Optimal Spectral Nudging for Global Dynamic Downscaling

Abstract

This study analyzes a method of constructing a homogeneous, high-resolution global atmospheric hindcast. The method is the spectral nudging technique, which was applied to a state-of-the-art general circulation model (ECHAM6, T255L95). Large spatial scales of the global climate model prognostic variables were spectrally nudged toward a reanalysis dataset (NCEP-1, T62L28) for the past few decades. The main idea is the addition of dynamically consistent regional weather details to the coarse-grid NCEP-1 reanalysis. A large number of sensitivity experiments was performed, using different nudging e-folding times, vertical profiles, wavenumbers, and variables. Comparisons with observations and several reanalyses showed a high dependency on the variations of the nudging configuration. At the global scale, the accordance is very high for extratropical regions and lower in the tropics. A wavenumber truncation of 30, a relatively short e-folding time of 50 min, and a plateaushaped nudging profile applied only to divergence and vorticity generally yielded the best results. This is one of the first global spectral nudging hindcast studies and the first applying an altitude-dependent profile to selected prognostic variables. The method can be applied to reconstructing the history of extreme events such as intense storms within the context of ongoing climate change.

Evaluating Different Techniques for Constraining Lower Atmospheric Variability in an Upper Atmosphere General Circulation Model: A Case Study During the 2010 Sudden Stratospheric Warming

“…Although not implemented or investigated in this study, another possibility for coupling a lower/middle atmospheric model with an upper atmospheric model is by constraining dynamical model fields in spectral space. Such a technique developed for global dynamic downscaling was recently described by Schubert‐Frisius et al (). Better coupling between numerical models of different resolution and different physical parameterizations without the generation of spurious numerical artifacts might be achieved in spectral space.…”

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.…”

State and Perspectives of the Concept of Large‐Scale Conditioning of Regional Climate Modelling

“…A variety of parameters, such as the vertical profile of the nudging parameters, were tested (Schubert-Frisius et al, 2017). Also the performance of the global simulation with respect to regional simulations was confirmed -it turned out that if the global and the regional models had a comparable grid resolution, also the hindcasts were similar, as demonstrated by Figure 7, which compares in terms of the Brier-skill score the global simulation with two regional models-using the satellite product QUIKSCAT as reference.…”

“…This shows the large dependency of the regional climate model on its global forcing data and that for some cases spectral nudging cannot completely cure data inhomogeneities, which were presumably inherent in the forcing. (Schubert-Frisius et al, 2017) …”

“…The concept was also tested and implemented by Schubert-Frisius et al, (2017) and, using the ECHAM6 global atmospheric models with a T255 grid resolution (corresponding to about 60 km), and the large-scale NCEP/NCAR reanalysis as constraint. As an example, Figure 6 shows the description of a typhoon in the NCEP/NCAR reanalysis together with the downscaling achieved by the constrained global simulation.…”