Abstract
The aim of this study is to investigate if the representation of Northern Hemisphere blocking is sensitive to resolution in current-generation atmospheric global circulation models (AGCMs). An evaluation is conducted of how well atmospheric blocking is represented in four AGCMs whose horizontal resolution is increased from a grid spacing of more than 100 km to about 25 km. It is shown that Euro-Atlantic blocking is simulated overall more credibly at higher resolution (i.e., in better agreement with a 50-yr reference blocking climatology created from the reanalyses ERA-40 and ERA-Interim). The improvement seen with resolution depends on the season and to some extent on the model considered. Euro-Atlantic blocking is simulated more realistically at higher resolution in winter, spring, and autumn, and robustly so across the model ensemble. The improvement in spring is larger than that in winter and autumn. Summer blocking is found to be better simulated at higher resolution by one model only, with little change seen in the other three models. The representation of Pacific blocking is not found to systematically depend on resolution. Despite the improvements seen with resolution, the 25-km models still exhibit large biases in Euro-Atlantic blocking. For example, three of the four 25-km models underestimate winter northern European blocking frequency by about one-third. The resolution sensitivity and biases in the simulated blocking are shown to be in part associated with the mean-state biases in the models’ midlatitude circulation.
Highlights
Blocking refers to the occurrence of quasi-stationary high pressure systems at midlatitudes and can be described by a number of key characteristics (Barriopedro et al 2010); blocking highs persist for several days to weeks and often divert cyclones traveling in the storm track poleward or equatorward (Rex 1950; Woollings et al 2010; Zappa et al 2014)
Recent advances in computing power and investment in higher model resolution have enabled several modeling centers to run atmospheric general circulation models (GCMs) (AGCMs) at about 25-km grid spacing for the simulation lengths and ensemble sizes required for the evaluation of blocking in these higher-resolution climate models
This study aims to use an ensemble of present-day climate simulations from four atmospheric global circulation models (AGCMs) with about 25-km grid spacing at midlatitudes to (i) quantify biases in the representation of blocking throughout the year and (ii) assess the sensitivity of these biases to the model resolution
Summary
SHAFFREY, JANE STRACHAN,a AND PIER LUIGI VIDALE. National Centre for Atmospheric Science, Department of Meteorology, University of Reading, Reading, United Kingdom. European Centre for Medium-Range Weather Forecasts, Reading, United Kingdom, and Alfred Wegener Institute, Bremerhaven, Germany (Manuscript received 28 January 2016, in final form 12 September 2016)
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