint from the driving fields and fine‐scale contribution from the downscaling model.…”

Evaluation of the moisture sources in two extreme landfalling atmospheric river events using an Eulerian WRF tracers tool

“…Since spectral nudging has been used in the simulations, the large-scale circulation in the model closely follows ERAInterim and no further validation is required (Gómez and Miguez-Macho, 2017). Water vapor is not nudged to ensure the mass conservation needed for the traceability of humidity from different sources.…”

The Use of a Spectral Nudging Technique to Determine the Impact of Environmental Factors on the Track of Typhoon Megi (2010)

“…The experimental design in this study is different from that of many previous studies [19][20][21][22][23] in which SN is often used to correct large-scale circulation in the upper atmosphere levels. Therefore, the experimental design in this study has certain limitations and is unlikely to get optimal simulation results.…”