Abstract

In recent years much attention has been devoted to the investigation of the impact of increasing the horizontal resolution of global climate models. In the present work, a set of atmosphere-only idealized sensitivity simulations with EC-Earth3 have been designed to disentangle the relative roles of increasing the resolution of the resolved orography and of the atmospheric grid. Focusing on the winter Northern Hemisphere, it is shown that if the grid is refined while keeping the resolved orography unchanged, model biases are reduced only in some specific occasions. Conversely, increasing the resolved (or mean) orography is found to clearly reduce several important systematic model errors, including synoptic transient eddies, the North Atlantic jet stream variability and atmospheric blocking frequency and duration. From an analysis of the radiation budget it is concluded that the large changes in radiative fluxes caused by the resolution increase – something commonly observed in climate models – have a relevant impact on the atmospheric circulation, partially offsetting the benefits obtained from the increase in orographic resolution. These findings point to the necessity of always tuning climate models to fully exploit the benefits of high horizontal resolution.

Highlights

  • Global climate models (GCMs) have been shown to be powerful tools for understanding Earth’s climate variability and for estimating its future evolution

  • From an analysis of the radiation budget it is concluded that the large changes in radiative fluxes caused by the resolution increase - something commonly observed in climate models - have a relevant impact on the atmospheric circulation, partially offsetting the benefits obtained from the 10 increase in orographic resolution

  • The analysis presented in the previous sections showed that while the increase in orographic resolution improves the representation of the Northern Hemisphere winter mean climate and variability, the effect of the increase in atmospheric resolution is more complex and provides contrasting results

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Summary

Introduction

Global climate models (GCMs) have been shown to be powerful tools for understanding Earth’s climate variability and for estimating its future evolution. Orography is well known to considerably impact the Northern Hemisphere winter circulation from daily to climate timescales (Held et al, 2002; Sandu et al, 2016; Pithan et al, 2016; van Niekerk 45 et al, 2017). This is due both to the direct forcing on the planetary waves induced by the large-scale orographic barriers such as Rocky Mountains or Tibetan Plateau (Brayshaw et al, 2009; White et al, 2021) and to the small-scale processes, such as turbulent orographic form drag, blocking of the flow at low levels and breaking of orographically generated gravity waves in the upper-troposphere and stratosphere (Lott and Miller, 1997; Beljaars et al, 2004; Sandu et al, 2019). Many questions related to orographic processes and their impacts on the atmospheric flow remain open

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