Spectral nudging

Spectral nudging is a technique to keep the larger spatial scales of a limited area model (LAM) close to the larger spatial scales of its driving model, whereas the smaller spatial scales can develop freely. To do this, one gently "nudges" the larger spatial scales of a few dynamic fields (usually u- and v-wind and sometimes temperature as well) towards the larger spatial scales of the driving model. The nudging is usually stronger at higher altitudes than at lower altitudes. In GEM, the strength of the spectral nudging can get adjusted though parameters in the gem_settings.nml 'spn' namelist.

Spectral nudging parameters in GEM:

   GEM 3.3.3.1 (CRCM5)
   GEM 4 and newer versions

References:

• Xu Z. and Z.-L. Yang (2015): A new dynamical downscaling approach with GCM bias corrections and spectral nudging. J. Geophys. Res. Atmos., 120, 3063–3084

• Berg P., R. Döscher, and T. Koenigk (2013): Impacts of using spectral nudging on regional climate model RCA4 simulations of the Arctic Geosci. Model Dev., 6, 849–859, 2013

• Wang J., and V.R. Kotamarthi (2013): Assessment of Dynamical Downscaling in Near-Surface Fields with Different Spectral Nudging Approaches Using the Nested Regional Climate Model (NRCM) JAMC Vol 52, 1576-1591

• Bowden J.H., C.G. Nolte, and T.L. Otte (2012): Simulating the impact of the large-scale circulation on the 2-m temperature and precipitation climatology. Clim Dyn (2013) 40:1903–1920

• Šeparović L., R. de Elía, and R. Laprise (2012) Impact of spectral nudging and domain size in studies of RCM response to parameter modification. Clim Dyn 38(7-8): 1325-1343
  Exerpt:
  "Nested limited-area Regional Climate Models (RCMs) are models that dynamically downscale global General Circulation Model (GCM) simulations or objective analyses to high-resolution computational grids, using a high-resolution representation of the surface forcing and model dynamics. RCMs require the information on some prognostic variables as their lateral boundary conditions (LBC). The choices of integration domains and nesting techniques are free parameters of RCMs. The optimal integration domain depends on the particular situation, although there are some general recommendations that can facilitate user’s judgment (e.g., Laprise et al. 2008). For example, Leduc and Laprise (2008) showed that the use of a too small domain could result in the simulations being deficient in fine-scale variance. It has been also noted that in large continental-scale domains RCM large-scale variables can considerably drift from the driving fields, which can then result in appearance of large spurious gradients in the vicinity of the outflow boundaries. Spectral nudging (SN; von Storch et al. 2000; Biner et al. 2000) has been employed to ensure that the model solution remains close to the large-scale components of the driving fields over the entire domain. However, the use of spectral nudging remains an open issue. …"

• Mohino E., S. Janicot, H. Douville, L.Z.X. Li (2011): Impact of the Indian part of the summer MJO on West Africa using nudged climate simulations. Clim Dyn (2012) 38:2319–2334

• Alexandru A. (2009): Sensitivity Study of Regional Climate Model Simulations to Large-Scale Nudging Parameters. Mon Weather Rev 137: 1666-1686

• Omrani H., P. Drobinski, and T. Dubos (2009): Spectral nudging in regional climate modeling: how strong should we nudge? Q.J.R. Meteorol. Soc. 00: 1–6 (2009)

• Radu R., M. Déqué, and S. Somot (2008): Spectral nudging in a spectral regional climate model Tellus (2008), 60A, 898–910

• Riette S., and D. Caya (2002): Sensitivity of short simulations to the various parameters in the new CRCM spectral nudging. Research Activities in Atmospheric and Oceanic Modelling WMO/TD-1105, Rep. 32, H. Ritchie, Ed., WMO, 7.39–7.40

• Riette S. (2000): Technique de Pilotage Spectral du Modele Regional Canadien de Climat

• Biner S., D. Caya, R. Laprise, L. Spacek (2000) Nesting of RCMs by imposing large scales. Research activities in Atmospheric and Oceanic Modelling, WMO/TD 987, Report 30: 7.3-7.4

• von Storch H., H. Langenberg, and F. Feser (2000): A spectral nudging technique for dynamical downscaling purposes. Mon Weather Rev 128: 3664–3673