Response of gap wind-driven freezing rain and ice accretion in the Willamette Basin, Oregon, to global warming

Topographically channeled winds such as gap winds can be key drivers of freezing rain and ice accretion. Understanding the effect of these winds on the response of freezing rain and ice accretion to global warming is challenging because the spatial resolution of global, and most regional, climate models is too coarse to accurately simulate these winds. To examine the effect of global warming in a region influenced by strong gap winds (the northern Willamette Basin, Oregon, United States), we used 13 year retrospective and pseudo-global warming simulations from a high-resolution, convection-permitting climate model capable of reproducing the easterly gap winds through the Columbia River Gorge. We compared results from the high-resolution model to those from a large ensemble of simulations generated with a coarser-resolution climate model without a well-defined Gorge. Generally, the future projected occurrence of freezing rain decreased at lower elevations and increased at higher elevations. Easterly, low-level winds that bring cold air into the basin were stronger during projected future freezing rain because in a warmer climate, weaker easterly winds were less likely to decrease the basin’s near-surface air temperatures to sub-freezing. Because of the stronger gap winds, more of the projected ice events had longer durations and accreted more ice, even at low elevations downwind of the Gorge. The coarser resolution model also projected stronger easterly winds during freezing rain, which implies that the necessity for stronger easterly winds during future freezing rain is not limited to the gap winds but is a regional, if not more widespread.