Resolution Dependence of Extreme Wind Speed Projections in the Great Lakes Region

The effect of anthropogenic climate change on extreme near -surface wind speeds is uncertain. Observed trends are weak and dif fi cult to disentangle from internal variability, and model projections disagree on the sign and magnitude of trends. Standard coarse -resolution climate models represent the fi ne structures of relevant physical phenomena such as extratropical cyclones (ETCs), upper -level jet streaks, surface energy fl uxes, and land surface variability less skillfully than their high -resolution counterparts. Here, we use simulations with the NCAR Community Earth System Model with both uniform (110 km) resolution and the variable -resolution con fi guration (VR-CESM-SONT, from 110 to 7 km) to study the effect of re fi ned spatial resolution on projections of extreme strong and weak wind speeds in the Great Lakes region under end -of -century RCP8.5 forcing. The variable -resolution con fi guration projects strengthening of strong -wind events in the re fi ned region with the opposite occurring in the uniform -resolution simulation. The two con fi gurations provide consistent changes to synoptic -scale circulations associated with high -wind events. However, only the variable -resolution con fi guration projects weaker static stability, enhanced turbulent vertical mixing, and consequentially enhanced surface wind speeds because boundary layer dynamics are better captured in the re fi ned region. Both models project increased frequency of extreme weak winds, though only VR-CESM-SONT resolves the cold -season inversions and summertime high temperatures associated with stagnant wind events. The identi fi able mechanism of the changes to strong winds in VR-CESM-SONT provides con fi dence in its projections and demonstrates the value of enhanced spatial resolution for the study of extreme winds under climate change.