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

Abstract

Spectral nudging is a method which 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 kilometers. In this article, we examine if spectral nudging is having an effect on simulations with model regions of the size of about 700 km x 500 km at mid-latitudes located mainly over flat terrain. We first compare two pairs of simulations, two runs each with and without spectral nudging, and find 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 grid-point scale, irrespective if spectral nudging is employed or not. We conclude that the application of spectral nudging has little effect on the simulation when the model region is relatively small.