Orographic resolution driving the improvements associated with horizontal resolution increase in the Northern Hemisphere winter mid-latitudes

Abstract

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.

Toward Eliminating the Decades‐Old “Too Zonal and Too Equatorward” Storm‐Track Bias in Climate Models

“…In this respect, the “too zonal and too equatorward” bias is actually closely related to the underestimation of the frequency of high‐intensity growth in many climate models (Priestley et al., 2020; Seiler & Zwiers, 2015), and thus all three biases are associated with insufficiently resolved diabatic processes. The bias is thus best summarized as “too zonal, too equatorward, too weak” and at least partly resulting from too small diabatic‐growth contributions in addition to low‐resolution SSTs (Lee et al., 2018; Woollings et al., 2010) and orography (Davini et al., 2022; Kanehama et al., 2019). Finally, it is worth emphasizing that the tropopause sharpness, as measured by the PV gradient, also benefits from higher resolution and a better storm track representation is thus to be expected even in a hypothetically dry atmosphere (Gray et al., 2014).…”

“…This study addresses the question of whether kilometer‐scale simulations have the potential to overcome the critical “too zonal and too equatorial” circulation bias observed over the North Atlantic storm track in CMIP models. Among the potential mechanisms underlying the bias, sea‐surface temperature anomalies (Keeley et al., 2012), underresolved orography (Berckmans et al., 2013; Davini et al., 2022) and low model resolution in general (Curtis et al., 2020; Zappa et al., 2013) are all discussed in the literature. To test the role of resolution and the question if processes on the storm scale and thus internal to storm tracks account for the bias, a 120‐month perpetual boreal winter simulation (corresponding to 40 winter seasons) is conducted at a range of resolutions from a global resolution of about 20–10 km over the storm track to 5 km over an idealized sea surface temperature (SST) front using regional resolution refinements via bi‐directional interacting grid nests.…”