Atmospheric Blocking, Forecast Model Resolution, and Extreme Winter Weather Conditions in the U.S.

An atmospheric block is defined as a large-scale obstruction of zonal flow in the form of 500mb quasi-stationary cyclones and anticyclones lasting a minimum of five days. Their persistent displacement of the jet stream coincide with a shift in storm tracks, influencing regional weather patterns, often in the form of temperature and precipitation extremes. With resulting impacts from extremes on human and natural systems at large, the significance in predictability of blocks is highlighted. Climate models are notorious for lack of skill in accurately capturing atmospheric blocking, primarily with strong underestimations of wintertime blocking frequencies over the North Atlantic basin. Suggestions to decrease model biases relating to blocking include increasing horizontal resolution and the use of a fully coupled ocean-atmosphere model. Therefore both 1.0ºx1.0º and 0.5ºx0.5º outputs of the Community Climate System Model, version 4 (CCSM4) have been evaluated in their ability to capture January-February blocking frequency, duration, and consequential up- and downstream impacts on the mean flow with associated regional precipitation and temperature extremes.

While duration and location of blocking events were accurately represented by the model, blocking frequencies were poorly captured for both higher and lower versions of CCSM4, with minimal differences between them and a strong underestimation over the North Atlantic. The higher resolution was able to outperform the lower with impacts on the mean flow during blocking events and better represent resulting regional precipitation and temperature extremes. These results reveal that increasing horizontal resolution may not improve blocking frequency bias, but do increase confidence in accurately predicting resulting regional impacts caused by blocking events.