The benefits of increasing resolution in global and regional climate simulations for European climate extremes

Abstract

Abstract. Many climate extremes, including heatwaves and heavy precipitation events, are projected to worsen under climate change, with important impacts for society. Future projections required for adaptation are often based on climate model simulations. Given finite resources, trade-offs must be made concerning model resolution, ensemble size, and level of model complexity. Here we focus on the resolution component. A given resolution can be achieved over a region using either global climate models (GCMs) or at lower cost using regional climate models (RCMs) that dynamically downscale coarser GCMs. Both approaches to increasing resolution may better capture small-scale processes and features (downscaling effect), but increased GCM resolution may also improve the representation of the large-scale atmospheric circulation (upscaling effect). The size of this upscaling effect is therefore important for deciding modelling strategies. Here we evaluate the benefits of increased model resolution for both global and regional climate models for simulating temperature, precipitation, and wind extremes over Europe at resolutions that could currently be realistically used for coordinated sets of climate projections at the pan-European scale. First we examine the benefits of regional downscaling by comparing EURO-CORDEX simulations at 12.5 and 50 km resolution to their coarser CMIP5 driving simulations. Secondly, we compare global-scale HadGEM3-A simulations at three resolutions (130, 60, and 25 km). Finally, we separate out resolution-dependent differences for HadGEM3-A into downscaling and upscaling components using a circulation analogue technique. Results suggest limited benefits of increased resolution for heatwaves, except in reducing hot biases over mountainous regions. Precipitation extremes are sensitive to resolution, particularly over complex orography, with larger totals and heavier tails of the distribution at higher resolution, particularly in the CORDEX vs. CMIP5 analysis. CMIP5 models underestimate precipitation extremes, whilst CORDEX simulations overestimate compared to E-OBS, particularly at 12.5 km, but results are sensitive to the observational dataset used, with the MESAN reanalysis giving higher totals and heavier tails than E-OBS. Wind extremes are somewhat stronger and heavier tailed at higher resolution, except in coastal regions where large coastal grid boxes spread strong ocean winds further over land. The circulation analogue analysis suggests that differences with resolution for the HadGEM3-A GCM are primarily due to downscaling effects.

The role of transient eddies and diabatic heating in the maintenance of European heat waves: a nonlinear quasi-stationary wave perspective

“…On one hand, the NSWM responses to the nonlinear interaction effects between orography and other forcings tend to be largest in the Pacific-North American sector, which is beyond the scope of this paper, in agreement with Held et al (2002). On the other hand, Iles et al (2019) suggest that increasing the resolution of the orography and coastlines in climate simulations have limited benefits for temperature extremes over Europe, except in reducing hot biases over mountainous regions.…”

The role of land cover in the climate of glacial Europe

“…Changes in land cover can only slightly influence temperature by an additional cooling of 0.66 • C in MAM and a warming of 0.85 • C in JJA, both statistically significant (Table 3). Similarly, Jahn et al (2005) found that the LGM-like vegetation cover produces colder temperatures (ca. −0.6 • C globally), especially in areas with the greatest decrease in tree cover.…”

“…One possible explanation for the disagreement is the coarse spatial resolution of the GCMs; at the continental scale, mountains, ice sheets, and water bodies have an important influence on regional circulation and climate that may not be represented appropriately at a typical GCM grid spacing of ca. 100 km (Rauscher et al, 2010;Gómez-Navarro et al, 2011Di Luca et al, 2012;Prein et al, 2013;Demory et al, 2020;Iles et al, 2020).…”

Projection of the Precipitation Extremes in Thailand Under Climate Change Scenario RCP8.5

“…For example, by comparing the ensemble mean of projected change by the regional climate model, with 25km grid spacing in Southeast Asia (Supari et al, 2020), their results showed decreasing the PRCPTOT in northern Thailand, but the PRCTOT was generally located in eastern Thailand, with substantial decreasing patterns, while the rising pattern was in northern Thailand in this study. In general, increasing resolution may better capture small-scale processes and features (downscaling effect), but increased GCM resolution may also improve the representation of large-scale atmospheric circulation (upscaling effect) (Iles et al, 2020). Particularly, precipitation extremes are sensitive to resolution over complex orography.…”