Comparative analyses of the January 2004 cold air outbreak
Hornberger, Kelli Lynne
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Cold air outbreaks (CAOs) occur when large scale atmospheric circulations allow for the incursion of polar air masses into middle and lower latitudes, influencing wintertime temperatures regionally. The January 2004 CAO is identified as a major CAO in the Deep South of the United States in terms of wind chill equivalent temperature or a temperature-only criterion. Surface air temperature, horizontal winds, specific humidity, and Ertel potential vorticity are analyzed for this event using several reanalysis products: National Aeronautic and Space Administration Modern Era Retrospective-Analysis for Research and Application (MERRA), the National Centers for Environmental Prediction National Center for Atmospheric Research (NCEP-NCAR), and the National Centers for Environmental Prediction North American Regional Reanalysis (NARR). We perform an intercomparison of the reanalysis products and parallel surface station observations during the synoptic evolution of the leading cold front associated with CAO onset. The key synoptic, mesoscale, and dynamical features associated with onset are studied to determine the relative accuracy of the respective reanalysis products in representing the key features. The comparative evaluation revealed pronounced temperature and moisture biases in the NCEP-NCAR reanalysis products that limit its utility in portraying the synoptic features characteristic of CAO onset. Conversely, both MERRA and NARR accurately represent the detailed thermodynamic and moisture structural evolution associated with CAO onset indicating their utility in future observationally-based studies of CAO events. Ertel potential vorticity analyses indicate that the onset of the 2004 CAO is strongly linked to an incipient tropopause fold feature that developed over the Great Lakes region.