Does Spectral Nudging Have an Effect on Dynamical Downscaling Applied in Small Regional Model Domains?

Abstract

Spectral nudging is a method that was developed to constrain regional climate models so that they reproduce the development of the large-scale atmospheric state, while permitting the formation of regional-scale details as conditioned by the large scales. Besides keeping the large-scale development in the interior close to a given state, the method also suppresses the emergence of ensemble variability. The method is mostly applied to reconstructions of past weather developments in regions with an extension of typically 1000–8000 km. In this article, the authors examine if spectral nudging is having an effect on simulations with model regions of the size of about 700 km × 500 km at midlatitudes located mainly over flat terrain. First two pairs of simulations are compared, two runs each with and without spectral nudging, and it is found that the four simulations are very similar, without systematic or intermittent phases of divergence. Smooth fields, which are mainly determined by spatial patterns, such as air pressure, show hardly any differences, while small-scale and heterogeneous fields such as precipitation vary strongly, mostly on the gridpoint scale, irrespective if spectral nudging is employed or not. It is concluded that the application of spectral nudging has little effect on the simulation when the model region is relatively small.

Added value of regional reanalyses for climatological applications

“…meteorological parameter, applied method (e.g. Sheffield et al 2006, and references therein) and scale (Schaaf et al 2017).…”

High‐Resolution Climate Projections for the Northeastern United States Using Dynamical Downscaling at Convection‐Permitting Scales

“…Therefore, our model domain and nests are kept large, the regions of interest (primarily NH and MA) are kept as far away from the boundaries as is possible, and no nudging is applied at the boundaries to minimize numerical artifacts introduced from the boundaries to the innermost domain. Shaaf et al (2017) investigated the effects of spectral nudging using a regional climate model with a horizontal resolution and model time step similar to ours and found no differences compared to simulations that did not include nudging. There is still debate on nudging-related numerical artifacts entering inner domains.…”

“…One such methodology is dynamical downscaling using a regional climate model, where regional climate models are run with lateral and initial boundary conditions from ESM projections to produce higher resolution climate variables (e.g., Caldwell et al, 2009;Done et al, 2015;Dosio et al, 2014;Gao et al, 2012;Mearns et al, 2012;Pierce et al, 2012;Shaaf et al, 2017;Sun et al, 2016). Similarly, reanalysis products can be downscaled to assess regional phenomena and to understand historical changes in regional scales (e.g., Caldwell et al, 2009).…”

Is there added value of convection-permitting regional climate model simulations for storms over the German Bight and Northern Germany?

“…Test simulations, which are referred to the 2.8 km simulation, with and without SN showed no significant differences and therefore lead to the conclusion that spectral nudging is not necessary for high-resolution simulation. This is presumably a consequence of the relatively small model domain as spacious weather patterns which deviate from observed large weather systems have no time to develop inside this domain (Schaaf et al 2017). In addition, the area of the German Bight is topographically very flat and often dominated by strong westerlies and this prevailing exchange of air masses between the model boundaries and its interior leads to a smaller potential effect of SN.…”