A climatology of cold air outbreaks over North America: WACCM and ERA-40 comparison and analysis

A climatology of cold air outbreaks (CAOs) over North America is presented on the basis of a 50 year simulation of the Whole Atmosphere Community Climate Model (WACCM). This climatology is compared to a similar CAO climatology based on 45 years (1957-2002) of European Centre for Medium-Range Weather Forecasts 40 Year Re-Analysis Project (ERA-40) data. A CAO is identified at a given grid point if the following criteria are met: (1) the surface temperature is lower than 1.5 standard deviations below the 31 day climatological running mean, (2) the standard deviation in temperature is greater than 2 K, and (3) conditions 1 and 2 are satisfied over a contiguous area of similar to 5 degrees longitude by 5 degrees latitude. WACCM and ERA-40 comparisons are shown for CAO frequency, temperature anomaly from 31 day climatological mean, geographical location, minimum temperature, and areal extent. Overall, CAOs in WACCM occur similar to 30% less frequently than in ERA-40 but cover similar to 30% greater area and are 1-2 K lower. In midwinter, WACCM CAOs form at lower latitudes and penetrate to lower latitudes compared to CAOs in ERA-40.